1. Morgan Kaufmann Publishers Computer Organization and Assembly Language
January 18, 2019
CS 206 D
Computer Organization and Assembly Language
Chapter 1 — Computer Abstractions and Technology

2. The Stack and Procedures
Lecture 8
The Stack and Procedures

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

4. “Last-In-First-Out” LIFO
Introduction
The stack segment of a program is used for temporary dynamic storage of data and addresses.
PUSH and POP instructions are used to add and remove words from the stack.
The stack is a one-dimensional data structure.
Items are added and removed from one end of the structure; that is, it processes in:
“Last-In-First-Out” LIFO 2

5. The Stack
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

6. 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

7. 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

8. 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

9. 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

10. The Stack Segment
0A000H
00000H
0A00
SS:
0A100H SP
0100
SS:SP
Memory
0A00
Segment Register
Offset
Physical Address +
0100
0A100H
0FFFFFH
The stack is always referenced with respect to the stack segment register.
The offset is given by the SP register.
The stack grows toward decreasing memory locations.
The SP points to the last or top item on the stack.
PUSH - pre-decrement the SP
POP - post-increment the SP

11. 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

12. Terminology of Procedures
A subroutine or procedure (sometimes referred to as a subprogram) is a segment of a larger program that is typically relatively independent of that program.
The MAIN procedure, contains the entry point to the program.
To carry out a task, the main procedure calls one of the other procedures.
It is also possible for these procedures to call each other, or a procedure to call itself.
When one procedure calls another, control transfers to the called procedure and its instructions are executed; the called procedure usually returns control to the caller at the next instruction after the call statement. 9

13. Procedure Declaration
Syntax (except the main procedure):
name PROC type
; body of the procedure
RET
name ENDP
Name is the
user-defined
name of the
procedure.
The optional operand type is:
NEAR: the statement that
calls the procedure is in the
same segment as the procedure itself, or
FAR: the statement that
calls the procedure is in a
different segment.
If type is omitted, NEAR is assumed.
The RET (return)
instruction causes
control to transfer
back to the calling
procedure 15

14. Communication Between Procedures
Assembly language procedures do not have parameter lists.
It’s up to the programmer to devise a way for procedures to communicate.
E.g. If there are only few input and output values, they can be placed in registers.
The CALL Instruction
To invoke a procedure, the CALL instruction is used.
Stack is used by CALL instruction to keep return address for procedure, RET instruction gets this value from the stack and returns to that offset. Quite the same thing when INT and IRET 10

15. The CALL Instruction Before CALL After CALL Offset address 0010 0012
0200
Code segment
MAIN PROC
CALL PROC1
next instruction
PROC1 PROC
first instruction
RET
Offset address
0010
0012
0200
Code segment
MAIN PROC
CALL PROC1
next instruction
PROC1 PROC
first instruction
RET IP IP
Offset address
00FE
0100
Stack segment
Offset address
00FE
0100
Stack segment
0012 SP SP
Before CALL
After CALL 12

16. The RET Instruction Before RET After RET Offset address 0010 0012 0200
0300
Code segment
MAIN PROC
CALL PROC1
next instruction
PROC1 PROC
first instruction
RET
Offset address
0010
0012
0200
0300
Code segment
MAIN PROC
CALL PROC1
next instruction
PROC1 PROC
first instruction
RET IP IP
Offset address
00FE
0100
Stack segment
Offset address
00FE
0100
Stack segment
0012 SP SP
Before RET
After RET 13

17. CALL example ORG 100h MOV AL, 1 MOV BL, 2 CALL m2
RET ; return to operating system.
m2 PROC
MUL BL ; AX = AL * BL.
RET ; return to caller.
m2 ENDP
END
This example update AL every time the procedure is called, BL register stays unchanged, so this algorithm calculates 2 in power of 4, so final result in AX register is 16 (or 10h).

18. INDEC / OUTDEC Procedures
Procedures used to read and print decimal data
To invoke the two procedures, use CALL instruction inside the MAIN PROC .
Example
CALL INDEC
CALL OUTDEC 14

19. INCLUDE C:\ASM\ PGM9_3.ASM INCLUDE C:\ASM\ PGM9_1.ASM
INDEC / OUTDEC Procedures
INDEC ; Read character input from user and convert it to ; decimal stored in AX register
; Code of INDEC exist in file PGM9_3.ASM
OUTDEC ;Display the decimal number in register AX to ; output screen
; Code of OUTDEC exist in file PGM9_1.ASM
Include the two files using INCLUDE directive
Syntax:
INCLUDE C:\ASM\ PGM9_3.ASM
INCLUDE C:\ASM\ PGM9_1.ASM 15

20. INDEC / OUTDEC Procedures
@REPEAT1:
XOR DX,DX
DIV BX
PUSH DX
INC CX
OR AX,AX
MOV AH,2
@PRINT_LOOP:
POP DX
OR DL,30H
INT 21H
POP CX
POP BX
POP AX
RET
OUTDEC ENDP
OUTDEC PROC
PUSH AX
PUSH BX
PUSH CX
PUSH DX
OR AX,AX
MOV DL,'-'
MOV AH,2
INT 21H
POP AX
NEG AX
@END_IF1:
XOR CX,CX
MOV BX,10D 16

21. INDEC / OUTDEC Procedures
@MINUS:
MOV CX,1
@PLUS:
INT 21H
@REPEAT2:
CMP AL,'0'
CMP AL,'9'
AND AX,000FH
PUSH AX
MOV AX,10
MUL BX
POP BX
ADD BX,AX
MOV AH,1
CMP AL,0DH
INDEC PROC
; READ DECIMAL NUMBER
PUSH BX
PUSH CX
PUSH DX
@BEGIN:
MOV AH,2
MOV DL,'?'
INT 21H
XOR BX,BX
XOR CX,CX
MOV AH,1
CMP AL,'-'
CMP AL,'+'
MOV AX,BX
OR CX,CX
NEG AX
@EXIT:
POP DX
POP CX
POP BX
RET
@NOT_DIGIT:
MOV AH,2
MOV DL,0DH
INT 21H
MOV DL,0AH
INDEC ENDP
;END READ

22. INDEC / OUTDEC Procedures MAIN PROGRAM
MODEL SMALL
.STACK 100H
.CODE
MAIN PROC
CALL INDEC
CALL OUTDEC
MOV AH, 4CH ; exit to DOS
INT 21H
MAIN ENDP
INCLUDE C:ASM\PGM9_1.ASM
INCLUDE C:ASM\PGM9_3.ASM
END MAIN 19