1. Flow Control Instructions
ICS312 SET 8
Flow Control Instructions

2. Flow Control Instructions
Flow control, or branching, instructions for jumps, loops and conditional jumps allow us to control the execution of
program statements and to build high-level programming
structures such as if-else statements, case statements,
and loops in assembler programs

3. Jump Instructions (1)
The unconditional jump instruction, branches to a new address in the program code.     JMP NEXT        ; jumps to address of "NEXT"
Conditional jump instructions test the condition of one or two FLAGS in the flags register, then "jump" to a new location in the program based on the result of the test.
Example:
TOP:
SUB AX, BX ;AX = AX - BX
JC EXIT: ;IF AX < BX then jump to EXIT
JMP TOP ;else, repeat until AX < BX
EXIT:
MOV AX, 4C00H
INT 21H

4. Jump Instructions (2) CMP Instruction
The CMP instruction sets the flags the same as if a subtraction had been performed, but the destination operand is NOT changed.
Example (simulating the LOOP instruction):
MOV CX, 5
TOP_2:
CMP CX, 0 ; CX = 0 ?
JE EXIT_: ; IF CX = 0, then EXIT_
SUB AX, BX ; AX = AX - BX
DEC CX
JMP TOP_2 ;else, repeat until CX = 0
EXIT_:
MOV AX, 4C00H
INT 21H
Note: the above code simulates the effect of the LOOP instruction

5. Signed versus Unsigned Conditional Jumps
Note:In this course we will only make use of signed conditional jumps.
Unsigned Conditional Jumps:  JA and JB
Signed Conditional Jumps: JG or JL
Example 1. Will the JG instruction in the following code result in a jump?
MOV AL, 80h
MOV BL, 7FH
CMP AL, BL
JG EXIT_
...
Example 2. Will the JA instruction in the following code result in a jump?
JA EXIT_

6. Examples of Flow Control Using Jump Instructions (1)
IF-THEN-ELSE Structures
START:
MOV AH, 1
INT 21H ; input AL
CMP AL, 'Q' ; if AL = 'Q'
JE EXIT ; then, branch to EXIT
MOV AH, ; else output AL
MOV DL, AL
INT 21H
JMP START ; repeat
EXIT:
MOV AX, 4C00H

7. Examples of Flow Control Using Jump Instructions (2)
IF-THEN-ELSE
If AL ≤ BL, display BL
else display AL
CMP AL, BL ; AL ≤ BL?
JG ELSE_ ; no, go to "else"
MOV DL, BL ; display BL
JMP DISPLAY
ELSE_: ; else, AL > BL
MOV DL, AL ; display AL
DISPLAY: ; display result
MOV AH, 2
INT 21H

8. Examples of Flow Control Using Jump Instructions (3)
CASE Structure
MOV AH, 1
INT 21H
CMP AL, 'A'
JE CASE_A
CMP AL, 'B'
JE CASE_B
CMP AL, 'C'
JE CASE_C
CMP AL, 'D'
JE CASE_D
CASE_A:
; code for case A goes here
JMP NEXT_
CASE_B:
; code for case B goes here
CASE_C:
; code for case C goes here
CASE_D:
; code for case D goes here
NEXT_:
; code continues here

9. Examples of Flow Control Using Jump Instructions (4)
AND Conditions: Is AL >= 30H and <= 39H?
I.e. is it the code for a decimal digit?
MOV AH, 1
INT 21H
CMP AL, 30H ; AL >= 30h?
JL ERROR ; no, error
CMP AL, 39H ; AL <= 39h?
JG ERROR ; no, error
; continue: AL >= 30h and AL <= 39h
ERROR:
; code for handling an error

10. Examples of Flow Control Using Jump Instructions (5)
OR Conditions Input until the character is Q or q.
START:
MOV AH, 1
INT 21H
CMP AL, 'Q'
JE EXIT_ ; if AL = 'Q', then exit
CMP AL, 'q'
JNE START ; if AL != 'Q' and AL != 'q', repeat input
EXIT_: ; if AL = 'Q' or AL = 'q'
MOV AX, 4COOH

11. Examples of Flow Control Using Jump Instructions (6)
FOR Loops
Input the value of AL.
For I = 1 to 10 output this value+I.
START:
MOV AH, 1
INT 21H
MOV CX, 10
TOP_:
JCXZ EXIT_ ; If CX = 0, exit
MOV DL, AL
MOV AH, 2
INC AL
LOOP TOP_ ; repeats for 10 times
EXIT_:
MOV AX, 4C00H

12. Examples of Flow Control Using Jump Instructions (7)
WHILE Loops
input Al
while AL != ‘Q’
output AL
end while
START:
MOV AH, 1
INT 21H
CMP AL, 'Q'
JE EXIT_ ; repeat while AL != 'Q'
MOV DL, AL
MOV AH, 2
JMP start
EXIT_:
MOV AX, 4C00H

13. Examples of Flow Control Using Jump Instructions (8)
REPEAT-UNTIL Loops
Set Dl to ‘A’
TOP: output DL
add 1 to DL
repeat top until DL = ‘[‘ (ascii code 5Bh)
MOV AH, 2
MOV DL, 'A'
TOP_:
INT 21H
INC DL
CMP DL, '['
JNE top_ ; repeat until DL = '['
EXIT_:
MOV AX, 4C00H

14. Loop Instruction
Used to implement "for loops" more conveniently than above.
Decrements CX register and jumps to specified label as long as CX != 0 after being decremented.
Example: Input a number, and display that number of asterisks
; required for MOVSX or MOVZX instructions
.CODE
START:
MOV DL, '*"
MOV AH, 1
INT 21H
SUB AL, 30H
MOVSX CX, AL ; sign extend into CX register
MOV AH, 2
TOP_:
INT 21H ; display asterisks
LOOP TOP_ ; repeats CX times
EXIT_:
MOV AX, 4C00H

15. MOVSX Instruction
Move with sign extension. Copies a byte or word from a
source operand to a register, and sign extends into the
upper half of the destination. This is used to copy an 8-bit
or 16-bit operand into a larger destination.

16. JMP Instructions For signed arithmetic J[N] {G | L | Z } [E]
eg: JG, JGE, JNZ, JLE
For unsigned arithmetic
(not used in this course)
J[N] {A | B | Z } [E]
eg: JA, JAE, JNZ, JBE

17. Example on JE MOV BL, 0 REPEAT: MOV AH, 1 INT 21H CMP AL, 0DH JE OUT
Count the number of characters entered which terminate in a CR (0DH),
don’t count CR. Use BL to store that number.
MOV BL, 0
REPEAT: MOV AH, 1
INT H
CMP AL, 0DH
JE OUT
INC BL
JMP REPEAT
OUT: …

18. Example on JGE To put the absolute value of AX into AX. CMP AX, 0
JGE OUT ; jump if greater than or equal
NEG AX
OUT: …

19. Example on JE & JL
If AX < 0, output – ; If AX = 0, output 0; If AX > 0, output +
CMP AX, 0
JE zerocase
JL lesscase
MOV DL, ‘+’
JMP display
zerocase: MOV DL, 0
lesscase: MOV DL, ‘-’
display: MOV AH, 2
INT 21H

20. Example on JCXZ JCXZ – Jump if cx is zero, cx used as a counter
Assume that CX contains some number and you want to
Output this number of asterisks
JCXZ OUT
place: MOV AH, 2
MOV DL, ‘*’
INT 21H
LOOP place
OUT: …

21. Textbook Reading (Jones):
Sections
Homework: Ex 5.1, p. 109, nos. 1-4
Ex 5.3, p. 115, no. 1