1. Microprocessor and Assembly Language
Lecture-6-Control Flow Instructions
Muhammad Hafeez
Department of Computer Science
GC University Lahore

2. Today’s Agenda
Conditional Jump
Unconditional Jump

3. Conditional Jump
The control is transferred to another location if certain condition is met
Flags in the flag register indicate the condition
Syntax.
Jxxx destination_label

4. Conditional Jump
If jump condition is false, IP is not altered, thereby, execute next instruction after jump
Range of conditional jump is relative to current IP
Conditional jumps are sometimes referred to short jumps

5. Conditional Jump Three Categories of Conditional Jumps:
Singed Jumps [singed interpretation to the result]
Unsigned Jump [unsigned interpretation to the result]
Single flag jump [based on the value of single flag]

6. CMP Instruction
Single Flag jumps are taken by looking at the value of single flag change
But singed and unsigned jumps are taken with a comparison based result using CMP instruction
Syntax,
CMP Destination, Source

7. CMP Instruction
Works like SUB instruction and define the result by subtracting source from destination, however, Destination is not change in CMP
Source and Destination must not be memory locations, destination must not be constant

8. Single Flag Jump
Works like SUB instruction and define the result by subtracting source from destination, however, Destination is not change in CMP
Source and Destination must not be memory locations, destination must not be constant

9. Signed Jump

10. Unsigned Jump

11. Singed and Unsigned Jumps
Each singed jump has its counterpart in unsigned like JG has JA
Signed and unsigned jumps may be used by programmer based on interpretation of the result
When we compare character, use either of singed or unsigned jump, for Extended ASCII use unsinged
AX = 7FFFH , BX=8000H
CMP AX,BX
JA someotherlocation
Even though AX > BX in singed way, jump will not be taken, as its unsigned jump

12. Example of Conditional Jump

13. Unconditional Jump JMP instruction take an unconditional jump Syntax
JMP Destination
Used to get around limit problem of conditional jump

14. Unconditional Jump TOP:. ;body of the loop DEC CX ; decrement counter
JNZ DOWN ;keep looping if CX > 0
JMP EXIT
DOWN:
JMP TOP
EXIT:
MOV AX,BX

15. Unconditional Jump Three Types of Unconditional Jumps NEAR JUMP
SHORT JUMP
FAR JUMP

16. Unconditional Jump SHORT JUMP, Syntax is JMP SHORT label
In this jump the address of target location is within -128 to +127 bytes of memory relative to the address of current IP.
Opcode is EB and operand is 1 byte in the range 00 to FF
Coding SHORT makes jump more efficient

17. Unconditional Jump NEAR JUMP, Types of Near Jump
Direct Jump, Syntax JMP NEAR label
In this jump the address of target location is within to bytes of memory relative to the address of current IP, within current code segment.
Register In-Direct Jump, Syntax JMP BX
IP takes the value in BX register
Memory In-Direct Jump, Syntax JMP [DI]
IP takes the value in DI AND DI+1
FAR JUP, Syntax JMP FAR PTR label
Takes control out of current code segment, CS, IP both are changed

18. High Level Language Equivalence in Assembly
If-Then in HLL
If condition then
some statement
End if

19. High Level Language Equivalence in Assembly
EX: If AX contains a number less than 0 then place 41H in AX
Sol:
;If AX < 0
CMP AX, 0
JNL END_IF
;THEN
NEG AX
INC AX
END_IF:

20. High Level Language Equivalence in Assembly
If-Then-Else in HLL
If condition then Then
statement for true case
Else
statement for false case
End if

21. High Level Language Equivalence in Assembly
EX: Suppose two numbers in AL and BH, put the largest one in CL
Sol:
;If AL <= BH ;
CMP AL, BH
JNLE ELSE_
;THEN
MOV CL,BH
;IF AL >= BH
:ELSE_
MOV CL,AL
;END_IF

22. High Level Language Equivalence in Assembly
Multi-Branch Structure of HLL in assembly
Switch Statement
IF THEN
ELSEIF THEN
ELSE
END IF

23. High Level Language Equivalence in Assembly
EX: If AL contains a positive number, put -1 in BH, if AL contain 0 then put 0 in BH, if AL contains a positive number put +1 in BH?
Sol: Use Case structure from HLL

24. High Level Language Equivalence in Assembly
Compound Statements Using ‘AND’ ‘OR’
IF cond1 AND Condi2 THEN
statements
END IF
IF cond1 OR Condi2 THEN

25. High Level Language Equivalence in Assembly
Example of OR compound statement
MOV AH,1 ; CHARACTER INPUT FUNCTION
INT 21H ;TAKE INPUT INTO AL
CMP AL,'Y'
JE THEN
CMP AL,'y'
JMP ELSE_
THEN:
MOV DL,AL
MOV AH,2
INT 21H
ELSE_:

26. High Level Language Equivalence in Assembly
Example of AND compound statement
MOV AH,1
INT 21H
CMP AL,'A'
JNGE END_IF
CMP AL,'Z'
JNLE END_IF
MOV DL,AL
MOV AH,2
END_IF:

27. Iterative Structure
Looping/ Iterative structure is the most important in any programming language
We can implement loops by using jump instructions and labels, until some specific conditions is met
Besides implementing loops with jumps and labels we also have LOOP instruction in 8086/8088 assembly

28. While Loop While Loop While condition is true Do End While
statement for true case
End While

29. While Loop
EX: Input Number of Character, count them, untill Enter is pressed
Initialize count to 0
Read a Character
While character <> CR do
Count = count + 1
Read a character
End While

30. While Loop – Example MOV DX,0 MOV AH,1 ;READ CARACTER FUNCTION
INT 21H ;READ CHARACTER INTO AL
WHILE_:
CMP AL, 0DH
JE END_WHILE
INC DX
MOV AH,2
MOV DL,AL
INT 21H
MOV AH,1
JMP WHILE_:
END_WHILE:

31. Do – While Loop
Do-While Loop Do
statement
While (condition)

32. Do – While loop EX: Read text while spacebar is pressed Do
Read Character
While character <> spacebar

33. Do – While Example: MOV AH,1 ;DO DO: INT 21H
CMP AL,20H ;WHILE CHARACTER IS NOT A SPACEBAR
JNE DO

34. Loop Instruction
The LOOP instruction is the easiest way to repeat a block of statements a specific number of times.
CX is automatically used as a counter and is decremented each time the loop repeats.
Syntax: LOOP destination
First loop instruction subtracts 1 from CX, then if CX is greater than zero, control is transferred to destination.
The destination operand must be a short label, in range -128 to +127 bytes from the current location.

35. Loop Instruction - Example
What if CX is zero, even before the loop begins?
How to check CX value ?
Figure out how to implement nested loop

36. Questions
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