Slide 6.6: LAHF instruction

Slide 6.5: MOVSX instruction
Slide 6.7: SAHF instruction
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LAHF Instruction

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

LAHF — Load Register AH from Flags

Usage: LAHF



         
          
           Flag
           O
           D
           I
           S
           Z
           A
           P
           C
          

          
           Result
            
            
            
            
            
            
            
            
          

         
         



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


      AH := S Z ? A ? P ? C

	Clocks
Operands	286	386	486	Size Bytes
	2	2	3	1

The EFLAGS register is a 32-bit register. Nine of its bits are shown below. Use this instruction, you can save a copy of the flags in a memory variable or test several flags at once.

`EFLAGS` Register
Control Flags			Status Flags
`T`	`D`	`I`	`O`	`S`	`Z`	`A`	`P`	`C`
Trap	Direction	Interrupt -enable	Overflow	Sign	Zero	Auxiliary Carry	Parity	Carry

Checking for Zero Bit	Output
.code sub eax, eax lahf test ah, 01000000b jnz L1 mov eax, 0 ; not zero jmp L2 L1: mov eax, 1 ; zero L2: call WriteInt

^†The TEST instruction performs an implied AND operation between each pair of matching bits in two operands and sets the flags accordingly.