CHAPTER 9   DIRECTIVES IN A86


Segments in A86

The following discussion applies when A86 is assembling a .COM
See the next chapter for the discussion of segmentation for .OBJ
files.

A86 views the 86 computer's memory space as having two parts: The
first part is the program, whose contents are the object bytes
generated by A86 during its assembly of the source.   A86 calls
this area the CODE SEGMENT.  The second part is the data area,
whose contents are generated by the program after it starts
running.  A86 calls this area the DATA SEGMENT.

Please note well that the only difference between the CODE and
DATA segments is whether the contents are generated by the
program or the assembler.  The names CODE and DATA suggest that
program code is placed in the CODE segment, and data structures
go in the DATA segment.  This is mostly true, but there are
exceptions.  For example, there are many data structures whose
contents are determined by the assembler: pointer tables, arrays
of pre-defined constants, etc.  These tables are assembled in the
CODE segment.

In general, you will want to begin your program with the
directive DATA SEGMENT, followed by an ORG statement giving the
address of the start of your data area.  You then list all your
program variables and uninitialized data structures, using the
directives DB, DW, and STRUC.  A86 will allocate space starting
at the address given in the ORG statement, but it will not
generate any object bytes in that space.  After your data segment
declarations, you provide a CODE SEGMENT directive.  If the
program starts at any location other than the standard 0100, you
give an ORG giving the address of the start of your program.  You
follow this with the program itself, together with any
assembler-generated data structures.  A short program
illustrating this suggested usage follows:

DATA SEGMENT
ORG 08000
  ANSWER_BYTE  DB ?
  CALL_COUNT   DW ?

CODE SEGMENT
  JMP MAIN

TRAN_TABLE:
  DB 16,3,56,23,0,9,12,7

MAIN:
  MOV BX,TRAN_TABLE
  XLATB
  MOV ANSWER_BYTE,AL
  INC CALL_COUNT
  RET
                                                              9-2

A86 allows you to intersperse CODE SEGMENTs and DATA SEGMENTs
throughout your program; but in general it is best to put all
your DATA SEGMENT declarations at the top of your program, to
avoid problems with forward referencing.


CODE ENDS and DATA ENDS Statements

For compatibility with Intel/IBM assemblers, A86 provides the
CODE ENDS and DATA ENDS statements.  The CODE ENDS statement is
ignored; we assume that you have not nested a CODE segment inside
a DATA segment.  The DATA ENDS statement is equivalent to a CODE
SEGMENT statement.



The ORG Directive

Syntax:  ORG address

ORG moves the output pointer (the location counter at which
assembly is currently taking place within the current segment) to
the value of the operand, which should be an absolute constant,
or an expression evaluating to an absolute,
non-forward-referenced constant.

ORG is most often used in a DATA segment, to control the location
of the data area within the segment.  For example, in programs
that fit entirely into 64K, you provide an ORG directive as the
first line within your DATA segment at the top of your program.
The location given by the ORG is some location that you are sure
will be beyond the end of your program.  If you are sure that
your program will not go beyond 8K (02000 hex), your program can
look like this:

DATA SEGMENT
ORG 02000       ; data goes here, beyond the end of the program

(your data segment variable and buffer declarations go here)

DATA ENDS

(your program goes here)
                                                              9-3

There is a special side effect to ORG when it is used in the CODE
segment.  If you begin your code segment with ORG 0, then A86
knows that you are not assembling a .COM program; but are instead
assembling a code segment to be used in some other context
(examples: programming a ROM, or assembling a procedure for older
versions of Turbo Pascal).  The output file will start at 0, not
0100 as in a .COM file; and the default extension for the output
file will be .BIN, not .COM.

Other than in the above example, you should not in general issue
an ORG within the CODE segment that would lower the value of the
output pointer.  This is because you thereby put yourself in
danger of losing part of your assembled program.  If you
re-assemble over space you have already assembled, you will
clobber the previously-assembled code.  Also, be aware that the
size of the output program file is determined by the value of the
code segment output pointer when the program stops.  If you ORG
to a lower value at the end of your program, the output program
file will be truncated to the lower-value address.

Again, almost no program producing a .COM file will need any ORG
directive in the code segment.  There is an implied ORG 0100 at
the start of the program.  You just start coding instructions,
and the assembler will put them in the right place.


The EVEN Directive

Syntax:  EVEN

The EVEN directive coerces the current output pointer to an even
value.  In a DATA SEGMENT or STRUC, it does so by adding 1 to the
pointer if the pointer was odd; doing nothing if the pointer was
already even.  In a code segment, it outputs a NOP if the pointer
was odd.  EVEN is most often used in data segments, before a
sequence of DW directives. The 16-bit machines of the 86 family
fetch words more quickly when they are aligned onto even
addresses; so the EVEN directive insures that your program will
have the faster access to those DW's that follow it.  (This speed
improvement will not be seen on the 8-bit machines, most notably
the 8088 of the original IBM-PC.)


Data Allocation Using DB, DW, DD, DQ, and DT

The 86 computer family supports the three fundamental data types
BYTE, WORD, and DWORD.  A byte is eight bits, a word is 16 bits
(2 bytes), and a doubleword is 32 bits (4 bytes).  In addition,
the 87 floating point processor manipulates 8-byte quantities,
which we call Q-words, and 10-byte quantities, which we call
T-bytes.  The A86 data allocation statement is used to specify
the bytes, words, doublewords, Q-words, and T-bytes which your
program will use as data.  The syntax for the data allocation
statement is as follows:
                                                              9-4

(optional var-name)  DB  (list of values)
(optional var-name)  DW  (list of values)
(optional var-name)  DD  (list of values)
(optional var-name)  DQ  (list of values)
(optional var-name)  DT  (list of values)

The variable name, if present, causes that name to be entered
into the symbol table as a memory variable with type BYTE (for
DB), WORD (for DW), DWORD (for DD), QWORD (for DQ), or TBYTE (for
DT). The variable name should NOT have a colon after it, unless
you wish the name to be a label (instructions referring to it
will interpret the label as the constant pointer to the memory
location, not its contents).

The DB statement is used to reserve bytes of storage; DW is used
to reserve words.  The list of values to the right of the DB or
DW serves two purposes.  It specifies how many bytes or words are
allocated by the statement, as well as what their initial values
should be.  The list of values may contain a single value or more
than one, separated by commas.  The list can even be missing;
meaning that we wish to define a byte or word variable at the
same location as the next variable.

If the data initialization is in the DATA segment, the values
given are ignored, except as place markers to reserve the
appropriate number of units of storage.  The use of "?", which in
.COM mode is a synonym for zero, is recommended in this context
to emphasize the lack of actual memory initialization. When A86
is assembling .OBJ files, the ?-initialization will cause a break
in the segment (unless ? is embedded in a nested DUP containing
non-? terms, in which case it is a synonym for zero).

A special value which can be used in data initializations is the
DUP construct, which allows the allocation and/or initialization
of blocks of data.  The expression  n DUP x  is equivalent to a
list with x repeated n times.  "x" can be either a single value,
a list of values, or another DUP construct nested inside the
first one.  The nested DUP construct needs to be surrounded by
parentheses.  All other assemblers, and earlier versions of A86,
require parentheses around all right operands to DUP, even simple
ones; but this requirement has been removed for simple operands
in the current A86.

Here are some examples of data initialization statements, with
and without DUP constructs:

CODE SEGMENT
  DW 5                  ; allocate one word, init. to 5
  DB 0,3,0              ; allocate three bytes, init. to 0,3,0
  DB 5 DUP 0            ; equivalent to DB 0,0,0,0,0
  DW 2 DUP (0,4 DUP 7)  ; equivalent to DW 0,7,7,7,7,0,7,7,7,7
                                                              9-5

DATA SEGMENT
XX      DW ?            ; define a word variable XX
YYLOW   DB              ; no init value: YYLOW is low byte of word var YY
YY      DW ?
X_ARRAY DB  100 DUP ?   ; X_ARRAY is a 100-byte array
D_REAL  DQ ?            ; double precision floating variable
EX_REAL DT ?            ; extended precision floating variable

A character string value may be used to initialize consecutive
bytes in a DB statement.  Each character will be represented by
its ASCII code.  The characters are stored in the order that they
appear in the string, with the first character assigned to the
lowest-addressed byte.  In the DB statement that follows, five
bytes are initialized with the ASCII representation of the
characters in the string 'HELLO':

DB 'HELLO'

Note that except for string comparisons described in the previous
chapter, the DB directive is the only place in your program that
strings of length greater than 2 may occur.  In all other
contexts (including DW), a string is treated as the constant
number representing the ASCII value of the string; for example,
CMP AL,'@' is the instruction comparing the AL register with the
ASCII value of the at-sign.  Note further that 2-character string
constants, like all constants in the 8086, have their bytes
reversed.  Thus, while DB 'AB' will produce hex 41 followed by
hex 42, the similar looking DW 'AB' reverses the bytes: hex 42
followed by hex 41.

For compatibility, A86 now accepts double quotes, as well as
single quotes, for strings in DB directives.


The DD directive is used to initialize 32-bit doubleword pointers
to locations in arbitrary segments of the 86's memory space.
Values for such pointers are given by two numbers separated by a
colon.  The segment register value appears to the left of the
colon; and the offset appears to the right of the colon.  In
keeping with the reversed-bytes nature of memory storage in the
86 family, the offset comes first in memory.  For example, the
statement

   DD   01234:05678

appearing in a CODE segment will cause the hex bytes 78 56 34 12
to be generated, which is a long pointer to segment 01234, offset
05678.

DD, DQ, and DT can also be used to initialize large integers and
floating point numbers.  Examples:

  DD 500000   ; half million, too big for most 86 instructions
  DD 3.5      ; single precision floating point number
  DQ 3.5      ; the same number in a double precision format
  DT 3.5      ; the same number in an extended precision format
                                                              9-6

The STRUC Directive

The STRUC directive is used to define a template of data to be
addressed by one of the 8086's base and/or index registers.  The
syntax of STRUC is as follows:

(optional strucname)  STRUC  (optional effective address)

The optional structure name given at the beginning of the line
can appear in subsequent expressions in the program, with the
operator TYPE applied to it, to yield the number of bytes in the
structure template.

The STRUC directive causes the assembler to enter a mode similar
to DATA SEGMENT: assembly within the structure declares symbols
(the elements of the structure), using a location counter that
starts out at the address following STRUC.  If no address is
given, assembly starts at location 0.  An option not available to
the DATA SEGMENT is that the address can include one base
register [BX] or [BP] and/or one index register [SI] or [DI]. The
registers are part of the implicit declaration of all structure
elements, with the offset value increasing by the number of bytes
allocated in each structure line. For example:

LINE STRUC [BP]        ; the template starts at [BP]
       DB 80 DUP (?)   ; these 80 bytes advance us to [BP+80]
LSIZE  DB ?            ; this 1 byte advances us to [BP+81]
LPROT  DB ?
     ENDS

The STRUC just given defines the variables LSIZE, equivalent to
B[BP+80], and LPROT, equivalent to B[BP+81].  You can now issue
instructions such as MOV AL,LSIZE; which automatically generates
the correct indexing for you.

The mode entered by STRUC is terminated by the ENDS directive,
which returns the assembler to whatever segment (CODE or DATA) it
was in before the STRUC, with the location counter restored to
its value within that segment before the STRUC was declared.



Forward References

A86 allows names for a variety of program elements to be forward
referenced.  This means that you may use a symbol in one
statement and define it later with another statement.  For
example:

  JNZ TARGET
  .
  .
TARGET:
  ADD AX,10
                                                              9-7

In this example, a conditional jump is made to TARGET, a label
farther down in the code.  When JNZ TARGET is seen, TARGET is
undefined, so this is a forward reference.

Earlier versions of A86 were much more restricted in the kinds of
forward references allowed.  Most of the restrictions have now
been eased, for convenience as well as compatibility with other
assemblers.  In particular, you may now make forward references
to variable names.  You just need to see to it that A86 has
enough information about the type of the operand to generate the
correct instruction.  For example, MOV FOO,AL will cause A86 to
correctly deduce that FOO is a byte variable.  You can even code
a subsequent MOV FOO,1 and A86 will remember that FOO was assumed
to be a byte variable.  But if you code MOV FOO,1 first, A86
won't know whether to issue a byte or a word MOV instruction; and
will thus issue an error message.  You then specify the type by
MOV FOO B,1.

In general, A86's compatibility with That Other assembler has
improved dramatically for forward references.  Now, for most
programs, you need only sprinkle a very few B's and W's into your
references.  And you'll be rewarded: in many cases the word form
is longer than the byte form, so that the other assembler winds
up inserting a wasted NOP in your program.  You'll wind up with
tighter code by using A86!


Forward References in Expressions

A86 now allows you to add or subtract a constant number from a
forward reference symbol; and to append indexing registers to a
forward reference symbol.  This covers a vast majority of
expressions formerly disallowed.  For the remaining, more
complicated expressions, there is a trick you can use to work
your way around almost any case where you might run into a
forward reference restriction.  The trick is to move the
expression evaluation down in your program so that it no longer
contains a forward reference; and forward reference the
evaluation answer.  For example, suppose you wish to advance the
ES segment register to point immediately beyond your program. If
PROG_SIZE is the number of bytes in your program, then you add
(PROGSIZE+15)/16 to the program's segment register value. This
value is known at assembly time; but it isn't known until the end
of the program.  You do the following:

     MOV AX,CS        ; fetch the program's segment value
     ADD AX,SEG_SIZE  ; use a simple forward reference
     MOV ES,AX        ; ES is now loaded as desired

Then at the end of the program you evaluate the expression:

     PROG_SIZE EQU $
     SEG_SIZE EQU (PROG_SIZE+15)/16
                                                              9-8

The EQU Directive

Syntax:  symbol-name EQU expression
         symbol-name EQU built-in-symbol
         symbol-name EQU INT n

The expression field may specify an operand of any type that
could appear as an operand to an instruction.

As a simple example, suppose you are writing a program that
manipulates a table containing 100 names and that you want to
refer to the maximum number of names throughout the source file.
You can, of course, use the number 100 to refer to this maximum
each time, as in MOV CX,100, but this approach suffers from two
weaknesses.  First of all, 100 can mean a lot of things; in the
absence of comments, it is not obvious that a particular use of
100 refers to the maximum number of names.  Secondly, if you
extend the table to allow 200 names, you will have to locate each
100 and change it to a 200.  Suppose, instead, that you define a
symbol to represent the maximum number of names with the
following statement:

MAX_NAMES EQU 100

Now when you use the symbol MAX_NAMES instead of the number 100
(for example, MOV CX,MAX_NAMES), it will be obvious that you are
referring to the maximum number of names in the table.  Also, if
you decide to extend the table, you need only change the 100 in
the EQU directive to a 200 and every reference to MAX_NAMES will
reflect the change.

You could also take advantage of A86's strong typing, by changing
MAX_NAMES to a variable:

MAX_NAMES  DB ?

or even an indexed quantity:

MAX_NAMES EQU [BX+1]

Because the A86 language is strongly typed, the instruction for
loading MAX_NAMES into the CX register remains exactly the same
in all cases: simply MOV CX,MAX_NAMES.
                                                              9-9

Equates to Built-In Symbols

A86 allows you to define synonyms for any of the assembler
reserved symbols, by EQUating an alternate name of your choosing,
to that symbol.  For example, suppose you were coding a source
module that is to be incorporated into several different
programs.  In some programs, a certain variable will exist in the
code segment.  In others, it will exist in the stack segment. You
want to address the variable in the common source module, but you
don't know which segment override to use.  The solution is to
declare a synonym, QS, for the segment register.  QS will be
defined by each program: the code-segment program will have a QS
EQU CS at the top of it; the stack-segment program will have QS
EQU SS.  The source module can use QS as an override, just as if
it were CS or SS.  The code would be, for example, QS MOV
AL,VARNAME.


The NIL Prefix

A86 provides a mnemonic, NIL, that generates no code.  NIL can be
used as a prefix to another instruction (which will have no
effect on that instruction), or it can appear by itself on a
line.  NIL is provided to extend the example in the previous
section, to cover the possibility of no overrides.  If your
source module goes into a program that fits into 64K, so that all
the segment registers have the same value, then code QS EQU NIL
at the top of that program.


Interrupt Equates

A86 allows you to equate your own name to an INT instruction with
a specific interrupt number.  For example, if you place  TRAP EQU
INT 3  at the top of your program, you can use the name TRAP as a
synonym for INT 3  (the debugger trap on the 8086).


Duplicate Definitions

A86 contains the unique feature of duplicate definitions.   We
have already discussed local symbols, which can be redefined to
different values without restriction.  Local symbols are the only
symbols that can be redefined.  However, any symbol can be
defined more than once, as long as the symbol is defined to be
the same value and type in each definition.

This feature has two uses.  First, it eases modular program
development.  For example, if two independently-developed source
files both use the symbol ESC to stand for the ASCII code for
ESCAPE, they can both contain the declaration ESC EQU 01B, with
no problems if they are combined into the same program.
                                                             9-10

The second use for this feature is assertion checking.  Your
deliberate redeclaration of a symbol name is an assertion that
the value of the symbol has not changed; and you want the
assembler to issue you an error message if it has changed.
Example: suppose you have declared a table of options in your
DATA segment; and you have another table of initial values for
those options in your CODE segment.  If you come back months
later and add an option to your tables, you want to be reminded
to update both tables in the same way.  You should declare your
tables as follows:

DATA SEGMENT
  OPTIONS:
    .
    .
  OPT_COUNT EQU $-OPTIONS    ; OPT_COUNT is the size of the table

CODE SEGMENT
  OPT_INITS:
    .
    .
  OPT_COUNT EQU $-OPT_INITS  ; second OPT_COUNT had better be the same!



The = Directive

Syntax:  symbol-name = expression
         symbol-name = built-in-symbol
         symbol-name = INT n

The equals sign directive is provided for compatibility with That
Other assembler.  It is identical to the EQU directive, with one
exception: if the first time a symbol appears in a program is in
an = directive, that symbol will be taken as a local symbol.  It
can be redefined to other values, just like the generic local
symbols (letter followed by digits) that A86 supports. (If you
try to redefine an EQU symbol to a different value, you get an
error message.) The = facility is most often used to define
"assembler variables", that change value as the assembly
progresses.


The PROC Directive

Syntax:   name  PROC NEAR
          name  PROC FAR
          name  PROC

PROC is a directive provided for compatibility with Intel/IBM
assemblers.  I don't like PROC; and I recommend that you do not
use it, even if you are programming for those assemblers.
                                                             9-11

The idea behind PROC is to give the assembler a mechanism whereby
it can decide for you what kind of RET instruction you should be
providing.  If you specify NEAR in your PROC directive, then the
assembler will generate a near (same segment) return when it sees
RET.  If you specify FAR in your PROC directive, the assembler
will generate a far RETF return (which will cause both IP and CS
to be popped from the stack).  If you simply leave well enough
alone, and never code a PROC in your program, then RET will mean
near return throughout your program.

The reason I don't like PROC is because it is yet another attempt
by the assembler to do things "behind your back".  This goes
against the reason why you are programming in assembly language
in the first place, which is to have complete control over the
code generated by your source program.  It leads to nothing but
trouble and confusion.

Another problem with PROC is its verbosity.  It replaces a simple
colon, given right after the label it defines.  This creates a
visual clutter in the program, that makes the program harder to
read.

A86 provides an explicit RETF mnemonic so that you don't need to
use PROC to distinguish between near and far return instructions.
You can use RET or a near return and RETF for a far return. Even
if you are programming in that other assembler, and you need to
code a far return, I recommend that you create a RETF macro (it
would have the single line DB 0CBH), and stay away from PROCs
entirely.


The ENDP Directive

Syntax:  [name] ENDP

The only action A86 takes when it sees an ENDP directive is to
return the assembler to its (sane) default state, in which RET is
a near return.

NOTE that this means that A86 does not support nested PROCs, in
which anything but the innermost PROC has the FAR attribute.  I'm
sorry if I am blunt, but anybody who would subject their program
to that level of syntactic clutter has rocks in their head.


The LABEL Directive

Syntax:  name LABEL NEAR
         name LABEL FAR
         name LABEL BYTE
         name LABEL WORD

LABEL is another directive provided for compatibility with
Intel/IBM assemblers.  A86 provides less verbose ways of
specifying all the above LABEL forms, except for LABEL FAR.
                                                             9-12

LABEL defines "name" to have the type given, and a value equal to
the current output pointer.  Thus, LABEL NEAR is synonymous with
a simple colon following the name; and LABEL BYTE and LABEL WORD
are synonymous with DB and DW, respectively, with no operands.

LABEL FAR does have a unique functionality, not found in other
assemblers.  It identifies "name" as a procedure that can be
called from outside this program's code segment.  Such procedures
should have RETFs instead of RETs. Furthermore, I have provided
the following feature, unique to A86: if you CALL the procedure
from within your program, A86 will generate a PUSH CS instruction
followed by a NEAR call to the procedure.  Other assemblers will
generate a FAR call, having the same functional effect; but the
FAR call consumes more program space, and takes more time to
execute.

WARNING: you cannot use the above CALL feature as a forward
reference; the LABEL FAR definition must precede any CALLs to it.
This is unavoidable, since the assembler must assume that a CALL
to an undefined symbol takes 3 program bytes.  All assemblers
will issue an error in this situation.

