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Conditional Jump and Loop Instruction

Computer Organization and Assembly Language

The Jcc Instructions

Ø The Jcc (Conditional Jump) instructions are jumps that may or may not transfer control depending on the state of the status flags at the time the instruction is executed.

Ø Format: Jcc operand

Ø Action: If the specified condition cc is true, it will jump to the address specified in the operand; otherwise the next instruction is executed.





(CF or ZF) = 1below or equal/not aboveJBE/JNA

CF = 1below/not above nor equalJB/JNA

CF = 0above or equal/not belowJAE/JNB

(CF or ZF) = 0above/not below nor equalJA/JNBE

Condition“Jump if …”Mnemonic

Comparing Unsigned Numbers





ZF=1 or (SF≠OF)

less than or equal/not greater than

JLE/JNG

SF ≠ OFless than/not greater than or equal

JL/JNGE

SF = OFgreater than or equal/not less than

JGE/JNL

ZF=0 or (SF=OF)

greater than/not less than or equal

JG/JNLE


Comparing Signed Numbers





OF = 0no overflow JNO

OF = 1overflowJO

PF = 0no parity/parity oddJNP/JPO

PF = 1parity/parity evenJP/JPE

SF = 0not signJNS

SF = 1signJS

CF = 0not carryJNC

CF = 1carryJC

ZF = 0not equal to/not zeroJNE/JNZ

ZF = 1equal to/zeroJE/JZ


Test Flags




Example – Jcc Instruction

1. MOV AL, A5H

MOV BL, 73H

CMP AL, BL

JA L1





2. MOV AL, A5H

MOV BL, 73H

CMP AL, BL

JG L1





3. MOV AL, 00H

MOV CL, 03H

AGAIN: ADD AL, 02H

DEC CL

JNZ AGAIN





4. MOV AL, 7FH

MOV BL, 75H

ADD AL, BL

JO NEXT





5. MOV AL, 01H

DEC AL

JG L1





6. Circumventing the limitation of the short-label:

MOV BX, 0002HMOV CX, 0002HCMP BX, CXJE L1

:::

L1: ::




The JCXZ Instruction

Ø The JCXZ (Jump if CX is 0) instruction transfers control to the target operand if CX is 0.

Ø Format: JCXZ short-label

Ø Action: If CX = 0, then jump to the address specified by the operand.




Program Tracing Example

Unless otherwise stated, determine the contents of all the affected general-purpose registers and the flags after executing the following programs (assume that all registers and flags are initially 0):

MOV AL, 23HMOV CH, 6BHXOR AL, CHTEST AL, 77HJZ L1AND CH, 33H

L1: XOR CH, AAHTEST CH, ALJZ L2NOT ALOR CH, AL

L2: NOT CH

What will be AX and CX after executing this program?




The LOOP Instruction

Ø The LOOP instruction decrements CX by 1 and transfers control to the target location if CX is not 0; otherwise, the instruction following LOOP is executed.

Ø Format: LOOP short-label

Ø Action: CX [CX] - 1

If CX ≠ 0, then jump to the address specified by the operand; otherwise execute the instruction following the LOOP.




Example - LOOP Instruction

MOV CX, 0003H

MOV AX, 0000H

L1: INC AX

LOOP L1

What is the value of AX and CX?




The LOOPE/LOOPZ Instruction

Ø The LOOPE/LOOPZ (Loop while equal/Loop while zero) instruction decrements CX by 1 and transfers control to target location if CX is not zero and if ZF is 1; otherwise the instruction following LOOPE/LOOPZ is executed.

Ø Format: LOOPE short-labelLOOPZ short-label


If CX ≠ 0 and ZF = 1, then jump to the address specified by the operand; otherwise execute the instruction following the LOOPE/LOOPZ.




Example - LOOPE/LOOPZ Instruction

MOV CX, 0005HMOV SI, FFFFHMOV AL, 01H

L1: INC SICMP BETA [SI], ALLOOPE L1

01H

01H

03H

01H

01HBETA:




The LOOPNE/LOOPNZ Instruction

Ø The LOOPNE/LOOPNZ (Loop while not equal/Loop while not zero) instruction decrements CX by 1 and transfers control to the target location if CX is not zero and ZF is 0; otherwise the instruction following LOOPNE/LOOPNZ is executed.

Ø Format: LOOPNE short-labelLOOPNZ short-label


If CX ≠ 0 and ZF = 0, then jump to the address specified by the operand; otherwise execute the instruction following the LOOPNE/LOOPNZ.




Example - LOOPNE/LOOPNZ Instruction

MOV CX, 0005HMOV SI, FFFFHMOV AL, 01H

L1: INC SICMP BETA [SI], ALLOOPNE L1

04H

07H

01H

05H

03HBETA:




Program Tracing Example

Unless otherwise stated, determine the contents of all the affected general-purpose registers and the flags after executing the following programs (assume that all registers and flags are initially 0):

MOV AX, 000AHMOV CX, 0014H

L1: SUB AX, 0001HLOOP L1LOOPZ L1LOOPNZ L1

What will be AX and CX after executing this program?

Download - MELJUN CORTES It subjects conditional jump and loop instructions

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