* Property of STI Page 1 of 19 Conditional Jump and Loop Instruction Computer Organization and Assembly Language The Jcc Instructions 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. Format: Jcc operand Action: If the specified condition cc is true, it will jump to the address specified in the operand; otherwise the next instruction is executed.
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* Property of STIPage 1 of 19
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.
* Property of STIPage 2 of 19
Conditional Jump and Loop Instruction
Computer Organization and Assembly Language
The Jcc Instructions
(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
* Property of STIPage 3 of 19
Conditional Jump and Loop Instruction
Computer Organization and Assembly Language
The Jcc Instructions
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
Condition“Jump if …”Mnemonic
Comparing Signed Numbers
* Property of STIPage 4 of 19
Conditional Jump and Loop Instruction
Computer Organization and Assembly Language
The Jcc Instructions
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
Condition“Jump if …”Mnemonic
Test Flags
* Property of STIPage 5 of 19
Conditional Jump and Loop Instruction
Computer Organization and Assembly Language
Example – Jcc Instruction
1. MOV AL, A5H
MOV BL, 73H
CMP AL, BL
JA L1
* Property of STIPage 6 of 19
Conditional Jump and Loop Instruction
Computer Organization and Assembly Language
Example – Jcc Instruction
2. MOV AL, A5H
MOV BL, 73H
CMP AL, BL
JG L1
* Property of STIPage 7 of 19
Conditional Jump and Loop Instruction
Computer Organization and Assembly Language
Example – Jcc Instruction
3. MOV AL, 00H
MOV CL, 03H
AGAIN: ADD AL, 02H
DEC CL
JNZ AGAIN
* Property of STIPage 8 of 19
Conditional Jump and Loop Instruction
Computer Organization and Assembly Language
Example – Jcc Instruction
4. MOV AL, 7FH
MOV BL, 75H
ADD AL, BL
JO NEXT
* Property of STIPage 9 of 19
Conditional Jump and Loop Instruction
Computer Organization and Assembly Language
Example – Jcc Instruction
5. MOV AL, 01H
DEC AL
JG L1
* Property of STIPage 10 of 19
Conditional Jump and Loop Instruction
Computer Organization and Assembly Language
Example – Jcc Instruction
6. Circumventing the limitation of the short-label:
MOV BX, 0002HMOV CX, 0002HCMP BX, CXJE L1
:::
L1: ::
* Property of STIPage 11 of 19
Conditional Jump and Loop Instruction
Computer Organization and Assembly Language
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.
* Property of STIPage 12 of 19
Conditional Jump and Loop Instruction
Computer Organization and Assembly Language
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):
What will be AX and CX after executing this program?
* Property of STIPage 13 of 19
Conditional Jump and Loop Instruction
Computer Organization and Assembly Language
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.
* Property of STIPage 14 of 19
Conditional Jump and Loop Instruction
Computer Organization and Assembly Language
Example - LOOP Instruction
MOV CX, 0003H
MOV AX, 0000H
L1: INC AX
LOOP L1
What is the value of AX and CX?
* Property of STIPage 15 of 19
Conditional Jump and Loop Instruction
Computer Organization and Assembly Language
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
Ø Action: CX [CX] - 1
If CX ≠ 0 and ZF = 1, then jump to the address specified by the operand; otherwise execute the instruction following the LOOPE/LOOPZ.
* Property of STIPage 16 of 19
Conditional Jump and Loop Instruction
Computer Organization and Assembly Language
Example - LOOPE/LOOPZ Instruction
MOV CX, 0005HMOV SI, FFFFHMOV AL, 01H
L1: INC SICMP BETA [SI], ALLOOPE L1
01H
01H
03H
01H
01HBETA:
* Property of STIPage 17 of 19
Conditional Jump and Loop Instruction
Computer Organization and Assembly Language
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
Ø Action: CX [CX] - 1
If CX ≠ 0 and ZF = 0, then jump to the address specified by the operand; otherwise execute the instruction following the LOOPNE/LOOPNZ.
* Property of STIPage 18 of 19
Conditional Jump and Loop Instruction
Computer Organization and Assembly Language
Example - LOOPNE/LOOPNZ Instruction
MOV CX, 0005HMOV SI, FFFFHMOV AL, 01H
L1: INC SICMP BETA [SI], ALLOOPNE L1
04H
07H
01H
05H
03HBETA:
* Property of STIPage 19 of 19
Conditional Jump and Loop Instruction
Computer Organization and Assembly Language
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?
