Passing parameters using stack

Passing parameters using registers-

The data to be passed is stored in the registers and these registers are accessed in the procedure to process the data.

Example:

.model small
.data
MULTIPLICAND DW 1234H
MULTIPLIER DW 4232H
.code
MOV AX, MULTIPLICAND
MOV BX, MULTIPLIER
CALL MULTI
 :
 :
MULTI PROC NEAR
MUL BX              ; Procedure to access data from BX register
RET
MULTI ENDP
:
:
END

The disadvantage of using registers to pass parameters is that the number of registers limits the number of parameters you can pass.

E.g. An array of 100 elements can’t be passed to a procedure using registers.

Passing parameters using memory-

In the cases where few parameters have to be passed to and from a procedure, registers are convenient. But, in cases when we need to pass a large number of parameters to procedure, we use memory. This memory may be a dedicated section of general memory or a part of it.

Example:

.model small
.data
MULTIPLICAND DW 1234H       ; Storage for multiplicand value
MULTIPLIER DW 4232H     ; Storage for multiplier value
MULTIPLICATION DW ?     ; Storage for multiplication result
.code
MOV AX, @Data
MOV DS, AX
:
:
CALL MULTI
:
:
MULTI PROC NEAR
MOV AX, MULTIPLICAND
MOV BX, MULTIPLIER
:
:
MOV MULTIPLICATION, AX    ; Store the multiplication value in named memory location
RET
MULTI ENDP
END

Passing parameter using pointers-

A parameter passing method which overcomes the disadvantage of using data item names (i.e. variable names) directly in a procedure is to use registers to pass the procedure pointers to the desired data.

Example:

.model small
.data
MULTIPLICAND DB 12H     ; Storage for multiplicand value
MULTIPLIER DB 42H           ; Storage for multiplier value
MULTIPLICATION DW ?     ; Storage for multiplication result
.code
MOV AX, @Data
MOV DS, AX
MOV SI, OFFSET MULTIPLICAND
MOV DI, OFFSET MULTIPLIER
MOV BX, OFFSET MULTIPLICATION
CALL MULTI
:
:
MULTI PROC NEAR
:
:
MOV AL, [SI]            ; Get multiplicand value pointed by SI in accumulator
MOV BL, [DI]            ; Get multiplier value pointed by DI in BL
:
:
MOV [BX], AX            ; Store result in location pointed out by BX
RET
MULTI ENDP
END

Passing parameters using stack-

In order to pass the parameters using stack we push them on the stack before the call for the procedure in the main program. The instructions used in the procedure read these parameters from the stack. Whenever stack is used to pass parameters it is important to keep a track of what is pushed on the stack and what is popped off the stack in the main program.

Example:

.model small
.data
MULTIPLICAND DW 1234H
MULTIPLIER DW 4232H
.code
MOV AX, @data
MOV DS, AX
 :
:
PUSH MULTIPLICAND
PUSH MULTIPLIER
CALL MULTI
:
:
MULTI PROC NEAR
PUSH BP
MOV BP, SP              ; Copies offset of SP into BP
MOV AX, [BP + 6]            ; MULTIPLICAND value is available at
                ; [BP + 6] and is passed to AX
MUL WORD PTR [BP + 4]       ; MULTIPLIER value is passed
POP BP
RET                 ; Increments SP by 4 to return address
MULTI ENDP              ; End procedure
END