Working with the Stack Stack concept Stack is

Stack concept • Stack is a special memory area used to store data temporarily

PUSH & POP instructions • push REG/MEM 16 -bits only • push ax Store

e. g. using the stack to print an array in reverse way. model small.

Procedures. model small. data num db 4 str 1 db ‘hello’, ’$’. code main

Using the stack to print out 4 digits decimal number. model small. data number

Using the stack to print out 4 digits hexadecimal number. model small. data number

Question For the following assembly language program, find the content of the register AL

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Working with the Stack

Stack concept • Stack is a special memory area used to store data temporarily when calling a subprogram and nested loops. Stack mechanism works in style “Last In First Out” LIFO. The following figure shows the stack mechanism.

Stack mechanism

PUSH & POP instructions • push REG/MEM 16 -bits only • push ax Store AX in the top of stack memory area then decrement SP register. • pop REG/MEM 16 -bits only • pop bx Increment SP register then copy the value in the top of stack to BX.

e. g. using the stack to print an array in reverse way. model small. data x db 0, 1, 2, 3, 4, 5, 6, 7, 8, 9. code mov ax, @data mov ds, ax lea si, x mov cx, 10 lop 1: mov ah, 0 mov al, [si] push ax inc si loop lop 1 mov lop 2: mov pop add int cx, 10 ah, 02 h dx dl, '0' 21 h loop lop 2

Procedures. model small. data num db 4 str 1 db ‘hello’, ’$’. code main proc mov ax, @data mov ds, ax call printme mov ah, 4 ch int 21 h endp main printme proc lea dx, str 1 mov ah, 09 h int 21 h ret endp printme

Using the stack to print out 4 digits decimal number. model small. data number dw 2153. code mov ax, @data mov ds, ax mov cl, 4 loop 1: mov ax, number mov bl, 10 div bl mov dl, ah mov dh, 0 push dx mov ah, 0 mov number, ax dec cl cmp cl, 0 jg loop 1 mov cl, 4 loop 2: pop dx add dl, 30 h mov ah, 2 int 21 h dec cl cmp cl, 0 jg loop 2

Using the stack to print out 4 digits hexadecimal number. model small. data number dw 2 A 5 Ch. code mov ax, @data mov ds, ax mov cl, 4 loop 1: mov ax, number and ax, 000 Fh push ax shr number, 4 dec cl cmp cl, 0 jg loop 1 mov cl, 4 loop 2: pop dx cmp dl, 9 jg ABCDEF add dl, 30 h mov ah, 2 int 21 h jmp update ABCDEF: add dl, 37 h mov ah, 2 int 21 h update: dec cl cmp cl, 0 jg loop 2

Question For the following assembly language program, find the content of the register AL after the execution of each underlined instruction: . MODEL SMALL. DATA M DB 8, 7, 8, 3, 2, 9. CODE MOV AX, @data MOV DS, AX LEA BX, M MOV AL, [BX] SHR AL, 2 SHL AL, 1 OR AL, 0 INC BX ADD AL, [BX] ADD BX, 3 SUB AL, [BX] MOV DX, 12 PUSH DX MOV DL, 3 MOV AH, 0 DIV DL MOV DL, 2 MUL DL POP AX MOV AL, '0' MOV AH, 4 ch INT 21 h AL = 8 2 4 4 AL = 11 AL = 9 AL = 4 AL = 8 AL = 12 AL = 30 h