1. Stack and Subroutines
Module M17.1
Section 11.2

2. The Stack
A region of memory in the stack segment (SS) used for storing temporary data.
The stack is a Last-In-First-Out (LIFO) data structure (like a stack of plates).
The stack pointer points to the value on top of the stack.
The stack grows toward lower memory addresses. That is, the stack pointer is decremented before an element is written to memory.

3. Pushing AX on the Stack

4. Note: Can only PUSH and POP 16-bit values

6. Push and Pop Example 0000 B8 34 12 MOV AX,1234H
0003 BB MOV BX,5678H
PUSH AX
PUSH BX
POP AX
B POP BX

7. Subroutines

8. Subroutine Example 0000 E8 05 00 CALL 0008H 0003 E8 0A 00 CALL 0010H
0006 CC INT 3
0008 E CALL 0010H
000B C3 RET
0010 C3 RET
Note: Destination address = Displacement +
Offset address of next instruction
= 0008
000A = 0010
B = 0010

9. SCREEN.ASM Subroutines
deccur
decrement the screen cursor
pbyte
print the hex value of the byte in AL followed by a space
crlf
carriage return - line feed: move cursor to beginning of next line
quit
return to DOS

10. Quit to DOS
quit proc near
mov ax,4C00h
int 21h
quit endp

11. Stack segment
Data segment
Subroutines from file
SCREEN.ASM

12. numer 78 56
numer+2 34 12
denom BC 9A

13. Must always set DS to DATA
dx:ax = numer
bx = denom
quotient in ax
remainder in dx

14. ;save registers
;save ax in bx
;print high byte
;decrement cursor
;print low byte
;restore registers