Procedures and Macros:
When we need to use a group of instructions several times throughout a program there are two ways, we can
avoid having to write the group of instructions each time we want to use them.
1. One way is to write the group of instructions as a separate procedure.
2. Another way we can use macros.
Procedures:
The procedure is a group of instructions stored as a separate program in the memory and it is called from
the main program whenever required using CALL instruction.
For calling the procedure we have to store the return address (next instruction address followed by CALL)
onto the stack.
At the end of the procedure RET instruction used to return the execution to the next instruction in the main
program by retrieving the address from the top of the stack.
Machine codes for the procedure instructions put only once in memory.
The procedure can be defined anywhere in the program using assembly directives PROC and ENDP.
The four major ways of passing parameters to and from a procedure are:
1. In registers
2. In dedicated memory location accessed by name
3. With pointers passed in registers
4. With the stack
The type of procedure depends on where the procedure is stored in the memory.
If it is in the same code segment where the main program is stored the it is called near procedure otherwise it
is referred to as far procedure.
For near procedure CALL instruction pushes only the IP register contents on the stack, since CS register
contents remains unchanged for main program.
But for Far procedure CALL instruction pushes both IP and CS on the stack.
Syntax:
Procedure name PROC near instruction 1
instruction 2 RET
Procedure name ENDP Example:
near procedure: far procedure: Procedures segment Assume CS : Procedures
ADD2 PROC near ADD2 PROC far
ADD AX, BX ADD AX, BX
RET RET ADD2 ENDP
ADD2 ENDP Procedures ends
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Depending on the characteristics, the procedures are of two types
1. Re-entrant Procedures
2. Recursive Procedures
Reentrant Procedures
The procedure which can be interrupted, used and “reentered” without losing or writing over anything.
Recursive Procedure
A recursive procedure is procedure which calls itself.
ALP for Finding Factorial of number using procedures
CODE SEGMENT
ASSUME CS: CODE
START: MOV AX,7
CALL FACT
MOV AH,4CH
INT 21H
FACT PROC NEAR
MOV BX, AX
DEC BX
BACK: MUL BX
DEC BX
JNZ BACK
RET
ENDP
CODE ENDS
END START
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Macros:
A macro is a group of repetitive instructions in a program which are codified only once and can be used as many
times as necessary.
A macro can be defined anywhere in program using the directives MACRO and ENDM
Each time we call the macro in a program, the assembler will insert the defined group of instructions in place of the
call.
The assembler generates machine codes for the group of instructions each time the macro is called.
Using a macro avoids the overhead time involved in calling and returning from a procedure.
Syntax of macro:
macroname MACRO
instruction1
instruction2
ENDM
Example:
Read MACRO Display MACRO
mov ah,01h mov dl,al
int 21h Mov ah,02h
ENDM int 21h
ENDM
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ALP for Finding Factorial of number using procedures
CODE SEGMENT
ASSUME CS: CODE
FACT MACRO
MOV BX, AX
DEC BX
BACK: MUL BX
DEC BX
JNZ BACK
ENDM
START: MOV AX,7
FACT
MOV AH,4CH
INT 21H
CODE ENDS
END START
Advantage of Procedure and Macros:
Procedures:
Advantages
The machine codes for the group of instructions in the procedure only have to be put once.
Disadvantages
Need for stack
Overhead time required to call the procedure and return to the calling program.
Macros:
Advantages
Macro avoids overhead time involving in calling and returning from a
procedure. Disadvantages
Generating in line code each time a macro is called is that this will make the program take up more
memory than using a procedure.
Differences between Procedures and Macros:
PROCEDURES MACROS
Accessed by CALL and RET mechanism during program Accessed by name given to macro when defined during
execution assembly
Machine code generated for instructions each time
Machine code for instructions only put in memory once
called
Parameters are passed in registers, memory locations Parameters passed as part of statement which calls
or stack macro
Procedures uses stack Macro does not utilize stack
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A procedure can be defined anywhere in program using A macro can be defined anywhere in program using the
the directives PROC and ENDP directives MACRO and ENDM
Procedures takes huge memory for CALL (3 bytes each Length of code is very huge if macro’s are called for more
time CALL is used) instruction number of times
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