Shareware Overload/ShartewareOverload.cdr/progm/chasm2.zip/PRIMER.DOC

Text File | 1986-03-07 | 47KB | 1,783 lines

                    An Assembly Language Primer

                     (c) 1983 by David Whitman






















































                  TABLE OF CONTENTS


    Introduction.......................................2

    The Computer As A Bit Pattern Manipulator..........3

    Digression: A Notation System for Bit Patterns.....5

    Addressing Memory..................................7

    The Contents of Memory: Data and Programs..........8

    The Dawn of Assembly Language......................9

    The 8088..........................................11

    Assembly Language Syntax..........................14

    The Stack.........................................17

    Software Interrupts...............................19

    Pseudo-Operations.................................21

    Tutorial..........................................23







































                                                                    2
    >>INTRODUCTION<<

    Many people requesting CHASM have indicated that they are
    interested in *learning* assembly language.  They are beginners,
    and have little idea just where to start.  This primer is
    directed to those users.  Experienced users will probably find
    little here that they do not already know.

    Being a primer, this text will not teach you everything there is
    to know about assembly language programming.  It's purpose is to
    give you some of the vocabulary and general ideas which will help
    you on your way.

    I must make a small caveat: I consider myself a relative beginner
    in assembly language programming.  A big part of the reason for
    writing CHASM was to try and learn this branch of programming
    from the inside out.  I think I've learned quite a bit, but it's
    quite possible that some of the ideas I relate here may have some
    small, or even large, flaws in them.  Nonetheless, I have
    produced a number of working assembly language programs by
    following the ideas presented here.












































                                                                    3
    >>THE COMPUTER AS A BIT PATTERN MANIPULATOR.<<

    We all have some conception about what a computer does.  On one
    level, it may be thought of as a machine which can execute BASIC
    programs.  Another idea is that the computer is a number
    crunching device.  As I write this primer, I'm using my computer
    as a word processor.

    I'd like to introduce a more general concept of just what sort of
    machine a computer is: a bit pattern manipulator.

    I'm certain that everyone has been introduced to the idea of a
    *bit*.  (Note: Throughout this primer, a word enclosed in
    *asterisks* is to be read as if it were in italics.)  A bit has
    two states: on and off, typically represented with the symbols
    "1"  and "0".  In this context, DON'T think of 1 and 0 as
    numbers.  They are merely convenient shorthand labels for the
    state of a bit.

    The memory of your computer consists of a huge collection of
    bits, each of which could be in either the 1 or 0 (on or off)
    state.

    At the heart of your computer is an 8088 microprocessor chip,
    made by Intel.  What this chip can do is manipulate the bits
    which make up the memory of the computer.

    The 8088 likes to handle bits in chunks, and so we'll introduce
    special names for the two sizes of bit chunks the 8088 is most
    happy with.  A *byte* will refer to a collection of eight bits.
    A *word* consists of two bytes, or equivalently, sixteen bits.

    A collection of bits holds a pattern, determined by the state of
    its individual bits.  Here are some typical byte long patterns:

    10101010         11111111         00001111

    If you've had a course in probability, it's quite easy to work
    out that there are 256 possible patterns that a byte could hold.
    similarly, a word can hold 65,536 different patterns.

























                                                                    4
    All right, now for the single most important idea in assembly
    language programming.  Are you sitting down?  These bit patterns
    can be used to represent other sets of things, by mapping each
    pattern onto a member of the other set.  Doesn't sound like much,
    but IBM has made *BILLIONS* off this idea.

    For example, by mapping the patterns a word can hold onto the set
    of integers, you can represent either the numbers from 0 to 65535
    or -32768 to 32767, depending on the exact mapping you use.  You
    might recognize these as the BASIC's range of possible line
    numbers, and possible values for integer variables.  This
    explains these somewhat arbitrary seeming limits: BASIC uses
    words of memory to hold line numbers and integer variables.

    As another example, you could map the patterns a byte can hold
    onto a series of arbitrarily chosen little pictures which might
    be displayed on a video screen.  If you look at the table in
    the back of your BASIC manual, you'll notice that there are
    *exactly* 256 different characters that can be displayed on your
    screen.  Your computer uses a byte of memory to tell it what
    character to display at each location of the video screen.

    Without getting too far ahead of myself, I'll just casually
    mention that there are about 256 fundamental ways the 8088 can
    manipulate the bit patterns stored in memory.  This suggests
    another mapping which we'll discuss in more detail later.

    The point of this discussion is that we can use bit patterns to
    represent anything we want, and by manipulating the patterns in
    different ways, we can produce results which have significance in
    terms of what we're choosing to represent.


































                                                                    5
    >>DIGRESSION: A NOTATION SYSTEM FOR BIT PATTERNS<<

    Because of their importance, it would be nice to have a
    convenient way to represent the various bit patterns we'll be
    talking about.  We already have one way, by listing the states of
    the individual bits as a series of 1's and 0's.  However, this
    system is somewhat clumsy, and error prone.  Are the following
    word patterns identical or different?

    1111111011111111                         1111111101111111

    You probably had trouble telling them apart.  It's easier to tell
    that they're different by breaking them down into more manageable
    pieces, and comparing the pieces.  Here are the same two patterns
    broken down into four bit chunks:

    1111 1110 1111 1111                      1111 1111 0111 1111

    Some clown has given the name *nybble* to a chunk of 4 bits,
    presumably because 4 bits are half a byte.  A nybble is fairly
    easy to handle.  There are only 16 possible nybble long patterns,
    and most people can distinguish between the patterns quite
    easily.

    Each nybble pattern has been given a unique symbol agreed upon by
    computer scientists.  The first 10 patterns were given symbols
    "0" through "9", and when they ran out of digit style symbols,
    they used the letters "A" through "F" for the last six patterns.
    Below is the "nybble pattern code":

    0000 = 0    0001 = 1    0010 = 2    0011 = 3

    0100 = 4    0101 = 5    0110 = 6    0111 = 7

    1000 = 8    1001 = 9    1010 = A    1011 = B

    1100 = C    1101 = D    1110 = E    1111 = F

    Using the nybble code, we can represent the two similar word
    patterns given above, with the following more manageable
    shorthand versions:

                   FEFF       FF7F






















                                                                    6
    Of course, the assignment of the symbols for the various nybble
    patterns was not so arbitrary as I've tried to make it appear.  A
    perceptive reader who has been exposed to binary numbers will
    have noticed an underlying system to the assignments.  If the 1's
    and 0's of the patterns are interpreted as actual *numbers*,
    rather than mere symbols for bit states, the first 10 patterns
    correspond to binary numbers whose decimal representation is the
    symbol assigned to the pattern.

    The last six patterns receive the symbols "A" through "F", and
    taken together, the symbols 0 through F constitute the digits of
    the *hexadecimal* number system.  Thus, the symbols assigned to
    the different nybble patterns were born out of historical
    prejudice in thinking of the computer as strictly a number
    handling machine.  Although this is an important interpretation
    of these symbols, for the time being it's enough to merely think
    of them as a shorthand way to write down bit patterns.

    Because some nybble patterns can look just like a number, it's
    often necessary to somehow indicate that we're talking about a
    pattern.  In BASIC, you do this by adding the characters &H to
    the beginning of the pattern: &H1234.  A more common convention
    is to just add the letter H to the end of the pattern: 1234H.  In
    both conventions, the H is referring to hexadecimal.

    Eventually you'll want to learn about using the hexadecimal
    number system, since it is an important way to use bit patterns.
    I'm not going to discuss it in this primer, because a number of
    books have much better treatments of this topic than I could
    produce.  Consider this an advanced topic you'll want to fill in
    later.


































                                                                    7
    >>ADDRESSING MEMORY<<

    As stated before, the 8088 chip inside your computer can
    manipulate the bit patterns which make up the computer's memory.
    Some of the possible manipulations are copying patterns from one
    place to another, turning on or turning off certain bits, or
    interpreting the patterns as numbers and performing arithmetic
    operations on them.  To perform any of these actions, the 8088
    has to know what part of memory is to be worked on.  A specific
    location in memory is identified by its *address*.

    An address is a pointer into memory.  Each address points to the
    beginning of a byte long chunk of memory.  The 8088 has the
    capability to distinguish 1,048,576 different bytes of memory.

    By this point, it probably comes as no surprise to hear that
    addresses are represented as patterns of bits.  It takes 20 bits
    to get a total of 1,048,576 different patterns, and thus an
    address may be written down as a series of 20 bits (or 5 nybble
    codes).  For example, DOS stores a pattern which encodes
    information about what equipment is installed on your IBM PC in
    the word which begins at location 00410.  Interpreting the
    address as a hex number, the second byte of this word has an
    address 1 greater than 00410, or 00411.

    The 8088 isn't very happy handling 20 bits at a time.  The
    biggest chunk that's convenient for it to use is a 16 bit word.
    The 8088 actually calculates 20 bit addresses as the combination
    of two words, a segment word and an offset word.  The combination
    process involves interpreting the two patterns as hexadecimal
    numbers and adding them.  The way that two 16 bit patterns can be
    combined to give one 20 bit pattern is that the two patterns are
    added out of alignment by one nybble:

        0040      4 nybble segment
         0010     4 nybble offset
       --------
        00410     5 nybble address

    Because of this mechanism for calculating addresses, they will
    often be written down in what's called segment:offset form.
    Thus, the address in above calculation could be written:

    0040:0010





















                                                                    8
    >>MEMORY CONTENTS: DATA AND PROGRAMS<<

    The contents of memory may be broken down into two broad classes.
    The first is *data*, just raw patterns of bits for the 8088 to
    work on.  The significance of the patterns is determined by what
    the computer is being used for at any given time.

    The second class of memory contents are *instructions*.  The 8088
    can look at memory and interpret a pattern it sees there as
    specifying one of the 200 some fundamental operations it knows
    how to do.  This mapping of patterns onto operations is called
    the *machine language* of the 8088.  A machine language *program*
    consists of a series of patterns located in consecutive memory
    locations, whose corresponding operations perform some useful
    process.

    Note that there is no way for the 8088 to know whether a given
    pattern is meant to be an instruction, or a piece of data to
    operate on.  It is quite possible for the chip to accidentally
    begin reading what was intended to be data, and interpret it as a
    program.  Some pretty bizarre things can occur when this happens.
    In assembly language programming circles, this is known as
    "crashing the system".










































                                                                    9
    >>THE DAWN OF ASSEMBLY LANGUAGE<<

    Unless you happen to be an 8088 chip, the patterns which make up
    a machine language program can be rather incomprehensible.  For
    example, the pattern which tells the 8088 to flip all the bits in
    the byte at address 5555 is:

    F6 16 55 55

    which is not very informative, although you can see the 5555
    address in there.  In ancient history, the old wood-burning and
    vacuum tube computers were programmed by laboriously figuring out
    bit patterns which represented the series of instructions
    desired.  Needless to say, this technique was incredibly tedious,
    and very prone to making errors.  It finally occurred to these
    ancestral programmers that they could give the task of figuring
    out the proper patterns to the computer itself, and assembly
    language programming was born.

    Assembly language represents each of the many operations that the
    computer can do with a *mnemonic*, a short, easy to remember
    series of letters.  For example, in boolean algebra, the logical
    operation which inverts the state of a bit is called "not", and
    hence the assembly language equivalent of the preceding machine
    language pattern is:

        NOTB [5555]

    The brackets around the 5555 roughly mean "the memory location
    addressed by".  The "B" at the end of "NOTB" indicates that we
    want to operate on a byte of memory, not a word.

    Unfortunately, the 8088 can't make head nor tail of the string of
    characters "NOTB".  What's needed is a special program to run on
    the 8088 which converts the string "NOTB" into the pattern F6 16.
    This program is called an assembler.  A good analogy is that an
    assembler program is like a meat grinder which takes in assembly
    language and gives out machine language.

    Typically, an assembler reads a file of assembly language and
    translates it one line at a time, outputting a file of machine
    language.  Often times the input file is called the *source file*
    and the output file is called the *object file*.  The machine
    language patterns produced are called the *object code*.





















                                                                   10
    Also produced during the assembly process is a *listing*, which
    summarizes the results of the assembly process.  The listing
    shows each line from the source file, along with the shorthand
    "nybble code" representation of the object code produced.  In the
    event that the assembler was unable to understand any of the
    source lines, it inserts error messages in the listing, pointing
    out the problem.

    The primeval assembly language programmers had to write their
    assembler programs in machine language, because they had no other
    choice.  Not being a masochist, I wrote CHASM in Turbo Pascal.
    When you think about it, there's a sort of circular logic in
    action here. Some programmers at Borland wrote the Turbo Pascal
    in assembly language, and I used Turbo to write an assembler.
    Someday, I hope to use the present version of CHASM to produce a
    machine language version, which will run about a ten times
    faster, and at the same time bring this crazy process full
    circle.















































                                                                   11
    >>THE 8088<<

    The preceding discussions have (I hope) given you some very
    general background, a world view if you will, about assembly and
    machine language programming.  At this point, I'd like to get
    into a little more detail, beginning by examining the internal
    structure of the 8088 microprocessor from the programmer's point
    of view.  This discussion is a condensation of information which
    I obtained from "The 8086 Book" which was written by Russell
    Rector and George Alexy, and published by Osborne/McGraw-Hill.
    Once you've digested this, I'd recommend going to The 8086 Book
    for a deeper treatment.

    Inside the 8088 are a number of *registers* each of which can
    hold a 16 bit pattern.  In assembly language, each of the
    registers has a two letter mnemonic name.  There are 14
    registers, and their mnemonics are:

    AX BX CX DX     SP BP    SI DI     CS DS SS ES    PC ST

    Each of the registers are a little different and have different
    intended uses, but they can be grouped into some broad classes.

    The *general purpose* registers (AX BX CX DX) are just that.
    These are registers which hold patterns pulled in from memory
    which are to be worked on within the 8088.  You can use these
    registers for just about anything you want.

    Each of the general purpose registers can be broken down into two
    8 bit registers, which have names of their own.  Thus, the CX
    register is broken down into the CH and CL registers.  The "H"
    and "L" stand for high and low respectively.  Each general
    purpose register breaks down into a high/low pair.

    The AX register, and its 8 bit low half, the AL register, are
    somewhat special.  Mainly for historical reasons, these registers
    are referred to as the 16 bit and 8 bit *accumulators*.  Some
    operations of the 8088 can only be carried out on the contents of
    the accumulators, and many others are faster when used in
    conjunction with these registers.

























                                                                   12
    Another group of registers are the *segment* registers (CS DS SS
    ES).  These registers hold segment values for use in calculating
    memory addresses.  The CS, or code segment register, is used
    every  time the 8088 accesses memory to read an instruction
    pattern.  The  DS, or data segment register, is used for bringing
    data patterns in.  The SS register is used to access the stack
    (more about the stack later).  The ES is the extra segment
    register.  A very few special instructions use the ES register to
    access memory, plus you can override use of the DS register and
    substitute the ES register if you need to maintain two separate
    data areas.

    The *pointer* (SP BP) and *index* (DI SI) registers are used to
    provide indirect addressing, which is an very powerful technique
    for accessing memory.  Indirect addressing is beyond the scope of
    this little primer, but is discussed in The 8086 Book.  The SP
    register is used to implement a stack in memory. (again, more
    about the stack later)  Besides their special function, the BP,
    DI and SI registers can be used as additional general purpose
    registers.  Although it's possible to directly manipulate the
    value in the SP register, it's best to leave it alone, since you
    could wipe out the stack.

    Finally, there are two registers which are relatively
    inaccessible to direct manipulation.  The first is the *program
    counter*, PC.  This register always contains the offset part of
    the address of the next instruction to be executed.  Although
    you're not allowed to just move values into this register, you
    *can* indirectly affect its contents, and hence the next
    instruction to be executed, using operations which are equivalent
    to BASIC's GOTO and GOSUB instructions.  Occasionally, you will
    see the PC referred to as the *IP*, which stands for instruction
    pointer.

    The last register is also relatively inaccessible.  This is the
    *status* register, ST.  This one has a *two* alternate names, so
    watch for FL (flag register) and PSW (program status word).  The
    latter is somewhat steeped in history, since this was the name
    given to a special location in memory which served a similar
    function on the antique IBM 360 mainframe.

























                                                                   13
    The status register consists of a series of one bit *flags* which
    can affect how the 8088 works.  There are special instructions
    which allow you to set or clear some of the flags.  In addition,
    many instructions affect the state of the flags, depending on
    their outcome.  For example, one of the bits of the status
    register is called the Zero flag.  Any operation which ends up
    generating a bit pattern of all 0's automatically sets the Zero
    flag on.

    Setting the flags doesn't seem to do much, until you know that
    there a whole set of conditional branching instructions which
    cause the equivalent to a BASIC GOTO if the particular flag
    pattern they look for is set.  In assembly language, the only way
    to make a decision and branch accordingly is via this flag
    testing mechanism.

    Although some instructions implicitly affect the flags, there are
    a series of instructions whose *only* effect is to set the flags,
    based on some test or comparison.  It's very common to see one
    of these comparison operations used to set the flags just before
    a conditional branch.  Taken together, the two instructions are
    exactly equivalent to BASIC's:

    IF (comparison) THEN GOTO (linenumber)









































                                                                   14
    >>ASSEMBLY LANGUAGE SYNTAX<<

    In general, each line of an assembly language program translates
    to a set of patterns which specify one fundamental operation for
    the 8088 to carry out.

    Each line may consist of one or more of the following parts:

    First, a label, which is just a marker for the assembler to use.
    If you want to branch to an instruction from some other part of
    the program, you put a label on the instruction.  When you want
    to branch, you refer to the label.  In general, the label can be
    any string of characters you want.  A good practice is to use a
    name which reminds you what that particular part of the program
    does. CHASM will assume that any string of characters which
    starts in the first column of a line is intended to be a label.

    After the label, or if the text of the line starts to the right
    of the first column, at the beginning of the text, comes an
    instruction mnemonic.  This specifies the operation that the line
    is asking for.  For a list of the 200-odd mnemonics, along with
    the instructions they stand for, see The 8086 Book.

    Most of the 8088 instructions require that you specify one or
    more *operands*.  The operands are what the operation is to work
    on, and are listed after the instruction mnemonic.

    There are a number of possible operands.  Probably the most
    common are registers, specified by their two letter mnemonics.

    Another operand type is *immediate data*, a pattern of bits to be
    put somewhere or compared or combined with some other pattern.
    Generally immediate data is specified by its nybble code
    representation, marked as such by following it with the letter
    "H".  Most assemblers allow alternate ways to specify immediate
    data which emphasize the pattern's intended use.  CHASM
    recognizes ten different ways of representing immediate data.




























                                                                   15
    A memory location can be used as an operand.  We've seen one way
    to do this, by enclosing its address in brackets.  (You can now
    see why the brackets are needed.  Without them, the assembler
    couldn't distinguish between an address and immediate data.)  If
    you've asked the assembler to set aside a section of memory for
    data (more on this latter), and put a label on the request, you
    can specify that point in memory by using the label.  Finally,
    there are a number of indirect ways to address memory locations,
    which you can read about in The 8086 Book.

    The last major type of operands are labels.  Branching
    instructions require an operand to tell them where to branch
    *to*.  In assembly language, you specify locations which may be
    branched to by putting a label on them.  You can then use the
    label as an operand on branches.

    Often times, the order in which the operands are listed can be
    important.  For example, when moving a pattern from one place to
    another, you need to specify where the pattern is to come from,
    and where it's going.  The convention in general use is that the
    first operand is the *destination* and the second is the
    *source*.  Thus, to move the pattern in the DX register into the
    AX register, you would write:

            MOV AX,DX

    This may take some getting used to, since when reading from left
    to right it seems reasonable to assume that the transfer goes in
    this direction as well.  However, since this convention is pretty
    well entrenched in the assembly language community, CHASM goes
    along with it.

    The last part of an assembly language line is a *comment*.  The
    comment is totally ignored by the assembler, but is *vital* for
    humans who are attempting to understand the program.  Assembly
    language programs tend to be very hard to follow, and so it's
    particularly important to put in lots of comments so that you'll
    remember just what it was you were trying to do with a given
    piece of code.  Professional assembly language programmers put a
    comment on *every* line of code, explaining what it does, plus
    devoting many entire lines for additional explanations.  For an
    example, you should examine the BIOS source listing given in the
    IBM Technical Reference manual.  Over *half* the text consists of
    comments!





















                                                                   16
    Since the assembler ignores the comments, they cost you nothing
    in terms of size or speed of execution in the resulting machine
    language program.  This is in sharp contrast to BASIC, where each
    remark slows your program down and eats up precious memory.

    Generally, a character is set aside to indicate to the assembler
    the beginning of a comment, so that it knows to skip over.  CHASM
    follows a common convention of reserving the semi-colon (;) for
    marking comments.
























































                                                                   17
    >>THE STACK<<

    I've been dropping the name *stack* from time to time.  The stack
    is just a portion of memory which has been temporarily set aside
    to be used in a special way.

    To get a picture of how the stack works, think of the spring
    loaded contraptions you sometimes see holding trays in a
    cafeteria.  As each tray is washed, the busboy puts it on top of
    the stack in the contraption.  Because the thing is spring
    loaded, the whole stack sinks down from the weight of the new
    tray, and the top of the stack ends up always being the same
    height off the floor.  When a customer takes a tray off the
    stack, the next one rises up to take its place.

    In the computer, the stack is used to hold data patterns, which
    are generally being passed from one program or subroutine to
    another.  By putting things on the stack, the receiving routine
    doesn't need to know a particular address to look for the
    information it needs, it just pulls them off the top of the
    stack.

    There is some jargon associated with use of the stack.  Patterns
    are *pushed* onto the stack, and *popped* off.  Accordingly,
    there are a set of PUSH and POP instructions in the 8088's
    repertoire.

    Because you don't need to keep track of where the patterns are
    actually being kept, the stack is often used as a scratch pad
    area, patterns being pushed when the register they're in is
    needed for some other purpose, then popped out when the register
    is free.  It's very common for the first few instructions of a
    subroutine to be a series of pushes to save the patterns which
    are occupying the registers it's about to use.   This is referred
    to as *saving the state* of the registers.  The last thing the
    subroutine will do is pop the patterns back into the registers
    they came from, thus *restoring the state* of the registers.

    Following the analogy of the cafeteria contraption, when you pop
    the stack, the pattern you get is the last one which was pushed.
    When you pop a pattern off, the next-to-last thing pushed
    automatically moves to the top, just as the trays rise up when a
    customer removes one.  Everything comes off the stack in the
    reverse order of which they went on.  Sometimes you'll see the
    phrase "last in, first out" or *LIFO stack*.




















                                                                   18
    Of course, there are no special spring loaded memory locations
    inside the computer.  The stack is implemented using a register
    which keeps track of where the top of the stack is currently
    located.  When you push something, the pointer is moved to the
    next available memory location, and the pattern is put in that
    spot.  When something is popped, it is copied from the location
    pointed at, then the pointer is moved back.  You don't have to
    worry about moving the pointer because it's all done
    automatically with the push and pop instructions.

    The register set aside to hold the pointer is SP, and that's why
    SP shouldn't be monkeyed with.  You'll recall that to form an
    address, two words are needed, an offset and a segment.  The
    segment word for the stack is kept in the SS register, so you
    should leave SS alone as well.  When you run the type of machine
    language program that CHASM produces, DOS will automatically set
    the SP and SS registers to reserve a stack capable of holding 128
    words.















































                                                                   19
    >>SOFTWARE INTERRUPTS<<

    I have been religiously avoiding talking about the various
    individual instructions the 8088 can carry out, because if I
    didn't, this little primer would soon grow into a rather long
    book.  However, there's one very important instruction, which
    when you read about it in The 8088 Book, won't seem particularly
    useful.  This section will discuss the *software interrupt*
    instruction, INT, and why it's so important.

    The 8088 reserves the first 1024 bytes of memory for a series of
    256 *interrupt vectors*.  Each of these two word long interrupt
    vectors is used to store the segment:offset address of a location
    in memory.  When you execute a software interrupt instruction,
    the the 8088 pushes the location of the next instruction of your
    program onto the stack, then branches to the memory location
    pointed at by the vector specified in the interrupt.

    This probably seems like a rather awkward way to branch around in
    memory, and chances are you'd never use this method to get from
    one part of your program to another.  The way these instructions
    become important is that IBM has pre-loaded a whole series of
    useful little (and not so little) machine language routines into
    your computer, and set the interrupt vectors to point to them.
    All of these routines are set up so that after doing their thing,
    they use the location pushed on the stack by the interrupt
    instruction to branch back to your program.

    Some of these routines are a part of DOS, and documentation for
    them can be found in the DOS Technical Reference manual.  The
    rest of them are stored in ROM (read only memory) and comprise
    the *BIOS*, or basic input/output system of the computer.
    Details of the BIOS routines can be found in the Hardware
    Technical Reference Manual.  DOS and BIOS interrupt data is also
    available in Peter Norton's book "Programmer's Guide to the IBM
    PC".

    The routines do all kinds of useful things, such as run the disk
    drive for you, print characters on the screen, or read data from
    the keyboard.  In effect, the software interrupts add a whole
    series of very powerful operations to the 8088 instruction set.
























                                                                   20
    A final point is that if you don't like the way that DOS or the
    BIOS does something, the vectored interrupt system makes it very
    easy to substitute your own program to handle that function.  You
    just load your program and reset the appropriate interrupt vector
    to point at your program rather than the resident routine.  This
    is how all those RAM disk and print spooler programs work.  The
    programs change the vector for disk drive or printer support to
    point to themselves, and carry out the operations in their own
    special way.

    To make things easy for you, one of the DOS interrupt routines
    has the function of resetting interrupt vectors to point at new
    code.  Still another DOS interrupt routine is used to graft new
    code onto DOS, so that it doesn't accidentally get wiped out by
    other programs.  The whole thing is really quite elegant and easy
    to use, and IBM is to be complimented for setting things up this
    way.
















































                                                                   21
    >>PSEUDO-OPERATIONS<<

    Up to this point, I've implied that each line of an assembly
    language program gets translated into a machine language
    instruction.  In fact, this is not the case.  Most assemblers
    recognize a series of *pseudo-operations* which are handled as
    embedded commands to the assembler itself, not as an instruction
    in the machine language program being built.  Almost invariably
    you'll see the phrase "pseudo-operation" abbreviated down to
    *pseudo-op*. Sometimes you'll see *assembler directive*, which
    means the same thing, but just doesn't seem to roll off the
    tongue as well.

    One very common pseudo-op is the *equate*, usually given mnemonic
    *EQU*.  What this allows you to do is assign a name to a
    frequently used constant.  Thereafter, anywhere you use that
    name, the assembler automatically substitutes the constant.  This
    process makes your program easier to read, since in place of the
    somewhat meaningless looking pattern, you see a name which tells
    you what the pattern is for.  It also makes your program easier
    to modify, since if you decide to change the constant, you only
    need to do it once, rather than all over the program.

    The only other type of pseudo-op I'll talk about here are those
    for setting aside memory locations for data.  These pseudo-ops
    tend to be quite idiosyncratic with each assembler.  CHASM
    implements three such pseudo-ops: DB (declare bytes), DW (declare
    words) and DS (declare storage).  DB is used to set aside small
    data areas, which can be initialized to any pattern, one byte at
    a time.  DW is an analogous pseudo-op for setting asides whole
    words of memory, one at a time.  DS sets up relatively large
    areas, but all the locations are filled with the same initial
    pattern.

    If you put a label on a pseudo-op which sets aside data areas,
    most assemblers allow you to use the label as an operand, in
    place of the actual address of the location.  The assembler
    automatically substitutes the address for the name during the
    translation process.

    Some assemblers have a great number of pseudo-ops.  CHASM
    implements a few more, which aren't discussed here.























                                                                   22
    >>TUTORIAL<<

    To conclude this primer, this section will walk through the
    process of writing, assembling, and running a very simple
    program.

    Our program will just print a message on the video screen, and
    then return to DOS.  Although very simple, this program will
    demonstrate a number of points, including a DOS function call,
    setting aside memory for storage, and good programming form.

    The DOS technical reference manual (or Norton's book) discusses
    the various DOS functions and interrupts available to the
    assembly language programmer.   To print a text string to the
    video screen, we'll use function 9.  You should read the
    documentation for this function at this time.


    Did it make any sense?  To use this function, we have to load the
    DX register with the address of a string in memory, specify
    function 9 by loading a "9" into the AH register, then ask DOS to
    do the printing by executing interrupt 21H.  Basically, we just
    set things up, and DOS does all the real work.

    Here's the code to do this:

         MOV AH, 9                 ;specify DOS function 9
         MOV DX, OFFSET(MESSAGE)   ;get address of string
         INT 21H                   ;call DOS

    Note that none of the lines starts at the left margin  (column
    one). If they did, CHASM would think that the instruction
    mnemonic was meant to be a label, and would get very confused.
    Also note that each line has a comment explaining what's going
    on.

    The second line needs a little explaining.  CHASM's OFFSET
    function returns the address of whatever is included in the
    parentheses, in this case, MESSAGE.  The assumption made here is
    that later in the program we will set aside a memory location
    containing our string to be printed, and give it the name
    "MESSAGE".























                                                                   23
    Once we've printed our string, we want to return to DOS.  If we
    don't explicitly transfer control back to DOS, the 8088 will
    happily continue to execute whatever random patterns are in
    memory after our stuff.  Remember "crashing the system"?  One of
    DOS's reserved interrupts handles program termination, returning
    you to DOS.  The proper instruction is:

           INT 20H        ;return to DOS

    All that's left at this point is to set aside a chunk of memory
    containing a string to be printed.  We'll use CHASM's DB (Declare
    Bytes) pseudo-op to do this:

    MESSAGE DB  'Hello, World!$'     ;message to be printed

    The memory location is given the name "MESSAGE" because the line
    started with a "MESSAGE" as a label.  Now CHASM will know that
    the preceding OFFSET was talking about this memory location.  You
    don't need to worry about what the actual address of MESSAGE is,
    CHASM takes care of that.

    Fourteen bytes of memory get set aside here, containing the ASCII
    codes for the characters in "Hello, World!$".  Note that the
    string ends in the character "$".  DOS function 9 prints
    characters until it encounters a "$", at which point it stops.
    If you forget to put the "$" at the end of your string, you'll
    have the less-than-amusing experience of watching DOS attempt to
    print out the entire contents of memory.





































                                                                   24
    Bringing everything together, and adding a few comments at the
    beginning, here's our complete program:

    ;=====================================;
    ; HELLO    Version 1.00               ;
    ;          1984 by David Whitman      ;
    ;                                     ;
    ; Sample source file for CHASM.       ;
    ; Prints a greeting on the console.   ;
    ;=====================================;

            MOV AH, 9                 ;specify DOS function 9
            MOV DX, OFFSET(MESSAGE)   ;get address of string
            INT 21H                   ;call DOS

            INT 20H                   ;return to DOS

    MESSAGE DB  'Hello, World!$'      ;message to be printed

    After writing all this, we need to create a text file which
    contains the lines of our program.  You do this with a text
    editor or word processor.  (Of course, in real life you write the
    program using the editor in the first place.)

    CHASM likes its source files in "standard DOS" format, what some
    word processors call "document" or "ASCII mode".  Most word
    processors, and all straight text editors work in this format
    automatically.  Wordstar and Easywriter (and probably a few other
    packages) have their own special formats, but their manual should
    tell you how to make standard DOS files.

    At this point, make a standard DOS file named HELLO.ASM which
    contains the above program lines.  If you're feeling lazy, or if
    you run into problems, the file EXAMPLE.ASM on your CHASM disk
    has these lines already entered for you.  Just copy EXAMPLE.ASM
    into a new file called HELLO.ASM and you're in business.

    It's now time to assemble the program.  If it's not already
    there, copy HELLO.ASM onto your CHASM disk. Put the CHASM disk
    into your default drive, and start up CHASM by typing its name:

       A> CHASM

    CHASM will respond by printing a hello screen, and ask you to
    press a key when you're done reading it.  When you do so, CHASM
    will ask you some questions:



















                                                                   25

        Source code file name? [.asm]

    Type in the HELLO.ASM, or just HELLO, then hit return.  (If you
    don't enter the file extension, CHASM assumes that it's .ASM)

        Direct listing to Printer (P), Screen (S), or Disk (D)?

    CHASM wants to know where to send the listing produced during the
    assembly process.  If you have a printer, turn it on, and then
    press P.  If you don't have a printer, press S.

    The last question is:

        Name for object file? [hello.com]

    CHASM is asking for the name you'd like to give to the machine
    language program which is about to be produced.  Just press enter
    here.  CHASM will name the program HELLO.COM

    At this point CHASM will start accessing the disk drive, reading
    in your program a line at a time.  A status line will appear at
    the bottom of your screen, telling you how far along the
    translation has gotten.  CHASM will make two passes over your
    file, outputing the translated program on the second pass.

    If the listing went to your printer, CHASM automatically returns
    you to DOS when finished.  If the listing went to the screen,
    CHASM waits for you to press a key to indicate that you're done
    reading.  Near the bottom of the listing will be the message:

    XXX Diagnostics Offered
    YYY Errors Detected

    If both numbers are 0, everything went fine.  If not, look up on
    the listing for error messages, which will point out the
    offending lines.  At this point, don't worry too much about what
    the error messages say, just fix the line in your input file to
    look like the text developed above.  Once you manage to get an
    assembly with no errors, you're ready to go on.

    Your disk will now contain a machine language program named
    HELLO.COM.  Confirm this by typing DIR to get a directory
    listing.  You should see the new program file listed.





















                                                                   26
    To run the machine language program, you just type its name,
    without the .COM extension.  (Note: even though you don't *enter*
    the "COM", the file has to have this extension for DOS to
    recognize it as a machine language program.)  Try it out now.
    From the DOS prompt, type: HELLO.  Your disk drive will whir for
    a second, then the message "Hello World!" will appear.

    For a further exercise, you might try printing a carriage return
    and then a line feed before the message, to space it down the
    screen a little.  Carriage return has ASCII code 13, and line
    feed is 10.  Read the CHASM User's Manual about the DB pseudo-op,
    and add these two characters at the beginning of STRING using
    their decimal representations.

    Try writing a new program called BEEP which writes the "bell"
    character to the screen.  You can use BEEP to signal the end of
    long batch files, or to annoy your co-workers.  Resist the urge
    to program a loop into BEEP.

    An even more advanced exercise would be to clear the screen
    before printing a message.  The easiest way to do this is to
    use a BIOS function, VIDEO_IO, documented on in the Hardware
    Technical Reference Manual.  The comments in the BIOS listing
    tell you what values to load into which registers to get VIDEO_IO
    to monkey with the screen for you.  Load the registers and
    execute INT 10H, once to blank the screen, and again to move the
    cursor to the upper left hand corner.

    If you've read all of this primer and run the above program,
    maybe modifying it a little, you're no longer a rank beginner.
    At this point you should have enough of a start to be able to
    digest the CHASM User's Manual and The 8086 Book, then begin to
    write your own programs.  Good Luck!