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
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Flag |
O
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D
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I
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S
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Z
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A
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P
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C
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Result |
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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
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Clocks |
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Operands |
286 |
386 |
486 |
Size Bytes |
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2 |
2 |
3 |
1 |
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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 |
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Control Flags |
Status Flags |
T |
D |
I |
O |
S |
Z |
A |
P |
C |
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Trap |
Direction |
Interrupt
-enable |
Overflow |
Sign |
Zero |
Auxiliary
Carry |
Parity |
Carry |
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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
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†The TEST instruction performs an implied AND operation between each pair of matching bits in two operands and sets the flags accordingly.
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― Unknown
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