Collection of Hack-Phreak Scene Programs/cleanhpvac.zip/cleanhpvac/A86V402.ZIP/A11.DOC

Text File | 1994-12-21 | 31KB | 824 lines

CHAPTER 11   MACROS AND CONDITIONAL ASSEMBLY


Macro Facility

A86 contains an easy-to-use, but very powerful macro facility.
The facility subsumes the capabilities of most assemblers,
including operand concatenation, repeat, indefinite repeat (often
called IRP), indefinite repeat character (IRPC), passing macro
operands by text or by value, comparing macro operands to
strings, and detecting blank macro operands.  Unlike other
assemblers, A86 integrates these functions into the main macro
facility; so they can be invoked without clumsy syntax, or
strange characters in the macro-call operands.


Simple Macro Syntax

All macros must be defined before they are used.  A macro
definition consists of the name of the macro, followed by the
word MACRO, followed by the text of the macro, followed by #EM,
which marks the end of the macro.

Many assembly languages require a list of dummy operand names to
follow the word MACRO.  A86 does not: the operands are denoted in
the text with the fixed names #1, #2, #3, ... up to a limit of
#9, for each operand in order.  If there is anything following
the word MACRO, it is considered part of the macro text.

Examples:

; CLEAR sets the register operand to zero.

CLEAR  MACRO  SUB #1,#1 #EM

  CLEAR AX      ; generates a SUB AX,AX instruction
  CLEAR BX      ; generates a SUB BX,BX instruction


; MOVM moves the second operand to the first operand.
;   Both operands can be memory variables.

MOVM  MACRO
  MOV AL,#2
  MOV #1,AL
#EM

VAR1  DB  ?
VAR2  DB  ?

MOVM VAR1,VAR2  ; generates MOV AL,VAR2 followed by MOV VAR1,AL
                                                             11-2

Formatting in Macro Definitions and Calls

The format of a macro definition is flexible.  If the macro text
consists of a single instruction, the definition can be given in
a single line, as in the CLEAR macro given above.  There is no
particular advantage to doing this, however: A86 prunes all
unnecessary spaces, blank lines, and comments from the macro text
before entering the text into the symbol table.  I recommend the
more spread-out format of the MOVM macro, for program
readability.

All special macro operators within a macro definition begin with
a hash sign # (a hex 23 byte).  The letters following the hash
sign can be given in either upper case or lower case.  Hash-sign
operators are recognized even within quoted strings.  If you wish
the hash sign to be treated literally, and not as the start of a
special macro operator, you must give 2 consecutive hash signs:
##.  For example:

FOO MACRO
  DB '##1'
  DB '#1'
#em

FOO abc      ; produces  DB '#1'  followed by  DB 'abc'

The format of the macro call line is also flexible.  A macro call
consists of the name of the macro, followed by the operands to be
plugged into the macro.  A86 prunes leading and trailing blanks
from the operands of a macro call.  The operands to a macro call
are always separated by commas.  Also, as in all A86 source
lines, a semi-colon occurring outside of a quoted string is the
start of a comment, ignored by A86.  If you want to include
commas, blanks, or semi-colons in your operands, you must enclose
your operand in single quotes.



Macro Operand Substitution

Some macro assemblers expect the operands to macro calls to
follow the same syntax as the operands to instructions.  In those
assemblers, the operands are parsed, and reduced to numeric
values before being plugged into the macro definition text.  This
is called "passing by value".  As its default, A86 does not pass
by value, it passes by text.  The only parsing of operands done
by the macro processor is to determine the start and the finish
of the operand text.  That text is substituted, without regard
for its contents, for the "#n" that appears in the macro
definition.  The text is interpreted by A86 only after a complete
line is expanded and as it is assembled.
                                                             11-3

If the first non-blank character after the macro name is a comma,
then the first operand is null: any occurrences of #1 in the
macro text will be deleted, and replaced with nothing.  Likewise,
any two consecutive commas with no non-blanks between them will
result in the corresponding null operand.  Also, out-of-range
operands are null; for example, #3 is a null operand if only two
operands are provided in the call.

Null operands to macros are not in themselves illegal.  They will
produce errors only if the resulting macro expansion is illegal.

The method of passing by text allows operand text to be plugged
anywhere into a macro, even within symbol names.  For example:

; KF_ENTRY creates an entry in the KFUNCS table, consisting of a
;   pointer to a KF_ action routine.  It also declares the
;   corresponding CF_ symbol, which is the index within the table
;   for that entry.

KF_ENTRY  MACRO
  CF_#1  EQU  ($-KFUNCS)/2+080
  DW  KF_#1
#EM

KFUNCS:
  KF_ENTRY  UP
  KF_ENTRY  DOWN

; The above code is equivalent to:
;
;  KFUNCS:
;    DW KF_UP
;    DW KF_DOWN
;
;  CF_UP    EQU  080
;  CF_DOWN  EQU  081



Quoted String Operands

As mentioned before, if you want to include blanks, commas, or
semicolons in your operands, you enclose the operand in single
quotes.  In the vast majority of cases in which these special
characters need to be part of operands, the user wants them to be
quoted in the final, assembled line also.  Therefore, the quotes
are passed in the operand.  To override this, and strip the
quotes from the string, you precede the quoted string with a hash
sign.  Examples:
                                                             11-4

DBW  MACRO
  DB  #1
  DW  #2
#EM

DBW  'E', E_POINTER
DBW  'W', W_POINTER

; note that if quotes were not passed, the above lines would have
;   to be   DBW  '''E''', E_POINTER;    DBW  '''W''', W_POINTER

FETCH_CHAR  MACRO
  LODSB
  #1
  CALL PROCESS_CHAR
#EM

FETCH_CHAR  STOSB      ; generates STOSB as second instruction
FETCH_CHAR  #'INC DI'  ; generates INC DI as second instruction



Looping by Operands in Macros

A86's macro facility contains two kinds of loops: you can loop
once for each operand in a range of operands; or you can loop
once for each character within an operand.  The first kind of
loop, the R-loop, is discussed in this section; the second kind,
the C-loop, is discussed later.

An R-loop is a stretch of macro-definition code that is repeated
when the macro is expanded.  In addition to the fixed operands #1
through #9, you can specify a variable operand, whose number
changes each time through the loop.  You give the variable
operand one of the 4 names #W, #X, #Y, or #Z.

An R-loop begins with #R, followed immediately by the letter
W,X,Y, or Z naming the variable, followed by the number of the
first operand to be used, followed by the number of the last
operand to be used.  After the #Rxnn is the text to be repeated.
The R-loop ends with #ER.  For example:

STORE3 MACRO
  MOV AX,#1
#RY24               ; "repeat for Y running from 2 through 4"
  MOV #Y,AX
#ER
#EM

STORE3  VAR1,VAR2,VAR3,VAR4

; the above call produces the 4 instructions MOV AX,VAR1; MOV VAR2,AX;
;   MOV VAR3,AX; MOV VAR4,AX.
                                                             11-5

The #L Last Operator and Indefinite Repeats

A86 recognizes the special operator #L, which is the last operand
in a macro call.  #L can appear anywhere in macro text; but its
big power occurs in conjunction with R-loops, to yield an
indefinite-repeat facility.

A common example is as follows: you can take any macro that is
designed for one operand, and easily convert it into a macro that
accepts any number of operands.  You do this by placing the
command #RX1L, "repeat for X running from 1 through L", at the
start of the macro, and the command #ER at the end just before
the #EM.  Finally, you replace all instances of #1 in the macro
with #X.  We see how this works with the CLEAR macro:

CLEAR MACRO #RX1L
  SUB #X,#X
#ER
#EM

CLEAR AX,BX  ; generates both SUB AX,AX and SUB BX,BX in one macro!

It is possible for R-loops to iterate zero times.  In this case,
the loop-text is skipped completely.  For example, CLEAR without
any operands would produce no expanded text.


Character Loops

We have seen the R-loop; now we discuss the other kind of loop in
macros, the character loop, or C-loop.  In the C-loop, the
variable W,X,Y, or Z does not represent an entire operand; it
represents a character within an operand.

You start a C-loop with #C, followed by one of the 4 letters
W,X,Y, or Z, followed by a single operand specifier-- a digit,
the letter L, another one of W,X, Y, or Z defined in an outer
loop, or one of the more complicated specifiers defined later in
this chapter.  Following the #Cxn is the text of the C-loop.  The
C-loop ends with #EC. The macro will loop once for every
character in the operand. That single character will be
substituted for each instance of the indicated variable operand.
For example:

PUSHC  MACRO  #CW1
  PUSH #WX
#EC#EM

PUSHC ABC        ; generates PUSH AX | PUSH BX | PUSH CX

If the C-operand is quoted in the macro call, the quotes ARE
removed from the operand before passing characters to the loop.
It is not necessary to precede the quoted string with a hash sign
in this case.  If you do, the hash sign will be passed as the
first character.
                                                             11-6

If the C-operand is a null operand (no characters in it), the
loop text is skipped completely.


The "B"-Before and "A"-After Operators

So far, we have seen that you can specify operands in your macro
in fourteen different ways: 1,2,3,4,5,6,7,8,9,W,X,Y,Z,L.  We now
multiply these 14 possibilities, by introducing the "A" and "B"
operators.  You can precede any of the 14 specifiers with "A" or
"B", to get the adjacent operand after or before the specified
operand.  For example, BL means the operand just before the last
operand; in other words, the second-to-the-last operand.  AZ
means the operand just after the Z operand.  You can even repeat,
up to a limit of 4 "B"s or 3 "A"s:  for example, BBL is the
third-to-last operand.

Note that any operand specifier can appear in contexts other than
by itself following a # within a macro.  For example, BBL could
appear as the upper limit to an R-loop: #RZ1BBL loops with Z
running from the first operand to the third-to-last operand.

In the case of the variable operand to a C-loop, the "A" and "B"
specifiers denote the characters before or after the current
looping-character.  An example of this is given in the next
section.


Multiple Increments within Loops

We have seen that you end an R-loop with a #ER, and you end a
C-loop with a #EC.  We now present another way to end these
loops; a way that lets you specify a larger increment to the
macro's loop counter.  You can end your loops with one of the 4
additional commands #E1, #E2, #E3, or #E4.

For R-loops terminated by #ER, the variable operand advances to
the next operand when the loop is made.  If you end your R-loop
with #E2, the variable operand advances 2 operands, not just one.
For #E3, it advances 3 operands; for #E4, 4 operands.  The #E1
command is the same as #ER.

The most common usage of this feature is as follows:  You will
recall that we generalized the CLEAR macro with the #L-variable,
so that it would take an indefinite number of operands.  Suppose
we want to do the same thing with the DBW macro.  We would like
DBW to take any number of operands, and alternate DBs and DWs
indefinitely on the operands.  This is made possible by creating
an R-loop terminated by #E2:

DBW  MACRO  #RX1L
  DB  #X
  DW  #AX
#E2
#EM

DBW  'E',E_POINTER,  'W',W_POINTER   ; two pairs on same line!
                                                             11-7

The #E2 terminator means that we are looping on a pair of
operands.  Note the crucial usage of the "A"-after operator to
specify the second operand of the operand pair.

A special note applies to the DBW macro above: A86 just happens
to accept a DW directive with no operands (it generates no object
code, and issues no error).  This means that DBW will accept an
odd number of operands with no error, and do the expected thing
(it alternates bytes and words, ending with a byte).

You could likewise generalize a macro with 3 or 4 operands, to an
indefinite number of triples or quadruples; by ending the R-loop
with #E3 or #E4.  The operands in each group would be specified
by #X, #AX, #AAX, and, for #E4, #AAAX.

For C-loops terminated by #E1 through #E4, the character pointer
is advanced the specified number of characters.  You use this in
much the same way as for R-loops, to create loops on pairs,
triplets, and quadruplets of characters.  For example:

PUSHC2  MACRO  #CZ1
  PUSH #Z#AZ
#E2
#EM

PUSHC2  AXBXSIDI  ; generates PUSH AX | PUSH BX | PUSH SI | PUSH DI


Negative R-loops

We now introduce another form of R-loop, called the Q-loop-- the
negative repeat loop.  This loop is the same as the R-loop,
except that the operand number decrements instead of increments;
and the loop exits when the number goes below the finish-number,
not above it.  The Q-loop is specified by #Qxnn instead of #Rxnn,
and #EQ instead of #ER.  You can also use the multiple-decrement
forms #E1 #E2 #E3 or #E4 to terminate an Q-loop.

Example:

MOVN MACRO #QXL2   ; "negative repeat X from L down to 2"
  MOV #BX,#X
#EQ#EM

MOVN AX,BX,CX,DX   ; generates the three instructions:
                   ;    MOV CX,DX
                   ;    MOV BX,CX
                   ;    MOV AX,BX

Note: the above functionality is already built into the MOV
instruction of A86.  The macro shows how you would implement it
if you did not already have this facility.
                                                             11-8

Nesting of Loops in Macros

A86 allows nesting of loops within each other. Since we provide
the 4 identifiers W,X,Y,Z for the loop operands, you can nest to
a level of 4 without restriction-- just use a different letter
for each nesting level.  You can nest even deeper, for example,
by having two nested R-loops that use W is its indexing letter.
The only restriction to this is that you cannot refer to the W of
the outer loop from within the inner W loop.  (I challenge anyone
to come up with an application in which these limitations /
restrictions cause a genuine inconvenience!)


Implied Closing of Loops

If you have a loop or loops ending when the macro ends, and if
the iteration count for those loops is 1, you may omit the #ER,
#EC, or #EQ.  A86 closes all open loops when it sees #EM, with no
error.

For example, if you omit the #ER for the loop version of the
CLEAR macro, it would make no difference-- A86 automatically
places an #ER code into the macro definition for you.


Passing Operands by Value

As already stated, A86's default mode for passing operands is by
text-- the characters of the operand are copied to the macro
expansion line as-is, without any evaluation.  You may override
this with the #V operator.  When A86 sees #Vn in a macro
definition, it will evaluate the expression given in the text of
operand n, and pass a string representing the decimal constant
answer, instead of the original text.  The operand must evaluate
to an absolute constant value, less than 65536.  For example:

JLV MACRO
  J#1 LABEL#V2
#EM

JINDEX = 3
JLV NC,JINDEX+1     ; generates JNC LABEL4
JINDEX = 6
JLV Z,JINDEX+2      ; generates JZ LABEL8


Passing Operand Size

The construct #Sn is translated by A86 into the decimal string
representing the number of characters in operand n.  One use of
this would be to make a conditional-assembly test of whether an
operand was passed at all, as we'll see later in this chapter.
Another use is to generate a length byte preceding a string, as
required by some high-level languages such as Turbo Pascal.
Example:
                                                             11-9

LSTRING MACRO
  DB #S1,'#1'
#EM

LSTRING  SAMPLE     ; generates  DB 6,'SAMPLE'


Generating the Number of an Operand

The construct #Nn is translated by A86 into the decimal string
represented by the position number n of the macro operand.  Note
that this value does not depend on the contents of the operand
that was passed to the macro.  Thus, for example, #N2 would
translate simply to 2; so this usage of #N is silly.  #N achieves
usefulness when n is variable: W,X,Y,Z, or L.  I give an example
of #N with a loop-control variable in the next section. Here is
an example of #NL, used to generate an array of strings, preceded
by a byte telling how many strings are in the array:

ZSTRINGS MACRO
  DB #NL           ; generates the number of operands passed
#RX1L
  DB '#X',0
#EM

ZSTRINGS  TOM,DICK,HARRY   ; generates DB 3 followed by strings


Parenthesized Operand Numbers

We've seen that macro operands are usually specified in your
macro definition by a single character: either a single digit or
one of the special letters W,X,Y,Z, or L.  A86 also allows you to
specify a constant operand number up to 255.  You do so by giving
an expression enclosed in parentheses, rather than a single
character.  The expression must evaluate at the time the macro is
defined, to a constant between 0 and 255.  You can use this
feature to translate many programs that use MASM's REPT
directive.  For example, if the following REPT construct occurs
within a MASM macro:

  TEMP = 0
REPT 100
  TEMP = TEMP + 1     ; MASM needs an explicitly-set-up counter
  DB TEMP
ENDM

you may translate it into an A86 loop, as follows:

#RX1(100)      ; the counter X is built into the A86 loop
  DB #NX
#ER

If the REPT does not occur within a macro, you must define a
macro containing the loop, which you may then immediately call.
                                                            11-10

Note that the expression enclosed in parentheses must not itself
contain any macro operators.  Thus, for example, you cannot
specify #(#NY+1) to represent the operand after Y-- you must use
#AY.


Exiting from the Middle of a Macro

For MASM compatibility, A86 offers the #EX operator, which is
equivalent to MASM's EXITM directive.  #EX is typically used in a
conditional-assembly block within a loop, to terminate the loop
early.  When the #EX code is seen in a macro expansion, the
expansion ceases at that point, and assembly returns to the
source file (or to the outer macro in a nested call).  You
couldn't use #EM to do this, because that would signal the end of
the macro definition, not just the call.


Local Labels in Macros

Some assemblers have a LOCAL pseudo-op that is used in
conjunction with macros.  Symbols declared LOCAL to a macro have
unique (and bizarre) symbol names substituted for them each time
the macro is called.  This solves the problem of duplicate label
definitions when a macro is called more than once.

In A86, the problem is solved more elegantly, by having a class
of generic local labels throughout assembly, not just in macros.
Recall that symbols consisting of a single letter, followed by
one or more decimal digits, can be redefined.  You can use such
labels in your macro definitions.

I have recommended that local labels outside of macros be
designated L1 through L9.  Within macro definitions, I suggest
that you use labels M1 through M9.  If you used an Ln-label
within a macro, you would have to make sure that you never call
the macro within the range of definition of another Ln-label with
the same name.  By using Mn-labels, you avoid such potential
conflicts.

The following example of a local label within a macro is taken
from the source of the macro processor itself:

; "JHASH label" checks to see if AL is a hash sign.  If it is,
;    it processes the hash sign term, and jumps to label.
;    Otherwise, it drops through to the following code.

JHASH MACRO
  CMP AL,'##'       ; is the scanned character a hash sign?
  JNE >M1           ; skip if not
  CALL MDEF_HASH    ; process the hash sign
  JMP #1            ; jump to the label provided
M1:
  #EM
                                                            11-11

  ...
L3:                 ; loop here to eat empty lines, leading blanks
  CALL SKIP_BLANKS  ; skip over the leading blanks of a line
  INC SI            ; advance source ptr beyond the next non-blank
  JHASH L3          ; if hash sign then process, and eat more blanks
  CMP AL,0A         ; were the blanks terminated by a linefeed?
  JE L3             ; loop if yes, nothing on this line
L5:                 ; loop here after a line is seen to have contents
  CMP AL,';'        ; have we reached the start of a comment?
  JE L1             ; jump if yes, to consume the comment
  JHASH >L6         ; if hash sign then process it; get next char
  ...
L6:
  LODSB             ; fetch the next definition char from the source
  CMP AL,' '        ; is it blank?
  JA L5             ; loop if not, to process it
  ...


Debugging Macro Expansions

There are two ways that A86 lets you debug your macro expansions.
First, if A86 encounters an error within a macro expansion, it
now includes the offending expansion line within the error
message.  This will often allow you to spot the problem.  If you
need a complete listing of the expanded macro, the A86 listing
will now give you that.  These facilities replace the old EXMAC
tool, which sometimes failed to expand excessively complicated
macros the way the assembler did.


Conditional Assembly

A86 has a conditional-assembly feature which allows you to
specify that blocks of source code will or will not be assembled,
according to the values of equated user symbols.  The controlling
symbols can be declared in the program (and can thus be the
result of assembly-time expressions), or they can be declared in
the assembler invocation.

You should keep in mind the difference between conditional
assembly, invoked by #IF, and the structured-programming feature,
invoked by IF without the hash sign.  #IF tests a condition at
assembly time, and can cause code to not be assembled and thus
not appear in the program.  IF causes code to be assembled that
tests a condition at run time, possibly jumping over code.  The
skipped code will always appear in the program.

All conditional-assembly lines are identified by a hash sign # as
the first non-blank character of a line.  The hash sign is
followed by one of the four reserved symbols IF, ELSEIF, ELSE or
ENDIF.
                                                            11-12

#IF starts a conditional-assembly block.  On the same line,
following the #IF, you provide either a single name, or an
arbitrary expression evaluating to an absolute constant.  In this
context, a single name evaluates to TRUE if it is defined and not
equal to the absolute constant zero.  A name is FALSE if it is
undefined, or if it has been equated to zero.  An expression is
TRUE if nonzero, FALSE if zero.

If the #IF expression evaluates to FALSE, then the following
lines of code are skipped, up to the next matching #ELSEIF,
#ELSE, or #ENDIF.  If the expression is TRUE, then the following
lines of code are assembled normally.  If a subsequent matching
#ELSEIF or #ELSE is encountered, then code is skipped up to the
matching #ENDIF.

#ELSEIF provides a multiple-choice facility for #IF-blocks.  You
can give any number of #ELSEIFs between an #IF and its matching
#ENDIF.  Each #ELSEIF has a name or expression following it on
the same line. If the construct following the #IF is FALSE, then
the assembler looks for the first TRUE construct following an
#ELSEIF, and assembles that block of code.  If there are no TRUE
#ELSEIFs, then the #ELSE-block (if there is one) is assembled.

You should use the ! instead of the NOT operator in
conditional-assembly expressions.  The ! operator performs the
correct translation of names into TRUE or FALSE values, and
handles the case !undefined without reporting an error.

#ELSE marks the beginning of code to be assembled if all the
previous blocks of an #IF have been skipped over.  There is no
operand after the #ELSE.  There can be at most one #ELSE in an
#IF-block, and it must appear after any #ELSEIFs.

#ENDIF marks the end of an #IF-block.  There is no operand after
#ENDIF.

It is legal to have nested #IF-blocks; that is, #IF-blocks that
are contained within other #IF-blocks.  #ELSEIF, #ELSE, and
#ENDIF always refer to the innermost nested #IF-block.

As an example of conditional assembly, suppose that you have a
program that comes in three versions: one for Texas, one for
Oklahoma, and one for the rest of the nation.  The three programs
differ in a limited number of places.  Instead of keeping three
different versions of the source code, you can keep one version,
and use conditional assembly on the boolean variables TEXAS and
OKLAHOMA to control the assembler output.  A sample block would
be:

#if TEXAS
  DB 0,1,2,3
#elseif OKLAHOMA
  DB 4,5,6,7
#else
  DB 8,9,10,11
#endif
                                                            11-13

If a block of code is to be assembled only if TEXAS is false,
then you would use the exclamation-point operator:

#if !TEXAS
  DB 0FF
#endif


Conditional Assembly and Macros

You may have conditional-assembly blocks either in macro
definitions or in macro expansions.  The only limitation is that
if you have an #IF-block in a macro expansion, the entire block
(i.e., the matching #ENDIF) must appear in the same macro
expansion.  You cannot, for example, define a macro that is a
synonym for #IF.

To have your conditional-assembly block apply to the macro
definition, you provide the block normally within the definition.
For example:

X1 EQU 0
  BAZ MACRO
#if X1
  DB 010
#else
  DB 011
#endif
#EM
  BAZ
X1 EQU 1
  BAZ

In the above sequence of code, the conditional-assembly block is
acted upon when the macro BAZ is defined.  The macro therefore
consists of the single line DB 011, with all the
conditional-assembly lines removed from the definition.  Thus,
both expansions of BAZ produce the object-code byte of 011, even
though the local label X1 has turned non-zero for the second
invocation.

To have your conditional-assembly block appear in the macro
expansion, you must literalize the hash sign on each
conditional-assembly line by giving two hash signs:

X1 EQU 0
  BAZ MACRO
##if X1
  DB 010
##else
  DB 011
##endif
#EM
  BAZ
X1 EQU 1
  BAZ
                                                            11-14

Now the entire conditional-assembly block is stored in the macro
definition, and acted upon each time the macro is expanded. Thus,
the two invocations of BAZ will produce the different object
bytes 011 and 010, since X1 has become non-zero for the second
expansion.

You will usually want your conditional-assembly blocks to be
acted upon at macro definition time, to save symbol table space.
You will thus use the first form, with the single hash signs.


Simulating MASM's Conditional Assembly Constructs

Microsoft's MASM assembler has an abundance of confusing
conditional-assembly directives, all of which are subsumed by
A86's #IF expression evaluation policies.  IFDEF is covered by
A86's #IF directive in conjunction with its DEF operator.  IFE
and IFNDEF are duplicated by #IF followed by the
exclamation-point (boolean negation) operator, followed by a DEF
operator.  IFB and IFNB test whether a macro operand has been
passed as blank -- they can be simulated by testing the size of
the operand with the #Sn operator.  Finally, IFIDN and IFDIF do
string comparisons of macro operands.  This is more generally
subsumed by the string-comparison capabilities of the operators
EQ, NE, and =.

Examples of translation of each of these constructs is given in
the next chapter, on compatibility with other assemblers.


Declaring Variables in the Assembler Invocation

To facilitate the effective use of conditional assembly, A86
allows you to declare boolean (true-false) symbols in the command
line that invokes the assembler.  The declarations can appear
anywhere in the list of source file names.  They are
distinguished from the file names by a leading equals sign =.  To
declare a symbol TRUE (value = 1), give the name after the equals
sign.  DO NOT put any spaces between the equals sign and the
name! To declare a symbol FALSE (value = 0), you can give an
equals sign, an exclamation point, then the name.  Again, DO NOT
embed any blanks!  Example: if your source files are src1.8,
src2.8, and src3.8, then you can assemble with TEXAS true by
invoking A86 as follows:

   a86 =TEXAS src1.8 src2.8 src3.8

You can assemble with TEXAS explicitly set to FALSE as follows:

   a86 =!TEXAS src1.8 src2.8 src3.8

Note that if TEXAS is used only as a conditional-assembly
control, then you do not need to include the =!TEXAS in the
invocation, because an undefined TEXAS will automatically be
interpreted as false.
                                                            11-15

A user pointed out to me that it's impossible to get an
equals-sign into an environment variable.  So A86 now accepts an
up-arrow (hex 5E) character in place of an equals-sign for an
invocation variable.


Null Invocation Variable Names

A86 will ignore an equals-sign by itself in the invocation line,
without error.  This allows you to generate assembler-invocation
lines using parameters that could be either boolean variable
names, or null strings. For example, in the previously-mentioned
TEXAS-OKLAHOMA-nation example, the program could be invoked via a
.BAT file called "AMAKE.BAT", coded as follows:

      A86 =%1 *.8

You invoke A86 by typing one of the following:

      amake texas
      amake oklahoma
      amake

The third line will produce the assembler invocation  A86 = *.8;
causing no invocation variables to be declared.  Thus both TEXAS
and OKLAHOMA will be false, which is exactly what you want for
the rest-of-the-nation version of the program.


Changing Values of Invocation Variables

The usual prohibition against changing the value of a symbol that
is not a local label does not apply to invocation variables.  For
example, suppose you have a conditional-control variable DEBUG,
which will generate diagnostic code for debugging when it is
true.  Suppose further that you have already debugged source
files src1.8 and src3.8; but you are still working on src2.8. You
may invoke A86 as follows:

   A86 src1.8 =DEBUG src2.8 =!DEBUG src3.8

The variable DEBUG will be TRUE only during assembly of src2.8,
just as you want.