Slide 6.7: SAHF instruction

Slide 6.6: LAHF instruction
Slide 6.8: XCHG instruction
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SAHF Instruction

The SAHF instruction (store AH into status flags) copies AH into the low byte of the EFLAGS register.

SAHF — Store AH Register into Flags

Usage: SAHF

Flag	O	D	I	S	Z	A	P	C
Result				*	*	*	*	*

Transfers bits 0-7 of AH into the flags register. This includes A, C, P, S, and Z.

    AH := S Z ? A ? P ? C

	Clocks
Operands	286	386	486	Size Bytes
	2	3	2	1

Setting Sign and Carry Flags	Output
.code mov ebx, 0 ; Clears ebx. mov ah, 10000001b ; Sets Sign and Carry flag. sahf ; Loads ah into flags. jnc L1 ; Jumps if not carry. inc ebx ; If carry, increases ebx. L1: jns L2 ; Jumps if not sign. inc ebx ; If sign, increases ebx. L2: mov eax, ebx ; Moves ebx to eax. call WriteInt ; Prints eax.

Setting Sign and Carry Flags

Output


      .code
      mov   ebx, 0             ; Clears ebx.
      mov   ah, 10000001b      ; Sets Sign and Carry flag.
      sahf                     ; Loads ah into flags.
      jnc   L1                 ; Jumps if not carry.
      inc   ebx                ; If carry, increases ebx.
 L1:  jns   L2                 ; Jumps if not sign.
      inc   ebx                ; If sign, increases ebx.
 L2:  mov   eax, ebx           ; Moves ebx to eax.
      call  WriteInt           ; Prints eax.

The following instructions will be covered in detail in Chapter 6.

test: It performs an implied and operation between each pair of matching bits in two operands and sets the flags accordingly. The only difference between test and and is the test does not modify the destination operand.
jnz: jumps if Zero flag is not set.
jnc: jumps if Carry flag is not set.
jns: jumps if Sign flag is not set.