Programming Tool Box/SIMS_2.iso/bp_6_93/bonus/winer/chap2.txt

Text File | 1994-09-03 | 105KB | 1,920 lines

                                 CHAPTER 2

                            VARIABLES AND DATA

DATA BASICS
===========

In Chapter 1 you examined the role of a compiler, and learned how it
translates BASIC source code into the assembly language commands a PC
requires.  But no matter how important the compiler is when creating a
final executable program, it is only half of the story.  This chapter
discusses the equally important other half: data.  Indeed, some form of
data is integral to the operation of every useful program you will ever
write.  Even a program that merely prints "Hello" to the display screen
requires the data "Hello".
     Data comes in many shapes and sizes, starting with a single bit,
continuing through eight-byte double precision variables, and extending all
the way to multi-megabyte disk files.  In this chapter you will learn about
the many types of data that are available to you, and how they are
manipulated in a BASIC program.  You will also learn how data is stored and
assigned, and how BASIC's memory management routines operate.
     Compiled BASIC supports two fundamental types of data (numeric and
string), two primary methods of storage (static and dynamic), and two kinds
of memory allocation (near and far).  Of course, the myriad of data types
and methods is not present to confuse you.  Rather, each is appropriate in
certain situations.  By fully understanding this complex subject, you will
be able to write programs that operate as quickly as possible, and use the
least amount of memory.
     I will discuss each of the following types of data: integer and
floating point numeric data, fixed-length and dynamic (variable-length)
string data, and user-defined TYPE variables.  Besides variables which are
identified by name, BASIC supports named constant data such as literal
numbers and quoted strings.
     I will also present a complete comparison of the memory storage
methods used by BASIC, to compare near versus far storage, and dynamic
versus static allocation.  It is important to understand that near storage
refers to variables and other data that compete for the same 64K data space
that is often referred to as Near Memory or Data Space.  By contrast, far
storage refers to the remaining memory in a PC, up to the 640K limit that
DOS imposes.
     The distinction between dynamic and static allocation is also
important to establish now.  Dynamic data is allocated in whatever memory
is available when a program runs, and it may be resized or erased as
necessary.  Static data, on the other hand, is created by the compiler and
placed directly into the .EXE file.  Therefore, the memory that holds
static data may not be relinquished for other uses.
     Each type of data has its advantages and disadvantages, as does each
storage method.  To use an extreme example, you could store all numeric
data in string variables if you really wanted to.  But this would require
using STR$ every time a value was to be assigned, and VAL whenever a
calculation had to be made.  Because STR$ and VAL are relatively slow,
using strings this way will greatly reduce a program's performance.
Further, storing numbers as ASCII digits can also be very wasteful of
memory.  That is, the double precision value 123456789.12345 requires
fifteen bytes, as opposed to the usual eight.
     Much of BASIC's broad appeal is that it lets you do pretty much
anything you choose, using the style of programming you prefer.  But as the
example above illustrates, selecting an appropriate data type can have a
decided impact on a program's efficiency.  With that in mind, let's examine
each kind of data that can be used with BASIC, beginning with integers.


INTEGERS AND LONG INTEGERS
==========================

An integer is the smallest unit of numeric storage that BASIC supports, and
it occupies two bytes of memory, or one "word".  Although various tricks
can be used to store single bytes in a one-character string, the integer
remains the most compact data type that can be directly manipulated as a
numeric value.  Since the 80x86 microprocessor can operate on integers
directly, using them in calculations will be faster and require less code
than any other type of data.  An integer can hold any whole number within
the range of -32768 to 32767 inclusive, and it should be used in all
situations where that range is sufficient.  Indeed, the emphasis on using
integers whenever possible will be a recurring theme throughout this book.
     When the range of integer values is not adequate in a given
programming situation, a long integer should be used.  Like the regular
integer, long integers can accommodate whole numbers only.  A long integer,
however, occupies four bytes of memory, and can thus hold more information.
This yields an allowable range of values that spans from -2147483648
through 2147483647 (approximately +/- 2.15 billion).  Although the PC's
processor cannot directly manipulate a long integer in most situations,
calculations using them will still be much faster and require less code
when compared to floating point numbers.
     Regardless of which type of integer is being considered, the way they
are stored in memory is very similar.  That is, each integer is comprised
of either two or four bytes, and each of those bytes contains eight bits.
Since a bit can hold a value of either 0 or 1 only, you can see why a
larger number of bits is needed to accommodate a wider range of values.
Two bits are required to count up to three, three bits to count to seven,
four bits to count to fifteen, and so forth.
     A single byte can hold any value between 0 and 255, however that same
range can also be considered as spanning from -128 to 127.  Similarly, an
integer value can hold numbers that range from either 0 to 65535 or -32768
through 32767, depending on your perspective.  When the range is considered
to be 0 to 65535 the values are referred to as *unsigned*, because only
positive values may be represented.  BASIC does not, however, support
unsigned integer values.  Therefore, that same range is used in BASIC
programs to represent values between -32768 and 32767.  When integer
numbers are considered as using this range they are called *signed*.
     If you compile and run the short program in the listing that follows,
the transition from positive to negative numbers will show how BASIC treats
values that exceed the integer range of 32767.  Be sure not to use the /d
debugging option, since that will cause an overflow error to be generated
at the transition point.  The BASIC environment performs the same checking
as /d does, and it too will report an error before this program can run to
completion.


Number% = 32760
FOR X% = 1 TO 14
    Number% = Number% + 1
    PRINT Number%,
NEXT

Displayed result:

 32761     32762     32763     32764     32765
 32766     32767    -32768    -32767    -32766
-32765    -32764    -32763    -32762    -32761


As you can see, once an integer reaches 32767, adding 1 again causes the
value to "wrap" around to -32768.  When Number% is further incremented its
value continues to rise as expected, but in this case by becoming "less
negative".  In order to appreciate why this happens you must understand how
an integer is constructed from individual bits.  I am not going to belabor
binary number theory or other esoteric material, and the brief discussion
that follows is presented solely in the interest of completeness.


BITS 'N' BYTES
==============

Sixteen bits are required to store an integer value.  These bits are
numbered 0 through 15, and the least significant bit is bit number 0.  To
help understand this terminology, consider the decimal number 1234.  Here,
4 is the least significant digit, because it contributes the least value to
the entire number.  Similarly, 1 is the most significant portion, because
it tells how many thousands there are, thus contributing the most to the
total value.  The binary numbers that a PC uses are structured in an
identical manner.  But instead of ones, tens, and hundreds, each binary
digit represents the number of ones, twos, fours, eights, and so forth that
comprise a given byte or word.
     To represent the range of values between 0 and 32767 requires fifteen
bits, as does the range from -32768 to -1.  When considered as signed
numbers, the most significant bit is used to indicate which range is being
considered.  This bit is therefore called the sign bit.  Long integers use
the same method except that four bytes are used, so the sign bit is kept in
the highest position of the fourth byte.
     Selected portions of the successive range from 0 through -1 (or 65535)
are shown in Table 2-1, to illustrate how binary counting operates.  When
counting with decimal numbers, once you reach 9 the number is wrapped
around to 0, and then a 1 is placed in the next column.  Since binary bits
can count only to one, they wrap around much more frequently.  The
Hexadecimal equivalents are also shown in the table, since they too are
related to binary numbering.  That is, any Hex value whose most significant
digit is 8 or higher is by definition negative.


 Signed   Unsigned
Decimal    Decimal          Binary          Hex
───────   ────────   ───────────────────   ────
     0          0    0000 0000 0000 0000   0000
     1          1    0000 0000 0000 0001   0001
     2          2    0000 0000 0000 0010   0002
     3          3    0000 0000 0000 0011   0003
     4          4    0000 0000 0000 0100   0004
     .          .                      .      .
     .          .                      .      .
 32765      32765    0111 1111 1111 1101   7FFD
 32766      32767    0111 1111 1111 1110   7FFE
 32767      32767    0111 1111 1111 1111   7FFF
-32768      32768    1000 0000 0000 0000   8000
-32767      32769    1000 0000 0000 0001   8001
-32766      32770    1000 0000 0000 0010   8002
     .          .                      .      .
     .          .                      .      .
    -4      65531    1111 1111 1111 1100   FFFB
    -3      65532    1111 1111 1111 1101   FFFC
    -2      65533    1111 1111 1111 1110   FFFD
    -1      65534    1111 1111 1111 1111   FFFE
     0      65535    0000 0000 0000 0000   FFFF

Table 2-1: When a signed integer is incremented past 32767, its value wraps
around and becomes negative.


MEMORY ADDRESSES AND POINTERS
=============================

Before we can discuss such issues as variable and data storage, a few terms
must be clarified.  A memory address is a numbered location in which a
given piece of data is said to reside.  Addresses refer to places that
exist in a PC's memory, and they are referenced by those numbers.  Every PC
has thousands of memory addresses in which both data and code instructions
may be stored.
     A *pointer* is simply a variable that holds an address.  Consider a
single precision variable named Value that has been stored by the compiler
at memory address 10.  If another variable--let's call it Address%--is then
assigned the value 10, Address% could be considered to be a pointer to
Value.  Pointer variables are the bread and butter of languages such as C
and assembler, because data is often read and written by referring to one
variable which in turn holds the address of another variable.
     Although BASIC shields you as the programmer from such details,
pointers are in fact used internally by the BASIC language library
routines.  This method of using pointers is sometimes called indirection,
because an additional, indirect step is needed to first go to one variable,
get an address, and then go to that address to access the actual data.  Now
let's see how these memory issues affect a BASIC program.


INTEGER STORAGE
===============

When a conventional two-byte integer is stored in the PC's memory, the
lower byte is kept in the lower memory address.  For example, if X% is said
to reside at address 10, then the least significant byte is at address 10
and the most significant byte is at address 11.  Likewise, a long integer
stored at address 102 actually occupies addresses 102 through 105, with the
least significant portion at the lowest address.  This is shown graphically
in Figure 2-1.


   ┌───────────── X% ────────────┐
   │     LSB             MSB     │
─ ╥─┬─┬─┬─┬─┬─┬─┬─╥─┬─┬─┬─┬─┬─┬─┬─╥── ─ ─
  ║1│0│0│1│1│0│1│0║0│0│1│0│1│1│0│1║
─ ╨─┴─┴─┴─┴─┴─┴─┴─╨─┴─┴─┴─┴─┴─┴─┴─╨── ─ ─
   ^               ^               ^
   └─ Address 10   └─ Address 11   └─ Address 12

Figure 2-1: An integer is stored in two adjacent memory locations, with the
Least Significant Byte at the lower address, and the Most Significant Byte
at the higher.


This arrangement certainly seems sensible, and it is.  However, some people
get confused when looking at a range of memory addresses being displayed,
because the values in lower addresses are listed at the left and the higher
address values are shown on the right.  For example, the DEBUG utility that
comes with DOS will display the Hex number ABCD as CD followed by AB.  I
mention this only because the order in which digits are displayed will
become important when we discuss advanced debugging in Chapter 4.
     In case you are wondering, the compiler assigns addresses in the order
in which variables are encountered.  The first address used is generally 36
Hex, so in the program below the variables will be stored at addresses 36,
38, 3A, and then 3C.  Hex numbering is used for these examples because
that's the way DEBUG and CodeView report them.


A% = 1         'this is at address &H36
B% = 2         'this is at address &H38
C% = 3         'this is at address &H3A
D% = 4         'this is at address &H3C


FLOATING POINT VALUES
=====================

Floating point variables and numbers are constructed in an entirely
different manner than integers.  Where integers and long integers simply
use the entire two or four bytes to hold a single binary number, floating
point data is divided into portions.  The first portion is called the
mantissa, and it holds the base value of the number.  The second portion is
the exponent, and it indicates to what power the mantissa must be raised to
express the complete value.  Like integers, a sign bit is used to show if
the number is positive or negative.
     The structure of single precision values in both IEEE and the original
proprietary Microsoft Binary Format (MBF) is shown in Figure 2-2.  For IEEE
numbers, the sign bit is in the most significant position, followed by
eight exponent bits, which are in turn followed by 23 bits for the
mantissa.  Double precision IEEE values are structured similarly, except
eleven bits are used for the exponent and 52 for the mantissa.
     Double precision MBF numbers use only eight bits for an exponent
rather than eleven, trading a reduced absolute range for increased
resolution.  That is, there are fewer exponent bits than the IEEE method
uses, which means that extremely large and extremely small numbers cannot
be represented.  However, the additional mantissa bits offer more absolute
digits of precision.


The IEEE format:

┌────────┬────────┬────────┬────────┐
│SEEEEEEE│EMMMMMMM│MMMMMMMM│MMMMMMMM│
└────────┴────────┴────────┴────────┘

The MBF format:

┌────────┬────────┬────────┬────────┐
│EEEEEEEE│SMMMMMMM│MMMMMMMM│MMMMMMMM│
└────────┴────────┴────────┴────────┘

Figure 2-2: A single precision value is comprised of a Sign bit, eight
Exponent bits, and 23 bits to represent the Mantissa.  Each letter shown
here represents one bit, and the bytes on the left are at higher addresses.


Notice that with IEEE numbers, the exponent spans a byte boundary.  This
undoubtedly contributes to the slow speed that results from using numbers
in this format when a coprocessor is not present.  Contrast that with
Microsoft's MBF format in which the sign bit is placed between the exponent
and mantissa.  This allows direct access to the exponent with fewer
assembler instructions, since the various bits don't have to be shifted
around.
     The IEEE format is used in QuickBASIC 4.0 and later, and BASIC PDS
unless the /fpa option is used.  BASIC PDS uses the /fpa switch to specify
an alternate math package which provides increased speed but with a
slightly reduced accuracy.  Although the /fpa format is in fact newer than
the original MBF used in interpreted BASIC and QuickBASIC 2 and 3, it is
not quite as fast.
     As was already mentioned, double precision data requires twice as many
bytes as single precision.  Further, due to the inherent complexity of the
way floating point data is stored, an enormous amount of assembly language
code is required to manipulate it.  Common sense therefore indicates that
you would use single precision variables whenever possible, and reserve
double precision only for those cases where the added accuracy is truly
necessary.  Using either floating point variable type, however, is still
very much slower than using integers and long integers.  Worse, rounding
errors are inevitable with any floating point method, as the following
short program fragment illustrates.


FOR X% = 1 TO 10000
    Number! = Number! + 1.1
NEXT
PRINT Number!

Displayed result:

10999.52


Although the correct answer should be 11000, the result of adding 1.1 ten
thousand times is incorrect by a small amount.  If you are writing a
program that computes, say, tax returns, even this small error will be
unacceptable.  Recognizing this problem, Microsoft developed a new Currency
data type which was introduced with BASIC PDS version 7.0.
     The Currency data type is a cross between an integer and a floating
point number.  Like a double precision value, Currency data also uses eight
bytes for storage.  However, the numbers are stored in an integer format
with an implied scaling of 10000.  That is, a binary value of 1 is used to
represent the value .0001, and a binary value of 20000 is treated as a 2.
This yields an absolute accuracy to four decimal places, which is more than
sufficient for financial work.  The absolute range of Currency data is plus
or minus 9.22 times 10 ^ 14 (± 9.22E14 or 922,000,000,000,000.0000), which
is very wide indeed.  This type of storage is called Fixed-Point, because
the number of decimal places is fixed (in this case at four places).
     Currency data offers the best compromise of all, since only whole
numbers are represented and the fractional portion is implied.  Further,
since a separate exponent and mantissa are not used, calculations involving
Currency data are extremely fast.  In practice, a loop that adds a series
of Currency variables will run about half as fast as the same loop using
long integers.  Since twice as many bytes must be manipulated, the net
effect is an overall efficiency that is comparable to long integers.
Compare that to double precision calculations, where manipulating the same
eight bytes takes more than six times longer.
     As you have seen, there is a great deal more to "simple" numeric data
than would appear initially.  But this hardly begins to scratch the surface
of data storage and manipulation in BASIC.  We will continue our tour of
BASIC's data types with conventional dynamic (variable-length) strings,
before proceeding to fixed-length strings and TYPE variables.


DYNAMIC STRINGS
===============

One of the most important advantages that BASIC holds over all of the other
popular high-level languages is its support for dynamic string data.  In
Pascal, for example, you must declare every string that your program will
use, as well as its length, before the program can be compiled.  If you
determine during execution of the program that additional characters must
be stored in a string, you're out of luck.
     Likewise, strings in C are treated internally as an array of single
character bytes, and there is no graceful way to extend or shorten them.
Specifying more characters than necessary will of course waste memory, and
specifying too few will cause subsequent data to be overwritten.  Since C
performs virtually no error checking during program execution, assigning to
a string that is not long enough will corrupt memory.  And indeed, problems
such as this cause untold grief for C programmers.
     Dynamic string memory handling is built into BASIC, and those routines
are written in assembly language.  BASIC is therefore extremely efficient
and very fast in this regard.  Since C is a high-level language, writing an
equivalent memory manager in C would be quite slow and bulky by comparison.
I feel it is important to point out BASIC's superiority over C in this
regard, because C has an undeserved reputation for being a very fast and
powerful language.
     Compiled BASIC implements dynamic strings with varying lengths by
maintaining a *string descriptor* for each string.  A string descriptor is
simply a four-byte table that holds the current length of the string as
well as its current address.  The format for a BASIC string descriptor is
shown in Figure 2-3.  In QuickBASIC programs and BASIC PDS when far strings
are not specified, all strings are stored in an area of memory called the
*near heap*.  The string data in this memory area is frequently shuffled
around, as new strings are assigned and old ones are abandoned.


┌──────┐   Higher addresses
│  64  │           ^
├──────┤ Address   │
│  B2  │           │
╞══════╡           │
│  00  │           │
├──────┤ Length
│  0A  │ ■──────────────── VARPTR(Work$)
└──────┘

Figure 2-3: Each string in a QuickBASIC program has a corresponding string
descriptor, which holds its current length and address.  The string in this
example has a length of ten characters (0A Hex) and its data is presently
at address 25778 (64B2 Hex).


The lower two bytes in a string descriptor together hold the current length
of the string, and the second two bytes hold its address.  The memory
location at the bottom of Figure 2-3 is at the lowest address.  The short
program below shows how you could access a string by peeking at its
descriptor.


DEFINT A-Z

Test$ = "BASIC Techniques and Utilities"
Descr = VARPTR(Test$)
Length = PEEK(Descr) + 256 * PEEK(Descr + 1)
Addr = PEEK(Descr + 2) + 256 * PEEK(Descr + 3)

PRINT "The length is"; Length
PRINT "The address is"; Addr
PRINT "The string contains ";
FOR X = Addr TO Addr + Length - 1
  PRINT CHR$(PEEK(X));
NEXT

Displayed result:

The length is 17
The address is 15646 (this will vary)
The string contains BASIC Techniques and Utilities


Each time a string is assigned or reassigned, memory in the heap is claimed
and the string's descriptor is updated to reflect its new length and
address.  The old data is then marked as being abandoned, so the space it
occupied may be reclaimed later on if it is needed.  Since each assignment
claims new memory, at some point the heap will become full.  When this
happens, BASIC shuffles all of the string data that is currently in use
downward on top of the older, abandoned data.  This heap compaction process
is often referred to colorfully as *garbage collection*.
     In practice, there are two ways to avoid having BASIC claim new space
for each string assignment--which takes time--and you should consider these
when speed is paramount.  One method is to use LSET or RSET, to insert new
characters into an existing string.  Although this cannot be used to make a
string longer or shorter, it is very much faster than a straight assignment
which invokes the memory management routines.  The second method is to use
the statement form of MID$, which is not quite as fast as LSET, but is more
flexible.
     Microsoft BASIC performs some additional trickery as it manages the
string data in a program.  For example, whenever a string is assigned, an
even number of bytes is always requested.  Thus, if a five-character string
is reassigned to one with six characters, the same space can be reused.
Since claiming new memory requires a finite amount of time and also causes
garbage collection periodically, this technique helps to speed up the
string assignment process.
     For example, in a program that builds a string by adding new
characters to the end in a loop, BASIC can reduce the number of times it
must claim new memory to only every other assignment.  Another advantage to
always allocating an even number of bytes is that the 80286 and later
microprocessors can copy two-byte words much faster than they can copy the
equivalent number of bytes.  This has an obvious advantage when long
strings are being assigned.
     In most cases, BASIC's use of string descriptors is much more
efficient than the method used by C and other languages.  In C, each string
has an extra trailing CHR$(0) byte just to mark where it ends.  While using
a single byte is less wasteful than requiring a four-byte table, BASIC's
method is many times faster.  In C the entire string must be searched just
to see how long it is, which takes time.  Likewise, comparing and
concatenating strings in C requires scanning both strings for the
terminating zero character.  The same operations in BASIC require but a
single step to obtain the current length.
     Pascal uses a method that is similar to BASIC's, in that it remembers
the current length of the string. The length is stored with the actual
string data, in a byte just before the first character.  Unfortunately,
using a single byte limits the maximum length of a Pascal string to only
255 characters.  And again, when a string is shortened in Pascal, the extra
characters are not released for use by other data.  But it is only fair to
point out that Pascal's method is both fast and compact.  And since strings
in C and Pascal never move around in memory, garbage collection is not
required.
     Although a BASIC string descriptor uses four bytes of additional
memory beyond that needed for the actual data, this is only part of the
story.  An additional two bytes are needed to hold a special "variable"
called a *back pointer*.  A back pointer is an integer word that is stored
in memory immediately before the actual string data, and it holds the
address of the data's string descriptor.  Thus, it is called a back pointer
because it points back to the descriptor, as opposed to the descriptor
which points to the data.
     Because of this back pointer, six additional bytes are actually needed
to store each string, beyond the number of characters that it contains.
For example, the statement Work$ = "BASIC" requires twelve bytes of data
memory--five for the string itself, one more because an even number of
bytes is always claimed, four for the descriptor, and two more for a back
pointer.  Every string that is defined in a program has a corresponding
descriptor which is always present, however a back pointer is maintained
only while the string has characters assigned to it.  Therefore, when a
string is erased the two bytes for its back pointer are also relinquished.
     I won't belabor this discussion of back pointers further, because
understanding them is of little practical use.  Suffice it to say that a
back pointer helps speed up the heap compaction process.  Since the address
portion of the descriptor must be updated whenever the string data is
moved, this pointer provides a fast link between the data being moved and
its descriptor.  By the way, the term "pointer" refers to any variable that
holds a memory address, regardless of what language is being considered.


FAR STRINGS IN BASIC PDS

BASIC PDS offers an option to specify "far strings", whereby the string
data is not stored in the same 64K memory area that holds most of a
program's data.  The method of storage used for far strings is of necessity
much more complex than near strings, because both an address and a segment
must be kept track of.  Although Microsoft has made it clear that the
structure of far string descriptors may change in the future, I would be
remiss if this undocumented information were not revealed here.  The
following description is valid as of BASIC 7.1 [it is still valid for
VB/DOS too].
     For each far string in a program, a four-byte descriptor is maintained
in near memory.  The lower two bytes of the descriptor together hold the
address of an integer variable that holds yet another address: that of the
string length and data.  The second pair of bytes also holds the address of
a pointer, in this case a pointer to a variable that indicates the segment
in which the string data resides.  Thus, by retrieving the address and
segment from the descriptor, you can locate the string's length and data,
albeit with an extra level of indirection.
     It is interesting to note that when far strings are being used, the
string's length is kept just before its data, much like the way Pascal
operates.  Therefore, the address pointer holds the address of the length
word which immediately precedes the actual string data.
     The short program that follows shows how to locate all of the
components of a far string based on examining its descriptor and related
pointers.  Notice that long integers are used to avoid the possibility of
an overflow error if the segment or addresses happen to be higher than
32767.  This way you can run the program in the QBX [or VB/DOS] editing
environment.  Figure 2-4 in turn illustrates the relationship between the
address and pointer information graphically.


DEF FNPeekWord& (A&)
  FNPeekWord& = PEEK(A&) + 256& * PEEK(A& + 1)
END DEF

Work$ = "This is a test"

DescAddr& = VARPTR(Work$)
AddressPtr& = FNPeekWord&(DescAddr&)
SegmentPtr& = FNPeekWord&(DescAddr& + 2)
Segment& = FNPeekWord&(SegmentPtr&)

DEF SEG = Segment&
DataAddr& = FNPeekWord&(AddressPtr&)
Length% = FNPeekWord&(DataAddr&)
StrAddr& = DataAddr& + 2

PRINT "The descriptor address is:"; DescAddr&
PRINT "      The data segment is:"; Segment&
PRINT "            The length is:"; Length%
PRINT "The string data starts at:"; StrAddr&
PRINT "   And the string data is: ";

FOR X& = StrAddr& TO StrAddr& + Length% - 1
  PRINT CHR$(PEEK(X&));
NEXT

Displayed result (the addresses may vary):

The descriptor address is: 17220
      The data segment is: 40787
            The length is: 14
The string data starts at: 106
   And the string data is: This is a test


Because two bytes are used to hold the segment, address, and length values,
we must PEEK both of them and combine the results.  This is the purpose of
the PeekWord function that is defined at the start of the program.  Note
the placement of an ampersand after the number 256, which ensures that the
multiplication will not cause an overflow error.  I will discuss such use
of numeric constants and type identifiers later in this chapter.


     ┌───┬────────────────── The string length
     │   │      ┌─────────── The string data
    ┌┴─┬─┴┬─────┴────────┐
┌──>│0A│00│This is a test│<── Segment &H8F00
│   └──┴──┴──────────────┘
│    ^
│    └─────── 8F00:0070
│
│   ┌──┬──┬──────────────┐
└───┤70│00│..............│<── Segment &H8F00
    └──┴──┴──────────────┘
     ^
     ├─────── 8F00:002E
     │
     │
     │ This is the "near" segment ──────┐
     └───────────────┬───┐              │
    ┌──┬──┬─────────┬┴─┬─┴┬──┬──┬───────┴────┐
    │00│8F│.........│2E│00│D4│03│............│
    └──┴──┴─────────┴──┴──┴┬─┴─┬┴────────────┘
     ^               ^     └─┬─┘
     ├─ Address 03D4 │       │
     │               └───────┼── VARPTR(Work$)
     └───────────────────────┘

Figure 2-4: A far string descriptor holds the addresses of other addresses,
in this case addresses that hold a far string's segment and its length and
actual data.


Even in a far-string program, some of the string data will be near.  For
example, DATA items and quoted string constants are stored in the same 64K
DGROUP data segment that holds simple numeric and TYPE variables.  The same
"indirect" method is used, whereby you must look in one place to get the
address of another address.  In this case, however, the "far" segment that
is reported is simply the normal near data segment.  [DATA items in VB/DOS
programs are still kept in near memory, but quoted strings are now kept in
a separate segment.]
     One final complication worth mentioning is that strings within a FIELD
buffer (and possibly in other special situations) are handled slightly
differently.  Since all of the strings in a FIELD buffer must be
contiguous, BASIC cannot store the length word adjacent to the string data.
Therefore, a different method must be used.  This case is indicated by
setting the sign bit (the highest bit) in the length word as a flag.  Since
no string can have a negative length, that bit can safely be used for this
purpose.  When a string is stored using this alternate method, the bytes
that follow the length word are used as additional pointers to the string's
actual data segment and address.


FIXED-LENGTH STRINGS

One of the most important new features Microsoft added beginning with
QuickBASIC 4.0 was fixed-length string and TYPE variables.  Although fixed-
length strings are less flexible than conventional BASIC strings, they
offer many advantages in certain programming situations.  One advantage is
that they are static, which means their data does not move around in memory
as with conventional strings.  You can therefore obtain the address of a
fixed-length string just once using VARPTR, confident that this address
will never change.  With dynamic strings, SADD must be used each time the
address is needed, which takes time and adds code.  Another important
feature is that arrays of fixed-length strings can be stored in far memory,
outside of the normal 64K data area.  We will discuss near and far array
memory allocation momentarily.
     With every advantage, however, comes a disadvantage.  The most severe
limitation is that when a fixed-length string is used where a conventional
string is expected, BASIC must generate code to create a temporary dynamic
string, and then copy the data to it.  That is, all of BASIC's internal
routines that operate on strings expect a string descriptor.  Therefore,
when you print a fixed-length string, or use MID$ or INSTR or indeed nearly
any statement or function that accepts a string, it must be copied to a
form that BASIC's internal routines can accept.  In many cases, additional
code is created to delete the temporary string afterward.  In others, the
data remains until the next time the same BASIC statement is executed, and
a new temporary string is assigned freeing the older one.
     To illustrate, twenty bytes of assembly language code are required to
print a fixed-length string, compared to only nine for a conventional
dynamic string.  Worse, when a fixed-length string is passed as an argument
to a subprogram or function, BASIC not only makes a copy before passing the
string, but it also copies the data back again in case the subroutine
changed it!  The extra steps the compiler performs are shown as BASIC
equivalents in the listing that follows.


'----- This is the code you write:

DIM Work AS STRING * 20
CALL TestSub(Work$)

'----- This is what BASIC actually does:

Temp$ = SPACE$(20)    'create a temporary string
LSET Temp$ = Work$    'copy Work$ to it
CALL TestSub(Temp$)   'call the subprogram
LSET Work$ = Temp$    'copy the data back again
Temp$ = ""            'erase the temporary data


As you can imagine, all of this copying creates an enormous amount of
additional code in your programs.  Where only nine bytes are required to
pass a conventional string to a subprogram, 64 are needed when a fixed-
length string is being sent.  But you cannot assume unequivocally that
conventional strings are always better or that fixed-length strings are
always better.  Rather, I can only present the facts, and let you decide
based on the knowledge of what is really happening.  In the discussion of
debugging later in Chapter 4, you will learn how to use CodeView to see the
code that BASIC generates.  You can thus explore these issues further, and
draw your own conclusions.


USER-DEFINED TYPE VARIABLES

As I mentioned earlier, the TYPE variable is an important and powerful
addition to modern compiled BASIC.  Its primary purpose is to let
programmers create composite data structures using any combination of
native data types.  C and Pascal have had such user-defined data types
since their inception, and they are called Structures and Records
respectively in each language.
     One immediately obvious use for being able to create a new, composite
data type is to define the structure of a random access data file.  Another
is to simulate an array comprised of varied types of data.  Obviously, no
language can support a mix of different data types within a single array.
That is, an array cannot be created where some of the elements are, say,
integer while others are double precision.  But a TYPE variable lets you do
something very close to that, and you can even create arrays of TYPE
variables.
     In the listing that follows a TYPE is defined using a mix of integer,
single precision, double precision, and fixed-length string components.
Also shown below is how a TYPE variable is dimensioned, and how each of its
components are assigned and referenced.


TYPE MyType
  I AS INTEGER
  S AS SINGLE
  D AS DOUBLE
  F AS STRING * 20
END TYPE

DIM MyData as MyType

MyData.I = 12       'assign the integer portion
MyData.S = 100.09   'and then the single part
MyData.D = 43.2E56  'and then the double
MyData.F = "Test"   'and finally the string

PRINT MyData.F      'now print the string


Once the TYPE structure has been established, the DIM statement must be
used to create an actual variable using that arrangement.  Although DIM is
usually associated with the definition of arrays, it is also used to
identify a variable name with a particular type of data.  In this case, DIM
tells BASIC to set aside an area of memory to hold that many bytes.  You
may also use DIM with conventional variable types.  For example, DIM
LastName AS STRING or DIM PayPeriod AS DOUBLE lets you omit the dollar sign
and pound sign when you reference them later in the program.  In my
opinion, however, that style leads to programs that are difficult to
maintain, since many pages later in the source listing you may not remember
what type of data is actually being referred to.
     As you can see, a period is needed to indicate which portion of the
TYPE variable is being referenced.  The base name is that given when you
dimensioned the variable, but the portion being referenced is identified
using the name within the original TYPE definition.  You cannot print a
TYPE variable directly, but must instead print each component separately.
Likewise, assignments to a TYPE variable must also be made through its
individual components, with two exceptions.  You may assign an entire TYPE
variable from another identical TYPE directly, or from a dissimilar TYPE
variable using LSET.
     For example, if we had used DIM MyData AS MyType and then DIM HisData
AS MyType, the entire contents of HisData could be assigned to MyData using
the statement MyData = HisData.  Had HisData been dimensioned using a
different TYPE definition, then LSET would be required.  That is, LSET
MyData = HisData will copy as many characters from HisData as will fit into
MyData, and then pad the remainder, if any, with blanks.
     It is important to understand that this behavior can cause strange
results indeed.  Since CHR$(32) blanks are used to pad what remains in the
TYPE variable being assigned, numeric components may receive some unusual
values.  Therefore, you should assign differing TYPE variables only when
those overlapping portions being assigned are structured identically.


Arrays Within Types

With the introduction of BASIC PDS, programmers may also establish static
arrays within a single TYPE definition.  An array is dimensioned within a
TYPE as shown in the listing that follows.  As with a conventional DIM
statement for an array, the number of elements are indicated and a non-zero
lower bound may optionally be specified.  Please understand, though, that
you cannot use a variable for the number of elements in the array.  That
is, using PayHistory(1 TO NumDates) would be illegal.


TYPE ArrayType
  AmountDue AS SINGLE
  PayHistory(1 TO 52) AS SINGLE
  LastName AS STRING * 15
END TYPE

DIM TypeArray AS ArrayType


There are several advantages to using an array within a TYPE variable.  One
is that you can reference a portion of the TYPE by using a variable to
specify the element number.  For example, TypeArray.PayHistory(PayPeriod) =
344.95 will assign the value 344.95 to element number PayPeriod.  Without
the ability to use an array, each of the 52 components would need to be
identified by name.  Further, arrays allows you to define a large number of
TYPE elements with a single program statement.  This can help to improve a
program's readability.


STATIC VS. DYNAMIC DATA
=======================

Preceding sections have touched only briefly on the concept of static and
dynamic memory storage.  Let's now explore this subject in depth, and learn
which methods are most appropriate in which situations.
     By definition, static data is that which never changes in size, and
never moves around in memory.  In compiled BASIC this definition is further
extended to mean all data that is stored in the 64K near memory area known
as DGROUP.  This includes all numeric variables, fixed-length strings, and
TYPE variables.  Technically speaking, the string descriptors that
accompany each conventional (not fixed-length) string are also considered
to be static, even though the string data itself is not.  The string
descriptors that comprise a dynamic string array, however, are dynamic
data, because they move around in memory (as a group) and may be resized
and erased.
     Numeric arrays that are dimensioned with constant (not variable)
subscripts are also static, unless the '$DYNAMIC metacommand has been used
in a preceding program statement.  That is, DIM Array#(0 TO 100) will
create a static array, while DIM Array#(0 TO MaxElements) creates a dynamic
array.  Likewise, arrays of fixed-length strings and TYPE variables will be
static, as long as numbers are used to specify the size.
     There are advantages and disadvantages to each storage method.  Access
to static data is always faster than access to dynamic data, because the
compiler knows the address where the data resides at the time it creates
your program.  It can therefore create assembly language instructions that
go directly to that address.  In contrast, dynamic data always requires a
pointer to hold the current address of the data.  An extra step is
therefore needed to first get the data address from that pointer, before
access to the actual data is possible.  Static data is also in the near
data segment, thus avoiding the need for additional code that switches
segments.
     The overwhelming disadvantage of static data, though, is that it may
never be erased.  Once a static variable or array has been used in a
program, the memory it occupies can never be released for other uses.
Again, it is impossible to state that static arrays are always better than
dynamic arrays or vice versa.  Which you use must be dictated by your
program's memory requirements, when compared to its execution speed.


DYNAMIC ARRAYS

You have already seen how dynamic strings operate, by using a four-byte
pointer table called a string descriptor.  Similarly, a dynamic array also
needs a table to show where the array data is located, how many elements
there are, the length of each element, and so forth.  This table is called
an array descriptor, and it is structured as shown in Table 2-2.
     There is little reason to use the information in an array descriptor
in a BASIC program, and indeed, BASIC provides no direct way to access it
anyway.  But when writing routines in assembly language for use with BASIC,
this knowledge can be quite helpful.  As with BASIC PDS far string
descriptors, none of this information is documented, and relying on it is
most certainly not endorsed by Microsoft.  Perhaps that's what makes it so
much fun to discuss!
     Technically speaking, only dynamic arrays require an array descriptor,
since static arrays do not move or change size.  But BASIC creates an array
descriptor for every array, so only one method of code generation is
necessary.  For example, when you pass an entire array to a subprogram
using empty parentheses, it is the address of the array descriptor that is
actually sent.  The subprogram can then access the data through that
descriptor, regardless of whether the array is static or dynamic.


Offset  Size        Description
──────  ────  ──────────────────────────────────
  00     02   Address where array data begins

  00     02   Segment where that address resides

  04     02   Far heap descriptor, pointer

  06     02   Far heap descriptor, block size

  08     01   Number of dimensions in the array

  09     01   Array type and storage method:
                Bit 0 set = far array
                Bit 1 set = huge (/ah) array
                Bit 6 set = static array
                Bit 7 set = string array

  0A     02   Adjusted Offset

  0C     02   Length in bytes of each element

  0E     02   Number of elements in the last
              dimension (UBOUND - LBOUND + 1)

  10     02   First element number in that
              dimension (LBOUND)

  12     02   Number of elements in the second
              from last dimension

  14     02   First element number in that
              dimension

    .    02   Repeat number of elements and
              first element number as necessary,
    .    02   through the first dimension

Table 2-2: Every array in a BASIC program has an associated array
descriptor such as the one shown here.  This descriptor contains important
information about the array.


The first four bytes together hold the segmented address where the array
data proper begins in memory.  Following the standard convention, the
address is stored in the lower word, with the segment immediately
following.
     The next two words comprise the Far Heap Descriptor, which holds a
pointer to the next dynamic array descriptor and the current size of the
array.  For static arrays both of these entries are zero.  When multiple
dynamic arrays are used in a program, the array descriptors are created in
static DGROUP memory in the order BC encounters them.  The Far Heap Pointer
in the first array therefore points to the next array descriptor in memory.
The last descriptor in the chain can be identified because it points to a
word that holds a value of zero.
     The block size portion of the Far Heap Descriptor holds the size of
the array, using a byte count for string arrays and a "paragraph" count for
numeric, fixed-length, and TYPE arrays.  For string arrays--whether near or
far--the byte count is based on the four bytes that each descriptor
occupies.  With numeric arrays the size is instead the number of 16-byte
paragraphs that are needed to store the array.
     The next entry is a single byte that holds the number of dimensions in
the array.  That is, DIM Array(1 TO 10) has one dimension and DIM Array(1
TO 10, 2 TO 20) has two.
     The next item is also a byte, and it is called the Feature byte
because the various bits it holds tell what type of array it is.  As shown
in the table, separate bits are used to indicate if the array is stored in
far memory, whether or not /ah was used to specify huge arrays, if the
array is static, and if it is a string array.  Multiple bits are used for
each of these array properties, since they may be active in combination.
However, BASIC never sets the far and huge bits for string arrays, even
when the PDS /fs option is used and the strings are in fact in far memory.
     Of particular interest is the Adjusted Offset entry.  Even though the
segmented address where the array data begins is the first entry in the
descriptor, it is useful only when the first element number in the array is
zero.  This would be the case with DIM Array(0 TO N), or simply DIM
Array(N).  To achieve the fastest performance possible when retrieving or
assigning a given element, the Adjusted Offset is calculated when the array
is dimensioned to compensate for an LBOUND other than 0.
     For example, if an integer array is dimensioned starting at element 1,
the Adjusted Offset is set to point two bytes before the actual starting
address of the data.  This way, the compiler can take the specified element
number, multiply that times two (each element comprises two bytes), and
then add that to the Adjusted Offset to immediately point at the correct
element in memory.  Otherwise, additional code would be needed to subtract
the LBOUND value each time the array is accessed.  Since the array's LBOUND
is simply constant information, it would be wasteful to calculate that
repeatedly at run time.  Of course, the Adjusted Offset calculation is
correspondingly more complex when dealing with multi-dimensional arrays.
     The remaining entries identify the length of each element in bytes,
and the upper and lower bounds.  String arrays always have a 4 in the
length location, because that's the length of each string descriptor.  A
separate pair of words is needed for each array subscript, to identify the
LBOUND value and the number of elements.  The UBOUND is not actually stored
in the array descriptor, since it can be calculated very easily when
needed.  Notice that for multi-dimensional arrays, the last (right-most)
subscript is identified first, followed by the second from the last, and
continuing to the first one.
     One final note worth mentioning about dynamic array storage is the
location in memory of the first array element.  For numeric arrays, the
starting address is always zero, within the specified segment.  (A new
segment can start at any 16-byte address boundary, so at most 15 bytes may
be wasted.)  However, BASIC sometimes positions fixed-length string and
TYPE arrays farther into the segment.  BASIC will not allow an array
element to span a segment boundary under any circumstances.  This could
never happen with numeric data, because each element has a length that is a
power of 2.  That is, 16,384 long integer elements will exactly fit in a
single 64K segment.  But when a fixed-length string or TYPE array is
created, nearly any element length may be specified.
     For example, if you use REDIM Array(1 TO 10) AS STRING * 13000,
130,000 bytes are needed and element 6 would straddle a segment.  To
prevent that from happening, BASIC's dynamic DIM routine fudges the first
element to instead be placed at address 536.  Thus, the last byte in
element 5 will be at the end of the 64K segment, and the first byte in
element 6 will fall exactly at the start of the second 64K code segment.
The only limitation is that arrays with odd lengths like this can never
exceed 128K in total size, because the inevitable split would occur at the
start of the third segment.  Arrays whose element lengths are a power of 2,
such as 32 or 4096 bytes, do not have this problem.  (Bear in mind that 1K
is actually 1,024 bytes, so 128K really equals 131,072 bytes).  This is
shown graphically below in Figure 2-5.


Element 10 is the last that evenly fits ─┐
Segment boundary ────┐                   │
                     ■                   ■
┌┬───┬───┬───┬───┬───╥───┬───┬───┬───┬───┬╥─ ─ ─
│├───┼───┼───┼───┼──■║■──┼───┼───┼───┼───┤║
└┴───┴───┴───┴───┴───╨───┴───┴───┴───┴───┴╨─ ─ ─
■■ ■   ■             ■
││ │   │             └──────■ Address 0
││ │   └────────────────────■ Element 2
││ └────────────────────────■ Element 1
│└──────────────────────────■ Address 536
└───────────────────────────■ Address 0

Figure 2-5


FAR DATA VERSUS NEAR DATA
=========================

You have already used the terms "near" and "far" to describe BASIC's data,
and now let's see exactly what they mean.  The 8086 family of
microprocessors that are used in IBM PC and compatible computers use what
is called a *segmented architecture*.  This means that while an 8086 can
access a megabyte of memory, it can do so only in 64K blocks at a time.
Before you think this is a terrible way to design a CPU, consider the
alternative.
     For example, the 68000 family used in the Apple Macintosh and Atari
computers use linear addressing, whereby any data anywhere may be accessed
without restriction.  But the problem is that with millions of possible
addresses, many bytes are needed to specify those addresses.  Because the
data segment is implied when dealing with an 80x86, a single integer can
refer to any address quickly and with very little code.  Therefore,
assembler instructions for the 68000 that reference memory tend to be long,
making those programs larger.
     Since being able to manipulate only one 64K segment is restrictive,
the 8086's designers provided four different segment registers.  One of
these, the DS (Data Segment) register, is set to specify a single segment,
which is then used by the program as much as possible.  This data segment
is also named DGROUP, and it holds all of the static data in a BASIC
program.  Again, data in DGROUP can be accessed much faster and with less
code than can data in any other segment.  In order to assign an element in
a far array, for example, BASIC requires two additional steps which
generates additional code.  The first step is to retrieve the array's
segment from the array descriptor, and the second is to assign the ES
(Extra Segment) register to access the data.
     Far data in a BASIC program therefore refers to any data that is
outside of the 64K DGROUP segment.  Technically, this could encompass the
entire 1 Megabyte that DOS recognizes, however the memory beyond 640K is
reserved for video adapters, the BIOS, expanded memory cards, and the like.
BASIC uses far memory (outside the 64K data segment but within the first
640K) for numeric, fixed-length string, and TYPE arrays, although BASIC PDS
can optionally store conventional strings there when the /fs (Far String)
option is used.  Communications buffers are also kept in far memory, and
this is where incoming characters are placed before your program actually
reads them.
     Near memory is therefore very crowded, with many varied types of data
competing for space.  Earlier I stated that all variables, static arrays,
and quoted strings are stored in near memory (DGROUP).  But other BASIC
data is also stored there as well.  This includes DATA items, string
descriptors, array descriptors, the stack, file buffers, and the internal
working variables used by BASIC's run-time library routines.
     When you open a disk file for input, an area in near memory is used as
a buffer to improve the speed of subsequent reads.  And like subprograms
and function that you write, BASIC's internal routines also need their own
variables to operate.  For example, a translation table is maintained in
DGROUP to relate the file numbers you use when opening a file to the file
handles that DOS issues.
     One final note on the items that compete for DGROUP is that in many
cases data is stored *twice*.  When you use READ to assign a string from a
DATA item, the data itself remains at the data statement, and is also
duplicated in the string being assigned.  There is simply no way to remove
the original data.  Similarly, when you assign a string from a constant as
in Message$ = "Press any key", the original quoted string is always
present, and Message$ receives a second copy.  When string space is very
tight, the only purely BASIC solution is to instead store the data in a
disk file.
     Speaking of DATA, bear in mind that reading numeric variables is
relatively slow and often even more wasteful.  Since all DATA items are
stored as strings, each time you use READ the VAL routine is called
internally by BASIC.  VAL is not a particularly fast operation, because of
the complexity of what it must do.  Worse, by storing numbers as strings,
even more memory can be wasted than you might think.  For example, storing
an integer value such as -20556 requires six bytes as a string, even though
it will be placed ultimately into a two-byte integer.


ASSESSING MEMORY WITH FRE()

Since memory is very important to the operation of most programs, it is
often useful to know how much of it is available at any given moment.
BASIC provides the FRE function to do this, however there are a number of
variations in its use.  Let's take an inside look at the various forms of
FRE, and see how they can be put to good use.
     There are no less than six different arguments that can be used with
FRE.  The first to consider is FRE(0), which reports the amount of free
string space but without first compacting the string pool.  Therefore, the
value returned by FRE(0) may be much lower than what actually could be
available.  FRE when used with a string argument, for example FRE("") or
FRE(Temp$), also returns the amount of DGROUP memory that is available,
however it first calls the heap compaction routines.  This guarantees that
the size reported accurately reflects what is really available.
     Although FRE(0) may seem to be of little value, it is in fact much
faster than FRE when a string argument is given.  Therefore, you could
periodically examine FRE(0), and if it becomes unacceptably low use FRE("")
to determine the actual amount of memory that is available.  With BASIC PDS
far strings, FRE(0) is illegal, FRE("") reports the number of bytes
available for temporary strings, and FRE(Any$) reports the free size of the
segment in which Any$ resides.  Temporary strings were discussed earlier,
when we saw how they are used when passing fixed-length string arguments to
procedures.
     FRE(-1) was introduced beginning with QuickBASIC 1, and it reports the
total amount of memory that is currently available for use with far arrays.
Thus, you could use it in a program before dimensioning a large numeric
array, to avoid receiving an "Out of memory" error which would halt your
program.  Although there is a distinction between near and far memory in
any PC program, BASIC does an admirable job of making available as much
memory as you need for various uses.  For example, it is possible to have
plenty of near memory available, but not enough for all of the dynamic
arrays that are needed.  In this case, BASIC will reduce the amount of
memory available in DGROUP, and instead relinquish it for far arrays.
     FRE(-1) is also useful if you use SHELL within your programs, because
at least 20K or so of memory is needed to load the necessary additional
copy of COMMAND.COM.  It is interesting to observe that not having enough
memory to execute a SHELL results in an "Illegal function call" error,
rather than the expected "Out of memory".
     FRE(-2) was added to QuickBASIC beginning with version 4.0, and it
reports the amount of available stack space.  The stack is a special area
within DGROUP that is used primarily for passing the addresses of variables
and other data to subroutines.  The stack is also used to store variables
when the STATIC option is omitted from a subprogram or function definition.
I will discuss static and non-static subroutines later in Chapter 3, but
for now suffice it to say that enough stack memory is necessary when many
variables are present and STATIC is omitted.
     FRE(-3) was added with BASIC PDS, mainly for use within the QBX
editing environment.  This newest variant reports the amount of expanded
(EMS) memory that is available, although EMS cannot be accessed by your
programs directly using BASIC statements.  However, QBX uses that memory to
store subroutines and optionally numeric, fixed-length, and TYPE arrays.
The ISAM file handler that comes with BASIC PDS can also utilize expanded
memory, as can the PDS overlay manager.


SETMEM AND STACK

Besides the various forms of the FRE function, SETMEM can be used to assess
the size of the far heap, as well as modify that size if necessary.  The
STACK function is available only with BASIC PDS, and it reports the largest
possible size the stack can be set to.  Let's see how these functions can
be useful to you.
     Although SETMEM is technically a function (because it returns
information), it is also used to re-size the far heap.  When given an
argument of zero, SETMEM returns the current size of the far heap.
However, this value is not the amount of memory that is free.  Rather, it
is the maximum heap size regardless of what currently resides there.  The
following short program shows this in context.


PRINT SETMEM(0)         'display the heap size
REDIM Array!(10000)     'allocate 40,000 bytes
PRINT SETMEM(0)         'the total size remains

Displayed result (the numbers will vary):

276256
276256


When a program starts, the far heap is set as large as possible by BASIC
and DOS, which is sensible in most cases.  But there are some situations in
which you might need to reduce that size, most notably when calling C
routines that need to allocate their own memory.  Also, BASIC moves arrays
around in the far heap as arrays are dimensioned and then erased.  This is
much like the near heap string compaction that is performed periodically.
If the far heap were not rearranged periodically, it is likely that many
small portions would be available, but not a single block sufficient for a
large array.
     In some cases a program may need to claim memory that is guaranteed
not to move.  Therefore, you could ask SETMEM to relinquish a portion of
the far heap, and then call a DOS interrupt to claim that memory for your
own use.  (DOS provides services to allocate and release memory, which C
and assembly language programs use to dimension arrays manually.)  Unlike
BASIC, DOS does not use sophisticated heap management techniques, therefore
the memory it manages does not move.  I will discuss using SETMEM this way
later on in Chapter 12.
     Finally, the STACK function will report the largest amount of memory
that can be allocated for use as a stack.  Like SETMEM, it doesn't reflect
how much of that memory is actually in use.  Rather, it simply reports how
large the stack could be if you wanted or needed to increase it.  Because
the stack resides in DGROUP, its maximum possible size is dependent on how
many variables and other data items are present.
     When run in the QBX environment, the following program fragment shows
how creating a dynamic string array reduces the amount of memory that could
be used for the stack.  Since the string descriptors are kept in DGROUP,
they impinge on the potentially available stack space.


PRINT STACK
REDIM Array$(1000)
PRINT STACK
ERASE Array$
PRINT STACK

Displayed result:

47904
43808
47904


Since BASIC PDS does not support FRE(0), the STACK function can be used to
determine how much near memory is available.  The only real difference
between FRE(0) and STACK is that STACK includes the current stack size,
where FRE(0) does not.  The STACK function is mentioned here because it
relates to assessing how much memory is available for data.  Sizing the
stack will be covered in depth in Chapter 3, when we discuss subprograms,
functions, and recursion.


VARPTR, VARSEG, AND SADD

One of the least understood aspects of BASIC programming is undoubtedly the
use of VARPTR and its related functions, VARSEG and SADD.  Though you
probably already know that VARPTR returns the address of a variable, you
might be wondering how that information could be useful.  After all, the
whole point of a high-level language such as BASIC is to shield the
programmer from variable addresses, pointers, and other messy low-level
details.  And by and large, that is correct.  Although VARPTR is not a
particularly common function, it can be invaluable in some programming
situations.
     VARPTR is a built-in BASIC function which returns the address of any
variable.  VARSEG is similar, however it reports the memory segment in
which that address is located.  SADD is meant for use with conventional
(not fixed-length) strings only, and it tells the address where the first
character in a string begins.  In BASIC PDS, SSEG is used instead of VARSEG
for conventional strings, to identify the segment in which the string data
is kept.  Together, these functions identify the location of any variable
in memory.
     The primary use for VARPTR in purely BASIC programming is in
conjunction with BSAVE and BLOAD, as well as PEEK and POKE.  For example,
to save an entire array quickly to a disk file with BSAVE, you must specify
the address where the array is located.  In most cases VARSEG is also
needed, to identify the array's segment as well.  When used on all simple
variables, static arrays, and all string arrays, VARSEG returns the normal
DGROUP segment.  When used on a dynamic numeric array, it instead returns
the segment at the which the specified element resides.
     The short example below creates and fills an integer array, and then
uses VARSEG and VARPTR to save it very quickly to disk.

REDIM Array%(1 TO 1000)

FOR X% = 1 TO 1000
  Array%(X%) = X%
NEXT

DEF SEG = VARSEG(Array%(1))
BSAVE "ARRAY.DAT", VARPTR(Array%(1)), 2000


Here, DEF SEG indicates in which segment the data that BSAVE will be saving
is located.  VARPTR is then used to specify the address within that
segment.  The 2000 tells BSAVE how many bytes are to be written to disk,
which is determined by multiplying the number of array elements times the
size of each element.  We will come back to using VARPTR repeatedly in
Chapter 12 when we discuss accessing DOS and BIOS services with CALL
Interrupt.  However, it is important to point out here exactly how VARPTR
and VARSEG work with each type of variable.
     When VARPTR is used with a numeric variable, as in Address =
VARPTR(Value!), the address of the first byte in memory that the variable
occupies is reported.  Value! is a single-precision variable which spans
four bytes of memory, and it is the lowest of the four addresses that is
returned.  Likewise, VARPTR when used with static fixed-length string and
TYPE variables reports the lowest address where the data begins.  But when
you ask for the VARPTR of a string variable, what is returned is the
address of the string's descriptor.
     To obtain the address of the actual data in a string requires the SADD
(String Address) function.  Internally, BASIC simply looks at the address
portion of the string descriptor to retrieve the address.  Likewise, the
LEN function also gets its information directly from the descriptor.  When
used with any string, VARSEG always reports the normal DGROUP data segment,
because that is where all strings and their descriptors are kept.
     Beginning with BASIC PDS and its support for far strings, the SSEG
function was added to return the segment where the string's data is stored.
But even when far strings are being used, VARSEG always returns the segment
for the descriptor, which is in DGROUP.
     SADD is not legal with a fixed-length string, and you must instead use
VARPTR.  Perhaps in a future version BASIC will allow either to be used
interchangeably.  SADD is likewise illegal for use with the fixed-length
string portion of a TYPE variable or array.  Again, VARPTR will return the
address of any component in a TYPE, within the segment reported by VARSEG.
     Another important use for VARPTR is to assist passing arrays to
assembly language routines.  When a single array element is specified using
early versions of Microsoft compiled BASIC, the starting address of the
element is sent as expected.  Beginning with QuickBASIC 4.0 and its support
for far data residing in multiple segments, a more complicated arrangement
was devised.  Here's how that works.
     When an element in a dynamic array is passed as a parameter, BASIC
makes a copy of the element into a temporary variable in near memory, and
then sends the address of the copy.  When the routine returns, the data in
the temporary variable is copied back to the original array element, in
case the called routine changed the data.  In many cases this behavior is
quite sensible, since the called routine can assume that the variable is in
near memory and thus operate that much faster.
     Further, BASIC subroutines *require* a non-array parameter (not passed
with empty parentheses) to be in DGROUP.  That is, any time a single
element in an integer array is passed to a routine, that routine would be
designed to expect a single integer variable.  This is shown in the brief
example below, where a single element in an array is passed, as opposed to
the entire array.


REDIM Array%(1 TO 100)
Array%(25) = -14
CALL MyProc(Array%(25))      'pass one element
.
.
.
SUB MyProc(IntVar%) STATIC   'this sub expects a
  PRINT IntVar%              '  single variable
END SUB

Displayed result:

-14


Unfortunately, this copying not only generates a lot of extra code to
implement, it also takes memory from DGROUP to hold the copy, and that
memory is taken permanently.  Worse still, *each* occurrence of an array
element passed in a CALL statement reserves however many bytes are needed
to store the element.  For a large TYPE structure this can be a lot of
memory indeed!
     So you won't think that I'm being an alarmist about this issue, here
are some facts based on programs compiled using BASIC 7.1 PDS.  These
examples document the amount of additional code that is generated to pass a
near string array element as an argument to a subprogram or function.
     Passing a string array element requires 56 bytes when a copy is made,
compared to only 17 when it is not.  The same operations in QuickBASIC 4.5
create 47 and 18 bytes respectively, so QB 4.5 is actually better when
making the copy, but a tad worse when not.  The code used in these examples
is shown below, and Array$ is a dynamic near string array.  (I will explain
the purpose of BYVAL in just a moment.)  Again, the difference in byte
counts reflects the additional code that BC creates to assign and then
delete the temporary copies.


CALL Routine(Array$(2))
CALL Routine(BYVAL VARPTR(Array$(2)))


Worse still, with either compiler 73 bytes of code are created to pass an
element in a TYPE array the usual way, compared to 18 when the copying is
avoided.  And this byte count does not include the DGROUP memory required
to hold the copy.  Is that reduction in code size worth working for?  You
bet it is!  And best of all, hardly any extra effort is needed to avoid
having BASIC make these copies--just the appropriate knowledge.
     The key, as you can see, is VARPTR.  If you are calling an assembly
language routine that expects a string and you want to pass an element from
a string array, you must use BYVAL along with VARPTR.  CALL Routine(BYVAL
VARPTR(Array$(Element))) is functionally identical to CALL
Routine(Array$(Element)), although they sure do look different!  In either
case, the integer address of a string is passed to the routine.
     Unlike the usual way that BASIC passes a variable by sending its
address, BYVAL instead sends the actual data.  In this case, the value of
an address is what we wanted to begin with anyway.  (Without the BYVAL,
BASIC would make a temporary copy of the integer value that VARPTR returns,
and send the address of that copy.)  Best of all, asking for the address
directly defeats the built-in copying mechanism.  Although creating a copy
of a far numeric array element is sensible as we saw earlier, it is not
clear to me why BC does this with string array data that is in DGROUP
already.
     Although you can't normally send an integer--which is what VARPTR
actually returns--to a BASIC subprogram that expects a string, you can if
that subprogram is in a different file and the files are compiled
separately.  This will also work if the BASIC code has been pre-compiled
and placed in a Quick Library.
     But there is another, equally important reason to use VARPTR with
array elements.  If you are calling an assembler routine that will sort an
array, it must have access to the array element's address, and not the
address of a copy.  All of the elements in any array are contiguous, and a
sort routine would need to know where in memory the first element is
located.  From that it can then access all of the successive elements.
With VARPTR we are telling BASIC that what is needed is the actual address
of the specified element.
     Bear in mind that this relates primarily to passing arrays to assembly
language (and possibly C) routines only.  After all, if you are designing a
sort routine using purely BASIC commands, you would pass and receive the
array using empty parentheses.  Indeed, this is yet another important
advantage that BASIC holds over C and Pascal, since neither of those
languages have array descriptors.  Writing a sort routine in C requires
that *you* do all of the work to locate and compare each element in turn,
based on some base starting address.
     There is one final issue that we must discuss, and that is passing far
array data to external assembly language routines.  I already explained
that by making a copy of a far array element, the called routine does not
have to be written to deal with far (two-word segmented) addresses.  But in
some cases, writing a routine that way will be more efficient.  Further,
like C, assembly language routines thrive on manipulating pointers to data.
Although an assembler routine could be written to read the segment and
address from the array descriptor, this is not a common method.  One reason
is that if Microsoft changes the format of the descriptor, the routine will
no longer work.  Another is that it is frankly easier to have the caller
simply pass the full segmented address of the first element.
     This brings us to the SEG directive, which is a combination of BYVAL
and VARPTR and also BYVAL and VARSEG. As with BYVAL VARPTR, using SEG
before a variable or array element in a call tells BASIC that the value of
the array's full address is needed.  A typical example would be CALL
Routine(SEG Array#(1)), and in this case, BASIC sends not one address word
but two to the routine.
     You could also pass the full address of an array element by value
using VARSEG and VARPTR, and this next example produces the identical
result: CALL Routine(BYVAL VARSEG(Array#(1)), BYVAL VARPTR(Array#(1))).
Using SEG results in somewhat less code, though, because BASIC will obtain
the segment and address in a single operation.  In fact, this is one area
where the compiler does a poor job of optimizing, because using VARSEG and
VARPTR in a single program statement generates a similar sequence of code
twice.
     There is one unfortunate complication here, which arises when SEG is
used with a fixed-length string array.  What SEG *should* do in that case
is pass the segmented address of the specified element.  But it doesn't.
Instead, BASIC creates a temporary copy of the specified element in a
conventional dynamic string, and then passes the segmented address of the
copy's descriptor.  Of course, this is useless in most programming
situations.
     There are two possible solutions to this problem.  The first is to use
the slightly less efficient BYVAL VARSEG and BYVAL VARPTR combination as
shown above.  The second solution is to create an equivalent fixed-length
string array by using a dummy TYPE that is comprised solely of a single
string component.  Since TYPE variables are passed correctly when SEG is
used, using a TYPE eliminates the problem.  Both of these methods are shown
in the listing that follows.


'----- this creates more code and looks clumsy

REDIM Array(1 TO 1000) AS STRING * 50
CALL Routine(BYVAL VARSEG(Array(1)), BYVAL VARPTR(Array(1)))


'----- this creates less code and reads clearly

TYPE FLen
  S AS STRING * 100
END TYPE
REDIM Array(1 TO 1000) AS FLen
CALL Routine(SEG Array(1))


Although SEG looks like a single parameter is being passed, in fact two
integers are sent to the called routine--a segment and an address.  This is
why a single SEG can replace both a VARSEG and a VARPTR in one call.
Chapter 13 will return to BYVAL, VARPTR, and SEG, though the purpose there
will be to learn how to write routines that accept such parameters.


CONSTANTS
=========

The final data type to examine is constants.  By definition, a constant is
simply any value that does not change, as opposed to a variable that can.
For example, in the statement I% = 10, the value 10 is a constant.
Similarly, the quoted string "Hello" is a constant when you write PRINT
"Hello".
     There are two types of constants that can appear in a BASIC program.
One is simple numbers and quoted strings as described above, and the other
is the named constant which is defined using a CONST statement.  For
example, you can write CONST MaxRows = 25 as well as CONST Message$ =
"Insert disk in drive", and so forth.  It is even possible to define one
CONST value based on a previous one, as in CONST NumRows = 25, ScrnSize =
NumRows * 80.  Then, you could use these meaningful names later in the
program, instead of the values they represent.
     It is important to understand that using named constants is identical
to using the numbers themselves.  The value of this will become apparent
when you see the relative advantages and disadvantages of using numbers as
opposed to variables.  Let's begin this discussion of numbers with how they
are stored by the compiler.  Or rather, how they are sometimes stored.
     When a CONST statement is used in a BASIC program, BASIC does
absolutely nothing with the value, other than to remember that you defined
it.  Therefore, you could have a hundred CONST statements which are never
used, and the final .EXE program will be no larger than if none had been
defined.  If a CONST value is used as an argument to, say, LOCATE or
perhaps as a parameter to a subroutine, BASIC simply substitutes the value
you originally gave it.  When a variable is assigned as in Value% = 100,
BASIC sets aside memory to hold the variable.  With a constant definition
such as CONST Value% = 100, no memory is set aside and BASIC merely
remembers that any use of Value% is to be replaced by the number 100.  But
how are these numbers represented internally.
     When you create an integer assignment such as Count% = 5, the BASIC
compiler generates code to move the value 5 into the integer variable, as
you saw in Chapter 1.  Therefore, the actual value 5 is never stored as
data anywhere.  Rather, it is placed into the code as part of an assembly
language instruction.
     Now, if you instead assign a single or double precision variable from
a number--and again it doesn't matter whether that number is a literal or a
CONST--the appropriate floating point representation of that number is
placed in DGROUP at compile time, and then used as the source for a normal
floating point assignment.  That is, it is assigned as if it were a
variable.
     There is no reasonable way to imbed a floating point value into an
assembly language instruction, because the CPU cannot deal with such values
directly.   Therefore, assigning X% = 3 treats the number 3 as an integer
value, while assigning Y# = 3 treats it as a double precision value.
Again, it doesn't matter whether the 3 is a literal number as shown here,
or a CONST that has been defined.  In fact, if you use CONST Three! = 3, a
subsequent assignment such as Value% = Three! treats Three! as an integer
resulting in less resultant code.  As you can see, the compiler is
extremely smart in how it handles these constants, and it understands the
context in which they are being used.
     In general, BASIC uses the minimum precision possible when
representing a number.  However, you can coerce a number to a different
precision with an explicit type identifier.  For example, if you are
calling a routine in a separate module that expects a double precision
value, you could add a pound sign (#) to the number like this: CALL
Something(45#).  Without the double precision identifier, BASIC would treat
the 45 as an integer, which is of course incorrect.
     Likewise, BASIC can be forced to evaluate a numeric expression that
might otherwise overflow by placing a type identifier after it.  One
typical situation is when constructing a value from two byte portions.  The
usual way to do this would be Value& = LoByte% + 256 * HiByte%.  Although
the result of this expression can clearly fit into the long integer no
matter what the values of LoByte% and HiByte% might be, an overflow error
can still occur.  (But as we saw earlier, this will happen only in the QB
environment, or if you have compiled to disk with the /d debugging option.)
     The problem arises when HiByte% is greater than 127, because the
result of multiplying HiByte% times 256 exceeds the capacity of a regular
integer.  Normally, BASIC is to be commended for the way it minimizes
overhead by reducing calculations to the smallest possible data type.  But
in this case it creates a problem, because the result cannot be expressed
as an integer.
     The solution, then, is to add an ampersand after the 256, as in Value&
= LoByte% + 256& * HiByte%.  By establishing the value 256 as a long
integer, you are telling BASIC to perform the calculation to the full
precision of a long integer.  And since the result of the multiplication is
treated as a long integer, so is the addition of that result to LoByte%.  A
single precision exclamation point could also be used, but that would
require a floating point multiplication.  Since a long integer multiply is
much faster and needs less code, this is the preferred solution.
     One final item worth noting is the way the QB and QBX editing
environments sometimes modify constants.  For example, if you attempt to
enter a statement such as Value! = 1.0, you will see the constant changed
to read 1! instead.  This happens when you press Enter to terminate the
line.  Similarly, if you write D# = 1234567.8901234, BASIC will add a
trailing pound sign to the number.  This behavior is your clue that these
numbers are being stored internally as single and double precision values
respectively.


PASSING NUMERIC CONSTANTS TO A PROCEDURE

Normally, any constant that could be an integer is passed to a subprogram
or function as an integer.  That is, calling an external procedure as in
CALL External(100) passes the 100 as an integer value.  If the called
routine has been designed to expect a variable of a different type, you
must add the appropriate type identifier.  If a long integer is expected,
for example, you must use CALL External(100&).  If, on the other hand, the
called routine is in the same module (that is, the same physical source
file), QB will create a suitable DECLARE statement automatically.  This
lets QB and BC know what is expected so they can pass the value in the
correct format.  Thus, BASIC is doing you a favor by interpreting the
constant's type in a manner that is relevant to your program.
     This "favor" has a nasty quirk, though.  If you are developing a
multi-module program in the QuickBASIC editor, the automatic type
conversion is done for you automatically, even when the call is to a
different module.  Your program uses, say, CALL Routine(25), and QB or QBX
send the value in the correct format automatically.  But when the modules
are compiled and linked, the same program that had worked correctly in the
environment will now fail.
     Since each module in a multi-module program is compiled separately, BC
has no way to know what the called routine actually expects.  In fact, this
is one of the primary purposes of the DECLARE statement--to advise BASIC as
to how arguments are to be passed.  For example, DECLARE SUB
Marine(Trident!) tells BASIC that any constant passed to Marine is to be
sent as a single precision value.  You could optionally use the AS SINGLE
directive, thus: DECLARE SUB Marine(Trident AS SINGLE).  In general, I
prefer the more compact form since it conveys the necessary information
with less clutter.
     Another important use for adding a type identifier to a numeric
constant is to improve a program's accuracy.  Running the short program
below will illustrate this in context.  Although neither answer is entirely
accurate, the calculation that uses the double precision constant is much
closer.  In this case, a decimal number that does not have an explicit type
identifier is assumed to have only single precision accuracy.  That is, the
value is stored in only four bytes instead of eight.


FOR X% = 1 TO 10000
  Y# = Y# + 1.1
  Z# = Z# + 1.1#
NEXT
PRINT Y#, Z#

Displayed result:
11000.00023841858     11000.00000000204


You have already learned that BASIC often makes a temporary copy of a
variable when calling a subprogram or function.  But you should know that
this also happens whenever a constant is passed as an argument.  For
example, in a function call such as Result = Calculate!(Value!, 100), where
Calculate! has been declared as a function, the integer value 100 is copied
to a temporary location.  Since BASIC procedures require the address of a
parameter, a temporary variable must be created and the address of that
variable passed.  The important point to remember is that for each
occurrence of a constant in a CALL or function invocation, a new area of
DGROUP is taken.
     You might think that BASIC should simply store a 100 somewhere in
DGROUP once, and then pass the address of that value.  Indeed, this would
save an awful lot of memory when many constants are being used.  The reason
this isn't done, however, is that subroutines can change incoming
parameters.  Therefore, if a single integer 100 was stored and its address
passed to a routine that changed it, subsequent calls using 100 would
receive an incorrect value.
     The ideal solution to this problem is to create a variable with the
required value.  For example, if you are now passing the value 2 as a
literal many times in a program, instead assign a variable, perhaps named
Two%, early in your program.  That is, Two% = 2.  Then, each time you need
that value, instead pass the variable.  For the record, six bytes are
needed to assign an integer such as Two%, and four bytes are generated each
time that variable is passed in a call.
     Contrast that to the 10 bytes generated to create and store a
temporary copy and pass its address, not including the two bytes the copy
permanently takes from near memory.  Even if you use the value only twice,
the savings will be worthwhile (24 vs. 30 bytes).  Because a value of zero
is very common, it is also an ideal candidate for being replaced with a
variable.  Even better, you don't even have to assign it!  That is, CALL
SomeProc(Zero%) will send a zero, without requiring a previous Zero% = 0
assignment.


STRING CONSTANTS
================

Like numeric constants, string constants that are defined in a CONST
statement but never referenced will not be added to the final .EXE file.
Constants that are used--whether as literals or as CONST statements--are
always stored in DGROUP.  If your program has the statement PRINT "I like
BASIC", then the twelve characters in the string are placed into DGROUP.
But since the PRINT statement requires a string descriptor in order to
locate the string and determine its length, an additional four bytes are
allocated by BASIC just for that purpose.  Variables are always stored at
an even-numbered address, so odd-length strings also waste one extra byte.
     Because string constants have a ferocious appetite for near memory, BC
has been designed to be particularly intelligent in the way they are
handled.  Although there is no way to avoid the storage of a descriptor for
each constant, there is another, even better trick that can be employed.
For each string constant you reference in a program that is longer than
four characters, BC stores it only once.  Even if you have the statement
PRINT "Press any key to continue" twenty-five times in your program, BC
will store the characters just once, and each PRINT statement will refer to
the same string.
     In order to do this, the compiler must remember each string constant
it encounters as it processes your program, and save it in an internal
working array.  When many string constants are being used, this can cause
the compiler to run out of memory.  Remember, BC has an enormous amount of
information it must deal with as it processes your BASIC source file, and
keeping track of string constants is but one part of the job.
     To solve this problem Microsoft has provided the /s (String) option,
which tells BC not to combine like data.  Although this may have the net
effect of making the final .EXE file larger and also taking more string
space, it may be the only solution with some large programs.  Contrary to
the BASIC documentation, however, using /s in reality often makes a program
*smaller*.  This issue will be described in detail in Chapter 5, where all
of the various BC command line options are discussed.



PASSING STRING CONSTANTS TO A PROCEDURE

As you have repeatedly seen, BASIC often generates additional code to
create copies of variables and constants.  It should come as no surprise,
therefore, to learn that this happens with string constants as well.  When
you print the same string more than once in a program, BASIC knows that its
own PRINT routine will never change the data.  But as with numeric
constants, if you send a string constant to a subprogram or function, there
is no such guarantee.
     For example, if you have a statement such as CALL PrintIt(Work$) in
your program, it is very possible--even likely--that the PrintIt routine
may change or reassign its incoming parameter.  Even if *you* know that
PrintIt will not change the string, BASIC has no way to know this.  To
avoid any possibility of that happening, BASIC generates code to create a
temporary copy of every string constant that is used as an argument.  And
this is done for every call.  If the statement CALL PrintMessage("Press a
key") appears in your program ten times, then code to copy that message is
generated ten times!
     Beginning with BASIC 7.1 PDS, you can now specify that variables are
to be sent by value to BASIC procedures.  This lets you avoid the creation
of temporary copies, and this subject will also be explored in more detail
in Chapter 3.
     With either QuickBASIC 4.5 or BASIC PDS, calling a routine with a
single quoted string as an argument generates 31 bytes of code.  Passing a
string variable instead requires only nine bytes.  Both of these byte
counts includes the five bytes to process the call itself.  The real
difference is therefore 4 bytes vs. 26--for a net ratio of 6.5 to 1.  (Part
of those 31 bytes is code that erases the temporary string.)  So as with
numeric constants that are used more than once, your programs will be
smaller if a variable is assigned once, and that variable is passed as an
argument.
     While we are on the topic of temporary variables, there is yet another
situation that causes BASIC to create them.  When the result of an
expression is passed as an argument, BASIC must evaluate that expression,
and store the result somewhere.  Again, since nearly all procedures require
the address of a parameter rather than its value, an address of that result
is needed.  And without storing the result, there can of course be no
address.
     When you use a statement such as CALL Home(Elli + Lou), BASIC
calculates the sum of Elli plus Lou, and stores that in a reserved place in
DGROUP which is not used for any other purpose.  That address is then sent
to the Home routine as if it were a single variable, and Home is none the
wiser.  Likewise, a string concatenation creates a temporary string, for
the same reason.  Although the requisite descriptor permanently steals four
bytes of DGROUP memory, the temporary string itself is erased by BASIC
automatically after the call.  Thus, the first example in the listing below
is similar in efficiency to the second.  The four-byte difference is due to
BASIC calling a special routine that deletes the temporary copy it created,
as opposed to the slightly more involved code that assigns Temp$ from the
null string ("") to erase it.


CALL DoIt(First$ + Last$)  'this makes 41 bytes

Temp$ = First$ + Last$     'this makes 45 bytes
CALL DoIt(Temp$)
Temp$ = ""


UNUSUAL STRING CONSTANTS

One final topic worth mentioning is that QuickBASIC also lets you imbed
control and extended characters into a string constant.  Consider the
program shown below.  Here, several of the IBM extended characters are used
to define a box, but without requiring CHR$ to be used repeatedly.
Characters with ASCII values greater than 127 can be entered easily by
simply pressing and holding the Alt key, typing the desired ASCII value on
the PC's numeric key-pad, and then releasing the Alt key.  This will not
work using the number keys along the top row of the keyboard.


DIM Box$(1 TO 4)          'define a box

Box$(1) = "╔══════════════════╗"
Box$(2) = "║                  ║"
Box$(3) = "║                  ║"
Box$(4) = "╚══════════════════╝"

FOR X = 1 TO 4            'now display the box
  PRINT Box$(X)
NEXT


To enter control characters (those with ASCII values less than 32) requires
a different trick.  Although the Alt-keypad method is in fact built into
the BIOS of all PCs, this next one is specific to QuickBASIC, QBX, and some
word processor programs.  To do this, first press Ctrl-P, observing the ^P
symbol that QB displays at the bottom right of the screen.  This lets you
know that the next control character you press will be accepted literally.
For example, Ctrl-P followed by Ctrl-L will display the female symbol, and
Ctrl-P followed by Ctrl-[ will enter the Escape character.
     Bear in mind that some control codes will cause unusual behavior if
your program is listed on a printer.  For example, an embedded CHR$(7) will
sound the buzzer if your printer has one, a CHR$(8) will back up the print
head one column, and a CHR$(12) will issue a form feed and skip to the next
page.  Indeed, you can use this to advantage to intentionally force a form
feed, perhaps with a statement such as REM followed by the Ctrl-L female
symbol.
     I should mention that different versions of the QB editor respond
differently to the Ctrl-P command.  QuickBASIC 4.0 requires Ctrl-[ to enter
the Escape code, while QBX takes either Ctrl-[ or the Escape key itself.  I
should also mention that you must never imbed a CHR$(26) into a BASIC
source file.  That character is recognized by DOS to indicate the end of a
file, and BC will stop dead at that point when compiling your program.  QB,
however, will load the file correctly.


WOULDN'T IT BE NICE IF DEPT.
============================

No discussion of constants would be complete without a mention of
initialized data.  Unfortunately, as of this writing BASIC does not support
that feature!  The concept is simple, and it would be trivial for BASIC's
designers to implement.  Here's how initialized data works.
     Whenever a variable requires a certain value, the only way to give it
that value is to assign it.  Some languages let you declare a variable's
initial value in the source code, saving the few bytes it takes to assign
it later.  Since space for every variable is in the .EXE file anyway, there
would be no additional penalty imposed by adding this capability.  I
envision a syntax such as DIM X = 3.9 AS SINGLE, or perhaps simply DIM Y% =
3, or even DIM PassWord$ = "GuessThis".  Where Y% = 3 creates a six-byte
code sequence to put the value 3 into Y%, what I am proposing would have
the compiler place that value there at the time it creates the program.
     Equally desireable would be allowing string constants to be defined
using CHR$ arguments.  For example, CONST EOF$ = CHR$(26) would be a
terrific enhancement to the language, and allowing code such as CONST CRLF$
= CHR$(13) + CHR$(10) would be even more powerful.  Again, we can only hope
that this feature will be added in a future version.
     Yet another constant optimization that BASIC could do but doesn't is
constant string function evaluation.  In many programming situations the
programmer is faced with deciding between program efficiency and
readability.  A perfect example of this is testing an integer value to see
whether it represents a legal character.  For instance, IF Char < 65 is not
nearly as meaningful as IF Char < ASC("A").
     Clearly, BC could and should resolve the expression ASC("A") while it
is compiling your program, and generate simple code that compares two
integers.  Instead, it stores the "A" as a one-byte string (which with its
descriptor takes five bytes), and generates code to call the internal ASC
function before performing the comparison.  The point here is that no
matter how intelligent BC is, folks like us will always find some reason to
complain!


BIT OPERATIONS
==============

The last important subject this chapter will cover is bit manipulation
using AND, OR, XOR, and NOT.  These logical operators have two similar, but
very different, uses in a BASIC program.  The first use--the one I will
discuss here--is to manipulate the individual bits in an integer or long
integer variable.  The second use is for directing a program's flow, and
that will be covered in Chapter 3.
     Each of the bit manipulation operators performs a very simple Binary
function.  Most of these functions operate on the contents of two integers,
using those bits that are in an equivalent position.  The examples shown in
Figure 2-6 use a single byte only, solely for clarity.  In practice, the
same operations would be extended to either the sixteen bits in an integer,
or the 32 bits in a long integer.


13 = 0000 1101
25 = 0001 1001
     ─────────
     0000 1001  result when AND is used
          ^  ^
          └──┴──────── both of the bits are set
                       in each column


13 = 0000 1101
25 = 0001 1001
     ─────────
     0001 1101  result when OR is used
        ^ ^^ ^
        └─┴┴─┴──────── one or both bits are set
                       in each column


13 = 0000 1101
25 = 0001 1001
     ─────────
     0001 0100  result when XOR is used
        ^  ^
        └──┴────────── the bits are different
                       in each column


13 = 0000 0000 0000 1101
     ───────────────────
     1111 1111 1111 0010  result after using NOT

Figure 2-6


The examples given here use the same decimal values 13 and 25, and these
are also shown in their Binary equivalents.  What is important when viewing
Binary numbers is to consider the two bits in each vertical column.  In the
first example, the result in a given column is 1 (or True) only when that
bit is set in the first number AND the same bit is also set in the second.
This condition is true for only two of the bits in these particular
numbers.  The result bits therefore represent the answer in Binary, which
in this case is 13 AND 25 = 9.  What is important here is not that 13 AND
25 equals 9, but how the bits interact with each other.
     The second example shows OR at work, and it sets the result bits for
any position where a given bit is set in one byte OR that bit is set in the
other.  Of course, if both are set the OR result is also true.  In this
case, four of the columns have one bit or the other (or both) set to 1.  By
the way, these results can be proven easily in BASIC by simply typing the
expression.  That is, PRINT 13 OR 25 will display the answer 29.
     The third example is for XOR, which stands for Exclusive Or.  XOR sets
a result bit only when the two bits being compared are different.  Here,
two of the bits are different, thus 13 XOR 25 = 20.  Again, it is not the
decimal result we are after, but how the bits in one variable can be used
to set or clear the bits in another.
     The NOT operator uses only one value, and it simply reverses all of
the bits.  Any bit that was a 1 is changed to 0, and any bit that had been
0 is now 1.  A full word is used in this example, to illustrate the fact
that NOT on any positive number makes it negative, and vice versa.  As you
learned earlier in this chapter, the highest, or left-most bit is used to
store the sign of a number.  Therefore, toggling this bit also switches the
number between positive and negative.  In this case, NOT 13 = -14.
     All of the logical operators can be very useful in some situations,
although admittedly those situations are generally when accessing DOS or
interfacing with assembly language routines.  For example, many DOS
services indicate a failure such as "File not found" by setting the Carry
flag.  You would thus use AND after a CALL Interrupt to test that bit.
Another good application for bit manipulation is to store True or False
information in each of the sixteen bits in an integer, thus preserving
memory.  That is, instead of sixteen separate Yes/No variables, you could
use just one integer.
     Bit operations can also be used to replace calculations in certain
situations.  One common practice is to use division and MOD to break an
integer word into its component byte portions.  The usual way to obtain the
lower byte is LoByte% = Word% MOD 256, where MOD provides the remainder
after dividing.  While there is nothing wrong with doing it that way, Word%
= LoByte% AND 255 operates slightly faster.  Division is simply a slower
operation than AND, especially on the 8088.  Newer chips such as the 80286
and 80386 have improved algorithms, and division is not nearly as slow as
with the older CPU.  Chapter 3 will look at some other purely BASIC uses of
AND and OR.


SUMMARY
=======

As you have seen in this chapter, there is much more to variables and data
than the BASIC manuals indicate.  You have learned how data is constructed
and stored, how the compiler manipulates that data, and how to determine
for yourself the amount of memory that is needed and is available.  In
particular, you have seen how data is copied frequently but with no
indication that this is happening.  Because such copying requires
additional memory, it is a frequent cause of "Out of memory" errors that on
the surface appear to be unfounded.
     You have also learned about BASIC's near and far heaps, and how they
are managed using string and array descriptors.  With its dynamic
allocation methods and periodic rearrangement of the data in your program,
BASIC is able to prevent memory from becoming fragmented.  Although such
sophisticated memory management techniques require additional code to
implement, they provide an important service that programmers would
otherwise have to devise for themselves.
     Finally, you have learned how the various bit manipulation operations
in BASIC work.  This chapter will prove to be an important foundation for
the information presented in upcoming chapters.  Indeed, a thorough
understanding of data and memory issues will be invaluable when you learn
about accessing DOS and BIOS services in Chapter 12.