Procedures and macros allow code to be reused in assembly language programs. Procedures are subroutines that are called using CALL and RET instructions. Macros allow short, repetitive code sequences to be defined once and reused by replacing the macro call with its body code. Some key differences are that procedures occupy less memory than macros since macro code is generated each time, while procedures' code is only stored once. Procedures are accessed using CALL while macros are accessed by name.

1. PROCEDURES & MACROS
While writing programs, sometimes it may be required to use same set of
instructions repeatedly. If these instructions are actually written again & again, the
program size increases and occupies more memory space. To avoid this, these
instructions can be written as a separate program (subprogram) and whenever this set
of instructions is required, an instruction can be written in the main program CALL
this subprogram. This saves the memory space. Such a subprogram which is called
from the main program to execute certain set of instructions is called subroutine or
procedure.
Declaring PROCEDURES:
The syntax for procedure declaration:
name PROC [NEAR/FAR]
; Instructions of the procedures
; are written here.
RET
name ENDP
Example:
Delay PROC
AGAIN: MOV CX,COUNT
NOP
LOOP AGAIN
RET
Delay ENDP
The above procedure introduces a delay in between the program statements, when it is
called from the main program.
A procedure in 8086 can be accessed with a CALL & RET instruction.
CALL instruction: This performs two operations
1. Saves the return address or the address of the instruction next to the CALL
instruction on the stack. Return address is the address where the program will
return to after the procedure completes execution.
a. If the call to procedure is in the same code segment, i.e. a near CALL,
then only the contents of IP are pushed on the stack.
b. If the call to procedure is in another code segment, i.e. a far CALL,
then the contents of IP as well as CS are pushed on the stack.
2. It loads the IP & CS register with a new starting address of the procedure and
then branches to the procedure.
RET instruction: When 8086 executes a CALL instruction, it stores the return
address of the CALLing routine on the stack. A RET instruction at the end of the
procedure copies the return address stored on the stack back into the CS and IP
registers and then returns execution to the main program.

2. Passing Parameters to procedures:
Procedures may require input data or constants for their execution. Their data
or constants may be passed to the procedure by the main program or some procedures
may access the readily available data of constants available in memory.
Generally following techniques are used to pass input data/parameter to procedures in
assembly language language programs,
1. Using global declared variable
2. Using registers of CPU architecture
3. Using memory locations
4. Using stack
Example : Using registers
CODE SEGMENT
START: MOV AX, 5555H
MOV BX,7272h
:
:
CALL PROC1
:
:
PROCEDURE PROC1
:
:
ADD AX,BX
:
:
RET
PROC1 ENDP
CODE ENDS
END START
Re-entrant Procedures :
A procedure is said to be re-entrant, if it can be interrupted, used and re-
entered without losing or writing over anything. To be a re-entrant,
 Procedure must first push all the flags and registers used in the procedure.
 It should also use only registers or stack to pass parameters.
The flow of re-entrant procedure for a multiply procedure when interrupt procedure is
executed, as shown below.
Call
Multiply
Interrupt
Here
Call
Multiply
Main Line
Interrupt
Procedure
Return to
Interrupt
Multiply
procedure
Return to Calling pgm

3. Recursive Procedures:
A recursive procedure is a procedure which calls itself. Here, the program sets
aside a few locations in stack for the storage of the parameters whicha re passed each
time the computation is done and the value is returned. Each value returned is then
obtained by popping back from the stack at every RET instruction when executed at
the end of the procedure.
Example : To calculate factorial using recursion
data segment
n db 04h
res dw ?
data ends
code segment
assume cs:code, ds:data
start: mov ax,data
mov ds,ax
mov al,n
mov ah,00h
call fact
int 3
fact proc
cmp ax,01 ;if n=1, fact=1 else fact=n*fact(n-1)
jz exit
push ax
dec ax ;n-1
call fact ;fact(n-1) , RECURSION
pop ax
mul res ;n*fact(n-1)
mov res,ax ;res=factorial
ret
exit:
mov res,01
ret
fact endp
code ends
end start
Advantages of Procedures:
1. Simple modular programming
2. Reduced workload and development time
3. Debugging of program and procedure is easier
4. Reduction in the size of the main program
5. Reuse of procedures in the same program many times or in another program.
Macros :
Small sequences of codes of the same pattern repeated frequently at different
places which perform the same operation on the different data of the same data type
are called MACRO.
Macro is also called as an Open subroutine. When Called, the code written
within macro are executed automatically. Macros should be used when it has few
program statements. This simplifies the programming process.

4. Procedures Vs Macros
 Procedure does occupy minimum memory space than macro.
 In macro machine code is generated for instructions each time when it is
called but in procedure machine code for instruction is put only once in the
memory
 Procedure is accessed by call Instruction whereas Macro is accessed with the
name given.
Advantages
 Simplify and reduce the amount of repetitive coding
 Reduce errors caused by repetitive coding
 Makes program more readable
 Execution time is less as compared to procedures as no extra instructions
required
Defining Macros
 The Directive MACRO indicates the beginning of a MACRO
 Name of the Macro followed by MACRO and arguments if any are specified.
 ENDM is always associated with MACRO which ends the macro.
General Form :
Macro_name MACRO [Arguement1, arguement2…]
:
:
ENDM
Example:
PRINT MACRO MES
MOV AX,09H
LEA DX, MES
INT 21H
ENDM
The above macro is used to display a string specified in the argument MES on
the screen, when evoked by the main program as given below
DATA SEGEMENT
STR DB 0DH,0AH,”Hello World$”
DATA ENDS
CODE SEGMENT
ASSUME DS:DATA, CS:CODE
START: MOV AX, DATA
MOV DS, AX
PRINT STR ; Calls Macro PRINT to display STR
; STR is the parameter passed which is
;taken as MES in the Macro PRINT.
:
:
CODE ENDS
END START
Note : Main difference between Macro and Procedure is that A call to Macro
will be replaced with its body during assembly time, whereas the call to procedure is
explicit transfer of control during run-time.