1. (Flow Control Instructions)
Assembly Language
Lecture 5 & 6
(Flow Control Instructions)

2. Lecture Outline Branching structures: IF-THEN IF-THEN-ELSE
Unconditional Jumps
Flow Control Instructions 1

3. Branching Structures
In high-level languages, branching structures enable a program to
take different paths, depending on conditions.
Branching structures:
IF-THEN
IF-THEN-ELSE
CASE
High-Level Language Structures 3

4. Introduction
Jump instructions can be used to implement branches and loops.
Because the jumps are so primitive, it is difficult to code an
algorithm with them without some guidelines.
High-level language structures could be used as a guideline when
coding in assembly language.
High-Level Language Structures 2

5. True-branch statements
IF-THEN
Pseudocode:
IF condition is true
THEN
execute true-branch statements
END_IF
true or false
False
condition
True
True-branch statements
High-Level Language Structures 4

6. IF-THEN Example: replace the number in AX by its absolute value.
Solution:
Pseudocode:
IF AX < 0
THEN
replace -AX by AX
END_IF
It can be coded as follows:
; if AX < 0
; then
CMP AX, 0 ; AX < 0
JNL END_IF ; no, exit
NEG AX ; yes, change sign
END_IF:
High-Level Language Structures 5

7. The CMP Instruction
The jump condition is often provided by the CMP (compare)
instruction
Syntax:
CMP destination, source
Compares by computing destination contents minus source contents.
The result is not stored, but the flags are affected.
Destination may not be a constant.
CMP is just like SUB, except that destination is not changed.
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8. Conditional Jumps Syntax Jxxx destination_label Example JNZ PRINT_LOOP
If the condition for the jump is true, the next instruction to be
executed is the one at destinaltion_label (PRINT_LOOP), which
may precede or follow the jump instruction itself.
If the condition is false, the instruction immediately following the
jump is done next.
For JNZ, the cindition is that the result of the previous operation is
not zero.
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9. Conditional Jumps Signed Jumps: used for signed interpretations.
Symbol Description Condition for Jumps
JG/JNLE jump if grater than ZF = 0 & SF = OF
jump if not less than or equal
JGE/JNL jump if grater than or equal SF = OF
jump if not less than
JL/JNGE jump if less than SF <> OF
jump if not greater than or equal
JLE/JNG jump if less than or equal ZF = 1 or SF <> OF
jump if not grater than
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10. Conditional Jumps Unsigned Jumps: used for unsigned interpretations.
Symbol Description Condition for Jumps
JA/JNBE jump if above CF = 0 & ZF = 0
jump if not below or equal
JAE/JNB jump if above or equal CF = 0
jump if not below
JB/JNAE jump if below CF = 1
jump if not above or equal
JBE/JNA jump if below or equal CF = 1 or ZF = 1
jump if not above
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11. Conditional Jumps
Single Flag Jumps: operates on settings of individual flags.
Symbol Description Condition for Jumps
JE/JZ jump if equal/ jump if equal to 0 ZF = 1
JNE/JNZ jump if not equal/ jump if not ZF = 0
JC jump if carry CF = 1
JNC jump if no carry CF = 0
JO jump if overflow OF = 1
JNO jump if no overflow OF = 0
JS jump if sign negative SF = 1
JNS jump if nonnegative sign SF = 0
JP/JPE jump if parity even PF = 1
JNP/JPO jump if parity odd PF = 0
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12. How the CPU Implements a Conditional Jump
The CPU looks at the FLAGS register.
If the conditions for the jump are:
True: the CPU adjusts the IP to point to the destination_label, so
that the instruction at this label will be done next.
False: the IP is not altered; this means that the next instruction in
line will be done.
Flow Control Instructions 6

13. Unconditional Jumps - The JMP Instruction
The JMP (jump) instruction causes an unconditional transfer of
control (unconditional jump).
Syntax:
JMP destination
a label in the same segment as the JMP itself
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14. Unconditional Jumps - The JMP Instruction
JMP can be used to get around the range restriction of a conditional
jump.
Example:
TOP:
; body of the loop
DEC CX
JNZ BOTTOM
JMP EXIT
BOTTOM:
JMP TOP
EXIT:
MOV AX, BX
If the loop body
contains so many
instructions that
label top is out of
range for JNZ
TOP:
; body of the loop
DEC CX
JNZ TOP
MOV AX, BX
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15. IF-THEN-ELSE Pseudocode: IF condition is true THEN
execute true-branch statements
ELSE
execute false-branch statements
END_IF
False
condition
True
False-branch statements
True-branch statements
High-Level Language Structures 6

16. IF-THEN-ELSE
Example: Suppose AL and BL contain extended ASCII characters.
Display the one that comes first in the character sequence.
Solution:
Pseudocode:
IF AL <= BL
THEN
display the character in AL
ELSE
display the character in BL
END_IF
continue
High-Level Language Structures 7

17. IF-THEN-ELSE It can be coded as follows: ; if AL <= BL
CMP AL, BL ; AL <= BL?
JNBE ELSE_ ; no, display char in BL
; AL <= BL
MOV DL, AL ; move char to be displayed
JMP DISPLAY ; go to display
ELSE_: ; BL < AL
MOV DL, BL
DISPLAY:
MOV AH, 2 ; prepare to display
INT 21h ; display it
High-Level Language Structures 8

18. Branches with compound Conditions
Sometimes the branching condition in an IF or CASE takes the
form:
condition_1 AND condition_2 or
condition_1 OR condition_2
where condition_1 and condition_2 are either true or false.
AND condition
OR condition
High-Level Language Structures 14

19. AND Condition
An AND condition is true if and only if all conditions are true.
Example: Read a character, and if it’s an uppercase letter, display it.
Solution:
Pseudocode:
Read a character (into AL)
IF ('A' <= character) and (character <= 'Z')
THEN
display character
END_IF
continue
High-Level Language Structures 15

20. AND Condition It can be coded as follows: ; read a character
; if ('A' <= char) and (char <='Z')
; then display char
MOV AH,1 ; prepare to read
INT 21h ; char in AL
CMP AL, 'A' ; char >= 'A'?
JNGE END_IF ; no, exit
CMP AL, 'Z' ; char <= 'Z'?
JNLE END_IF ; no, exit
MOV DL, AL ; get char
MOV AH, 2 ; prepare to display
INT 21h ; display char
END_IF:
High-Level Language Structures 16

21. OR Condition
An OR condition is true if at least one of the conditions is true.
Example: Read a character. If it’s 'y' or 'Y', display it; otherwise,
terminate the program.
Solution:
Pseudocode:
Read a character (into AL)
IF (character = 'y') or (character = 'Y')
THEN
display character
ELSE
terminate the program
END_IF
continue
High-Level Language Structures 17

22. OR Condition It can be coded as follows: ; read a character
; if (char = 'y') or (char = 'Y')
MOV AH,1 ; prepare to read
INT 21h ; char in AL
CMP AL, 'y' ; char = 'y'?
JE THEN ; yes, go to display it
CMP AL, 'Y' ; char = 'Y'?
JMP ELSE_ ; no, terminate
THEN:
MOV DL, AL ; get char
MOV AH, 2 ; prepare to display
INT 21h ; display char
JMP END_IF ; and exit
ELSE_:
MOV AH, 4Ch
INT 21h ; DOS exit
END_IF:
High-Level Language Structures 18

23. Working with Characters
In working with the standard ASCII character set, either signed or
unsigned jumps may be used.
Why?
Because the sign bit of a byte containing a character code is always
zero.
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