Bahasa Assembler :
Macro
|
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.
Macro definition:
name MACRO [parameters,...]
<instructions>
ENDM |
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
ENDM
ORG 100h
MyMacro 1, 2, 3
MyMacro 4, 5, DX
RET |
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, for example:
CALL MyProc
- When you want to use a macro, you can just type its name. For
example:
MyMacro
- 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 is
used.
- 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:
MyMacro2 MACRO
LOCAL label1, label2
CMP AX, 2
JE label1
CMP AX, 3
JE label2
label1:
INC AX
label2:
ADD AX, 2
ENDM
ORG 100h
MyMacro2
MyMacro2
RET |
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.
|