1. (The Stack and Procedures)
Lecture 8
(The Stack and Procedures)

2. Lecture Outline Introduction The Stack The PUSH Instruction
The POP Instruction
Terminology of Procedures
INDEC / OUTDEC procedures 1

3. Introduction
The stack segment of a program is used for temporary storage of
data and addresses.
PUSH and POP instructions are used to add and remove words from
the stack. 2

4. The Stack A stack is a one-dimensional data structure.
Items are added and removed from one end of the structure; that is,
it processes in a “last-in-first-out” manner.
A program must set aside a block of memory to hold the stack.
Ex: .STACK 100H
When the program is assembled and loaded in memory:
SS will contain the segment number of the stack segment.
SP is initialized to 100H, which represents the empty stack
position.
When the stack is not empty, SP contains the offset address
of the top of the stack. 3

5. The PUSH Instruction To add a new word to the stack we PUSH it on.
Syntax:
PUSH source
Execution of PUSH causes the following to happen:
SP is decreased/decremented by 2.
A copy of the source content is moved to the address specified by
SS:SP. The source is unchanged.
16-bit register or memory location 4

6. The PUSH Instruction 1234 5678 AX BX Offset 00F8 00FA 00FC 00FE 0100
Empty Stack SP
1234
Offset
00F8
00FA
00FC
00FE
0100
After PUSH AX SP
1234
Offset
00F8
00FA
00FC
00FE
0100
AFTER PUSH BX SP
5678 5

7. The POP Instruction To remove the top item from the stack, we POP it.
Syntax:
POP destination
Execution of POP causes the following to happen:
The content of SS:SP (the top of the stack) is moved to the
destination.
SP is increased by 2.
16-bit register (except IP) or memory location 6

8. The POP Instruction 5678 1234 Offset 00F8 00FA 00FC 00FE 0100
FFFF
0001 CX DX
5678
1234
Offset
00F8
00FA
00FC
00FE
0100
After POP CX SP
0001 CX DX
5678
1234
Offset
00F8
00FA
00FC
00FE
0100
After POP DX SP CX DX
1234
Offset
00F8
00FA
00FC
00FE
0100
Stack SP
5678 7

9. Exercise 1
Write assembly code that uses the stack operations to swap the
content of AX and DX.
PUSH AX
PUSH DX
POP AX
POP DX 8