8086 Assembler Tutorial for Beginners (Part 10)
Macros are just like procedures, but not really. Macros look like procedures,
but they exist only until your code is compiled, after compilation all macros
are replaced with real instructions. If you declared a macro and never used it
in your code, compiler will simply ignore it.
emu8086.inc is a good example of how macros can be used, this file
contains several macros to make coding easier for you.
name MACRO [parameters,...]
Unlike procedures, macros should be defined above the
code that uses it, for example:
MyMacro MACRO p1, p2, p3
MOV AX, p1
MOV BX, p2
MOV CX, p3
MyMacro 1, 2, 3
MyMacro 4, 5, DX
The above code is expanded into:
MOV AX, 00001h
MOV BX, 00002h
MOV CX, 00003h
MOV AX, 00004h
MOV BX, 00005h
MOV CX, DX
Some important facts about macros and procedures:
- When you want to use a procedure you should use CALL instruction,
When you want to use a macro, you can just type its name.
Procedure is located at some specific address in memory, and if you
use the same procedure 100 times, the CPU will transfer control
to this part of the memory. The control will be returned back to
the program by RET instruction. The stack is used
to keep the return address. The CALL instruction takes
about 3 bytes, so the size of the output executable file
grows very insignificantly, no matter how many time the procedure
Macro is expanded directly in program's code. So if you use the
same macro 100 times, the compiler expands the macro 100 times, making
the output executable file larger and larger, each time all
instructions of a macro are inserted.
You should use stack or any general purpose registers
to pass parameters to procedure.
To pass parameters to macro, you can just type them after the
macro name. For example:
MyMacro 1, 2, 3
- To mark the end of the macro ENDM directive is enough.
- To mark the end of the procedure, you should type the name of the
procedure before the ENDP directive.
Macros are expanded directly in code, therefore if there are labels inside the
macro definition you may get "Duplicate declaration" error when macro is used
for twice or more. To avoid such problem, use LOCAL directive followed
by names of variables, labels or procedure names. For example:
LOCAL label1, label2
CMP AX, 2
CMP AX, 3
ADD AX, 2
If you plan to use your macros in several programs, it may be a good idea to place
all macros in a separate file. Place that file in Inc folder and
use INCLUDE file-name directive to use macros.
See Library of common functions - emu8086.inc
for an example of such file.
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