Friday, December 25, 2009

Assembly Language Part 9

Procedures in Assembly

The Idea of procedure is to divide the main problem into sub problems and acquire the solution by solving the individual sub problems and combining through calling them from a mother problem.

The Idea is similar to functions() as used in High level languages. When a procedure call one another the control is transferred from the caller to the called, when the called procedure is done it usually returns the control to the caler function immediately after the calling statement.

The basic form of the procedure is

name PROC type
;  body of the procedure
RET
name ENDP

There are 2 types of procedures. NEAR and FAR. A procedure is called near if the procedure and the main program is in the same segment. If the procedure is in other segment then the proc is called FAR.

To invoke a procedure we write the following command


CALL name_of_proc


How is control returned?
The control is returned from the procedure to caller program when the RET instruction is executed. Every procedure should have a RET instruction in some place.

MAIN PROC
   ..............
   ..............
 CALL PROC_1
   statement_x
   ..............
   ..............




PROC_1 PROC
   ..............
   ..............
   RET

In the program mentioned above in a stage of execution of MAIN PROC the statement CALL PROC_1 will occur. This will transfer the control of the program to PROC_1 and the instructions in PROC_1 shall continue to execute. When the instruction RET is reached the control is returned to MAIN PROC and the program shall resume to execute from statement_x

Example ( factorial ! )
Let us describe the application of proc by finding the factorial

.MODEL SMALL
.STACK 100H
.DATA
        MSG1   DB    “This is from procedure_1$”
        MSG2  DB  0DH, 0AH, “This is from main procedure$”
.CODE
MAIN PROC
        MOV AX, @DATA
        MOV DS, AX

CALL PROCEDURE_1


        LEA DX,MSG2
        MOV AH,9

        INT 21H

     MOV AH, 4CH
     INT 21H
MAIN ENDP

PROCEDURE_1 PROC
        LEA  DX, MSG1
        MOV AH, 9
        INT 21H
  RET
PROCEDURE_1 ENDP

END MAIN

Macro in Assembly

The Idea of Macro is somewhat similar of procedure, divide the main problem into sub problems and acquire the solution by solving the individual sub problems. But macro at all is not like a procedure. It is just a block of program that has a name. The syntax for defining macro is -

macro_name      MACRO  operand1,operand2, . . . .
       statements
ENDM

Inside the Program we call the macro just by -

macro_name (operands)

When a macro is invoked, the program control does not jump as it does in the case of procedure. Instead the block of program defined by the macro name is copied to the place where it occurs before executing.

Example: Factorial using MACRO

.model small
fact macro 
    mov cx,var1
    xor ax,ax
    mov ax,1
    top:
    mul cx
    loop top   
    
    call outdec
 endm
    
.stack 100h
.data 
msg1 db 0ah,0dh,"Factorial is=$"  
msg2 db "Enter a number:$"  
;msg3 db 0ah,0dh,"your entered value is out of range!!$" 
var1 dw 0
;limit dw 0
.code    
main proc
    mov ax,@data
    mov ds,ax 
    lea dx,msg2
    mov ah,9
    int 21h
    call indec
    mov var1,ax  
    lea dx,msg1
    mov ah,9
    int 21h
    fact
    mov cx,ax
    mov ah,4ch
    int 21h
 
   main endp
    include outdec.asm
    include indec.asm   
end main

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