The flags register instruction: status and control bits — 8086 assembly guide

Overflow, direction, interrupt, trap (single step), sign, zero, auxiliary carry, parity, carry.

The flags register is a 16-bit register in which nine bits have a meaning. Six are status flags: arithmetic and logic instructions set or clear them to describe their result, and conditional jumps test them. Three are control flags that change how the CPU behaves. The other bits hold no flag.

Bit	Flag	Set (1) when
0	CF carry	an unsigned result carried out of, or borrowed into, the top bit; also the last bit shifted out
2	PF parity	the low byte of the result has an even number of 1 bits
4	AF auxiliary carry	a carry out of, or borrow into, bit 3 (used by the decimal adjust instructions)
6	ZF zero	the result is 0
7	SF sign	the top bit of the result is 1 (negative if signed)
8	TF trap	control: INT 1 after each instruction (single step)
9	IF interrupt	control: maskable hardware interrupts are accepted
10	DF direction	control: string instructions count down instead of up
11	OF overflow	a signed result does not fit the destination

Prints 110: CF = 1, ZF = 1, SF = 0.

MOV     AL, 0FFh
ADD     AL, 1    ; FFh + 1 = 100h: AL = 0, a carry out
LAHF             ; AH = SF ZF - AF - PF - CF
MOV     BL, AH
MOV     AL, BL
AND     AL, 1    ; CF, bit 0
ADD     AL, '0'  ; digit -> character
MOV     DL, AL
MOV     AH, 02h  ; DOS: print DL
INT     21h
MOV     AL, BL
MOV     CL, 6
SHR     AL, CL
AND     AL, 1    ; ZF, bit 6
ADD     AL, '0'  ; digit -> character
MOV     DL, AL
MOV     AH, 02h  ; DOS: print DL
INT     21h
MOV     AL, BL
MOV     CL, 7
SHR     AL, CL   ; SF, bit 7
ADD     AL, '0'  ; digit -> character
MOV     DL, AL
MOV     AH, 02h  ; DOS: print DL
INT     21h

Who changes what

Instructions	Flags
MOV XCHG LEA PUSH POP, jumps, LOOP	none
ADD ADC SUB SBB CMP NEG	OF SF ZF AF PF CF
INC DEC	OF SF ZF AF PF; CF unchanged
AND OR XOR TEST	SF ZF PF; CF = OF = 0
MUL IMUL	CF OF; the others undefined
DIV IDIV	all undefined

Signed and unsigned

The CPU does not know whether a byte is signed. It sets both CF (meaningful for unsigned numbers) and OF (meaningful for signed ones), and you pick the jump that matches your data: JB / JA read CF, JL / JG read SF and OF. FFh + 1 sets CF, since 255 + 1 does not fit; 7Fh + 1 sets OF, since 127 + 1 is not a signed byte.

Setting flags directly

CLC / STC / CMC change CF, CLD / STD change DF, CLI / STI change IF. LAHF / SAHF copy the low byte to and from AH; PUSHF / POPF save and load all of them, and are the only way to change TF.

Later CPUs

The 80286 uses bits 12-14 for the I/O privilege level and the nested task flag. The 80386 widens the register to 32-bit EFLAGS with the resume and virtual-8086 flags; later models add more.