Microprocessor MA Rahim Khan Computer Engineering and Networks Department

Flag Register Carry Flag (CF) - this flag is set to 1 when there is an unsigned overflow. For example when you add bytes (result is not in range ). When there is no overflow this flag is set to 0. Zero Flag (ZF) - set to 1 when result is zero. For none zero result this flag is set to 0. Sign Flag (SF) - set to 1 when result is negative. When result is positive it is set to 0. Actually this flag take the value of the most significant bit. Overflow Flag (OF) - set to 1 when there is a signed overflow. For example, when you add bytes (result is not in range ).

Flag Register Cont…. Parity Flag (PF) - this flag is set to 1 when there is even number of one bits in result, and to 0 when there is odd number of one bits. Even if result is a word only 8 low bits are analyzed! Auxiliary Flag (AF) - set to 1 when there is an unsigned overflow for low nibble (4 bits). Interrupt enable Flag (IF) - when this flag is set to 1 CPU reacts to interrupts from external devices. Direction Flag (DF) - this flag is used by some instructions to process data chains, when this flag is set to 0 - the processing is done forward, when this flag is set to 1 the processing is done backward.

Carry Flag include 'emu8086.inc' ORG 100h MOV AL, 255 ADD AL, 1 JC label1 PRINT 'no carry.' JMP exit label1: PRINT 'has carry.' exit: RET

Jump if CX Register is Zero include 'emu8086.inc' ORG 100h MOV CX, 0 JCXZ label1 PRINT 'CX is not zero.' JMP exit label1: PRINT 'CX is zero.' exit: RET

Jump if Parity Even include 'emu8086.inc' #make_com# ORG 100h MOV AL, b ; AL = 5 AND AL, B ; just set flags. JP label1 PRINT 'parity odd.' JMP exit label1: PRINT 'parity even.' exit: RET

Jump if Parity Even include 'emu8086.inc' ORG 100h MOV AL, b ; AL = 5 OR AL, 0 ; just set flags. JPE label1 PRINT 'parity odd.' JMP exit label1: PRINT 'parity even.' exit: RET

Jump if Parity Odd. include 'emu8086.inc' ORG 100h MOV AL, b ; AL = 7 OR AL, 0 ; just set flags. JPO label1 PRINT 'parity even.' JMP exit label1: PRINT 'parity odd.' exit: RET

Jump if Signed include 'emu8086.inc' ORG 100h MOV AL, b ; AL = -128 OR AL, 0 ; just set flags. JS label1 PRINT 'not signed.' JMP exit label1: PRINT 'signed.' exit: RET

Jump if Not Signed ORG 100h MOV AL, b ; AL = 7 OR AL, 0 ; just set flags. JNS label1 PRINT 'signed.' JMP exit label1: PRINT 'not signed.' exit: RET

Complement Carry flag(CMC) if CF = 1 then CF = 0 if CF = 0 then CF = 1 No operands

Clear Carry flag(CLC) Clear Carry flag. Algorithm: CF = 0 No operands

Signed divide.(IDIV) Signed divide. Algorithm: when operand is a byte: AL = AX / operand AH = remainder (modulus)when operand is a word: AX = (DX AX) / operand DX = remainder (modulus) Example: MOV AX, -203 ; AX = 0FF35h MOV BL, 4 IDIV BL ; AL = -50 (0CEh), AH = -3 (0FDh) RET

Signed Multiply when operand is a byte: AX = AL * operand. when operand is a word: (DX AX) = AX * operand. Example: MOV AL, -2 MOV BL, -4 IMUL BL ; AX = 8 RET