Q. Explain Passing Parameters Using Pointers ?

This method overcomes the drawback of using variable names directly in procedure. It uses registers to pass procedure pointers to desired data. Let's explain it more with the help of a program.

Program version 2:

DATA_SEG               SEGMENT

                        BCD DB   25h; Storage for BCD test value

BIN      DB?  ; Storage for binary value

DATA_SEG ENDS

STACK_SEG SEGMENT    STACK

DW 100 DUP (0)    ; Stack of 100 words

TOP_STACK LABEL    WORD

STACK_SEG ENDS

CODE_SEG SEGMENT

ASSUME CS: CODE_SEG, DS: DATA_SEG, SS: STACK_SEG

START:           MOV AX, DATA_SEG   ; Initialize data

MOV DS, AX   ; segment using AX register

MOV AX, STACK_SEG   ; initialize stack

MOV SS, AX; segment. Why stack?

MOV SP, OFFSET TOP_STACK; initialize stack pointer

; Put pointer to BCD storage in SI and DI prior to procedure call.

                        MOV SI, OFFSET BCD   ; SI now points to BCD_IN 

                        MOV DI, OFFSET BIN; DI points BIN_VAL 

                                                            ; (returned value)

                        CALL BCD_BINARY   ; Call the conversion 

                                                            ; Procedure

                        NOP     ; Continue with program

                                                            ; Here

; PROCEDURE     : BCD_BINARY Converts BCD numbers to binary. 

; INPUT     : SI points to location in memory of data

; OUTPUT     : DI points to location in memory for result

; DESTROYS    : Nothing

BCD_BINARY PROC NEAR

PUSHF     ; Save flag register 

PUSH AX; and AX registers

PUSH BX; BX  

PUSH CX; and CX

MOV AL, [SI]   ; Get BCD value from memory

                                     ; For conversion

MOV BL, AL   ; copy it in BL also

AND   BL, 0Fh   ; and mask to get lower 4 digits

AND AL, 0F0h; Separate upper 4 bits in AL

MOV CL, 04   ; initialize counter CL so that upper digit 

ROR AL, CL; in AL can be brought to lower 4 bit 

                                    ; Positions in AL

MOV BH, 0Ah   ; Load 10 in BH

MUL BH; Multiply upper digit in AL by 10

                                    ; The result is stored in AL

ADD AL, BL   ; Add lower BCD digit in BL to result of 

                                    ; Multiplication

; End of conversion, now restore the original values prior to call. All calls will be in 

; reverse order to save above. The result is in AL register.

MOV [DI], AL   ; Store binary value to memory

POP CX; Restore flags and

POP BX; registers

POP AX  

POPF    

RET    

BCD_BINARY ENDP

CODE_SEG ENDS

             END START

Discussion:

In the program written above DI points to BIN and SI points to the BCD. The instruction MOV AL, [SI] copies the byte pointed by SI to AL register. In the same manner MOV [DI], AL transfers result back to memory location which is pointed by DI. 

This scheme allows you to pass procedure pointers to data anywhere in memory. You can pass pointer to individual data element or a group of data elements such as strings and arrays. This scheme is used for parameters passing to BIOS procedures.