meljun cortes flag control and jump instructions

* Property of STI of 21

Flag Control and Jump Instructions

Computer Organization and Assembly Language

Flag Control Instructions

Ø The 8086/8088 provides a group of instructions that directly affects the state of the flags.

v LAHF Instruction v SAHF Instructionv CLC Instructionv STC Instruction v CMC Instructionv PUSHF Instructionv POPF Instruction




The LAHF Instruction

Ø The LAHF (Load AH from flags) instruction copies the SF, ZF, AF, PF, and CF into bits b7, b6, b4, b2, and b0 respectively of register AH.

Ø Format: LAHFØ Action:

AH SF ZF x AF x PF x CF




The SAHF Instruction

Ø The SAHF (Store AH into flags) instruction transfers bits b7, b6, b4, b2, and b0 from register AH into SF, ZF, AF, PF, and CF, respectively.

Ø Format: SAHFØ Action:

SF ZF x AF x PF x CF AH




The CLC Instruction

Ø The CLC (Clear Carry Flag) instruction clears the carry flag.

Ø Format: CLCØ Action: CF 0




The STC Instruction

Ø The STC (Set Carry Flag) instruction sets the carry flag to 1.

Ø Format: STCØ Action: CF 1




The CMC Instruction

Ø The CMC (Complement Carry Flag) instruction complements the carry flag.

Ø Format: CMCØ Action: CF [CF]




The PUSHF Instruction

Ø The PUSHF (Push Status Register onto Stack) instruction pushes the status register onto the stack. PUSHF decrements SP by 2 and then moves the status register to the top of the stack.

Ø Format: PUSHFØ Action: SP SP - 2

SP [status register]




The POPF Instruction

Ø The POPF (Pop Flags off the Stack) instruction transfers the word pointed to by SP to the status register, thereby altering the value of the flags.

Ø Format: POPFØ Action:

status register [SP]SP SP + 2




The CLI Instruction

Ø The CLI (Clear Interrupt Flag) instruction clears the interrupt flag, thereby disabling interrupt request. The 8086/8088 will therefore not recognize any external interrupt request with the exception of the non-maskable interrupt.

Ø Format: CLIØ Action: IF 0




The STI Instruction

Ø The STI (Set Interrupt Flag) instruction sets the interrupt flag to 1, thereby enabling the 8086/8088 to recognize any interrupt request.

Ø Format: STIØ Action: IF 1




The CLD Instruction

Ø The CLD (Clear Direction Flag) instruction clears the direction flag, thereby causing the string instruction to auto-increment the index registers (SI or DI).

Ø Format: CLDØ Action: DF 0




The STD Instruction

Ø The STD (Set Direction Flag) instruction sets the direction flag to 1, thereby causing the string instruction to auto-decrement the index registers (SI or DI).

Ø Format: STDØ Action: DF 1




Control Transfer Instructions

Ø The code segment and IP keep track of the next instruction to be executed. This means that programs are executed sequentially.

Ø The instructions that can alter the flow of a program are iteration control instructions, conditional transfers, unconditional transfers, and subroutines.




The JMP Instruction

Ø The JMP (Unconditional Jump) instruction unconditionally transfers control to the target location.

Ø Format: JMP operandØ Action: Jump is initiated to the

address specified by the operand.

JMP far [BX]Memptr32

JMP [BX]Memptr16

JMP DXRegptr16

JMP far L1Far-label

JMP L1Near-label

JMP short L1Short-label

ExampleOperand




Example – JMP Instruction

1. Short-label

MOV AX, 0001HJMP short L1MOV BX, 0002H

L1: MOV BX, 0001H





2. Far-LabelMOV AX, 0001HJMP far L1MOV BX, 0002HMOV BX, 0001H

.

.

.L1: MOV BX, 0003H





3. RegPtr16

MOV BX, 0000HJMP BX

4. MemPtr16

MOV BX, 0000HJMP [BX]

01H

00HDS:0000H

DS:0001H





5. MemPtr32

MOV BX, 0000HJMP far [BX]

02H

00HDS:0000H

DS:0001H

01H

00HDS:0002H

DS:0003H




The CALL Instruction

Ø The CALL (Subroutine Call) instruction transfers control to a subroutine.

Ø Format: CALL operandØ Action: Execution continues

from the address of the subroutine specified by the operand.

CALL far [BX]Memptr32

CALL [BX]Memptr16

CALL DXRegptr16

CALL far L1Far-proc

CALL L1Near-proc

ExampleOperand




The RET Instruction

Ø The RET (Return) instruction transfers control from a subroutine back to the instruction following the CALL instruction.

Ø Format: RET operand RET

Ø Action: Transfer control from a subroutine back to the instruction following the CALL instruction.

RET 2Disp16

RETNone

ExampleOperand




Program Tracing Exercise

MOV AX, 0000HL1: INC AX

CMP AX, 0098HJZ L2JMP short L1

L2: SUB AL, BL

1. How many times will the CMP AL, BL instruction be executed?

2. What will be the final contents of AL and BL after the program has finished executing?

meljun cortes flag control and jump instructions

Technology