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Flag control

The document discusses various flag control and flow control instructions in the 8086/8088 architecture. It describes instructions like LAHF, SAHF, CLC, STC, CMC, PUSHF, and POPF that directly affect the status of flags. It also explains conditional jump instructions, loop instructions, and how to implement control structures like IF-THEN, IF-THEN-ELSE, CASE, and loops using these instructions. Examples are provided to illustrate different branching scenarios and loop implementations.

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Flag Control Objective: • Identify the different flag control instruction • Apply the different flag control instructions
Flag Control Instructions The 8086/8088 provides a group of instructions that directly affects the state of the flags. • LAHF Instruction • SAHF Instruction • CLC Instruction • STC Instruction • CMC Instruction • PUSHF Instruction • POPF Instruction
The LAHF Instruction  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. Format: Action: AH LAHF SF x ZF x AF x PF x CF
The SAHF Instruction 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. SAHFFormat: Action: SF ZF x AF x PF x CF AH Example: MOV AH, 25H SAHF
The CLC Instruction  The CLC (Clear Carry Flag) instruction clears the carry flag.  Format: CLC  Action: CF 0
The STC Instruction The STC (Set Carry Flag) instruction sets the carry flag to 1. Format: STC Action: CF 1
The CMC Instruction  The CMC (Complement Carry Flag) instruction complements the carry flag.  Format: CMC  Action: CF CF’
Status & Flags Register • Carry flag (CF): CF=1 if there is – a carry out from most significant bit (msb) on addition – a borrow into msb on subtraction – CF also affected differently by shift and rotate instructions • Parity flag (PF): PF=1 if – low byte of result has an even number of one bits (even parity) 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 CFPFAFZFSFOF IFDF
Status & Flags Register - Cont. • Auxiliary carry flag (AF): AF=1 if there is – a carry out from bit 3 on addition – a borrow into bit 3 on subtraction • Zero flag (ZF): ZF=1 – if the result is zero • Sign flag (SF): SF=1 if – msb of result is 1 indicating that the result is negative for signed number interpretation • Overflow flag (OF): OF=1 – if signed overflow occurs
The PUSHF Instruction  The PUSHF (Push Status Register onto Stack) instruction pushes the flag register onto the stack. PUSHF decrements SP by 2 and then moves the flag register to the top of the stack.  Format: PUSHF  Action: SP SP - 2 SP [flag register]
The POPF Instruction  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. POPF  Format: Action: status register SP [SP] SP + 2
Stack Instructions • SP points at the the top of the stack • .STACK 100H – SP is initialized to 100H • PUSH operand – SP  SP - 2 – [SP+1:SP]  operand • POP operand – Operand  [SP+1:SP] – SP  SP + 2 • PUSHF – SP  SP - 2 – [SP+1:SP]  flags register • POPF – Flags register  [SP+1:SP] – SP  SP + 2
The CLI Instruction  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.  Format: CLI  Action: IF 0
The STI Instruction  The STI (Set Interrupt Flag) instruction sets the interrupt flag to 1, thereby enabling the 8086/8088 to recognize any interrupt request.  Format: STI  Action: IF 1
The CLD Instruction  The CLD (Clear Direction Flag) instruction clears the direction flag, thereby causing the string instruction to auto-increment the index registers (SI or DI).  Format: CLD  Action: DF 0
The STD Instruction  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).  Format: STD  Action: DF 1
Flow Control Instruction
Flow Control Instructions • Unconditional jump – JMP label ; IP  label • Conditional jump – Signed jumps – Unsigned jumps – Common jumps • Signed jumps – JG/JNLE jump if greater than, or jump if not less than or equal – JGE/JNL jump if greater than or equal, or jump if not less than – JL/JNGE jump if less than, or jump if not greater than or equal – JLE/JNG jump if less than or equal, or jump if not greater than
Flow Control Instructions
Flow Control Instructions
Flow Control Instructions • Single-flag jumps – JS jump if sign negative – JNS jump if nonnegative sign – JP/JPE jump if parity even – JNP/JPO jump if parity odd • Jump based on CX – JCXZ if CX = 0, then jump to the address specified by the operand • Loop Instructions – Loop short-label -loop decrement CX – Loopnz/Loopne -loop while not zero/loop while not equal – Loopz/Loope -loop while zero/loop while equal • All jump instructions have no effect on the flags.
Flow Control Instructions
Branching Structures: IF-Then • Example: If AX < 0 Then Replace AX by –AX ENDIF ; if AX < 0 CMP AX, 0 JNL END_IF ;then NEG AX END_IF:
IF-Then-Else • Example: If AL <= BL Then Display character in AL Else Display character in BL ENDIF MOV AH, 2 ; if AL<=BL CMP AL, BL JNBE ELSE_ ;then MOV DL, AL JMP DISPLAY ELSE_: MOV DL, BL DISPLAY: INT 21H END_IF:
CASE • Example: CASE AX <0: put –1 in BX =0: put 0 in BX >0: put 1 in BX END_CASE ; case AX CMP AX, 0 JL NEGATIVE JE ZERO JG POSITIVE NEGATIVE: MOV BX, -1 JMP END_CASE ZERO: MOV BX, 0 JMP END_CASE POSITIVE: MOV BX, 1 END_CASE:
CASE – Cont. • Example: CASE AL 1,3: display ‘o’ 2,4: display ‘e’ END_CASE ; case AL CMP AL, 1 ; 1, 3: JE ODD CMP AL, 3 JE ODD CMP AL, 2 ; 2, 4: JE EVEN CMP AL, 4 JE EVEN JMP END_CASE ODD: MOV DL, ‘o’ JMP DISPLAY EVEN: MOV DL, ‘e’ DISPLAY: MOV AH, 2 INT 21H END_CASE:
Branches with Compound Conditions • Example: If (‘A’ <= character) and (character <= ‘Z’) Then Display character END_IF ; read a character MOV AH, 1 INT 21H ; If (‘A’ <= character) and (character <= ‘Z’) Then CMP AL, ‘A’ JNGE END_IF CMP AL, ‘Z’ JNLE END_IF ; display character MOV DL, AL MOV AH, 2 INT 21H END_IF:
Branches with Compound Conditions • Example: If (character=‘y’) OR (character <= ‘Y’) Then Display character Else terminate program END_IF ; read a character MOV AH, 1 INT 21H ; If (character=‘y’) OR (character = ‘Y’) Then CMP AL, ‘y’ JE Then CMP AL, ‘Y’ JE Then JMP ELSE_ Then: MOV AH, 2 MOV DL, AL INT 21H JMP END_IF ELSE:MOV AH, 4CH INT 21H END_IF:
Loop Instructions • Loop Next – Dec Cx – If CX<>0 JMP Next • Loopz/loope Next – Dec Cx – If (CX<>0) AND (ZF=1) JMP Next • Loopnz/loopne Next – Dec Cx – If (CX<>0) AND (ZF=0) JMP Next
FOR LOOP • Example: For 80 times DO Display ‘*’ END_IF MOV CX, 80 MOV AH, 2 MOV DL, ‘*’ Next: INT 21H Loop Next
While Loop • Example: Initialize count to 0 Read a character While character <> Carriage Return DO Count = Count + 1 Read a character END_While MOV DX, 0 MOV AH, 1 INT 21H While_: CMP AL, 0DH JE End_While INC DX INT 21H JMP While_ End_While:
Repeat Loop • Example: Repeat Read a character Until character is blank MOV AH, 1 Repeat: INT 21H ; until CMP AL, ‘ ‘ JNE Repeat
Application of Loope • Example: Search for a number in a Table Table DB 1,2,3,4,5,6,7,8,9 XOR SI, SI MOV CX, 9 Next: INC SI CMP Table[SI-1], 7 Loopne Next