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Intel Pentium CPU Instruction Set Reference

AAS instruction - ASCII Adjust AL After Subtraction

Code Mnemonic Description
3F AAS ASCII adjust AL after subtraction


Adjusts the result of the subtraction of two unpacked BCD values to create a unpacked BCD result. The AL register is the implied source and destination operand for this instruction. The AAS instruction is only useful when it follows a SUB instruction that subtracts (binary subtraction) one unpacked BCD value from another and stores a byte result in the AL register. The AAA instruction then adjusts the contents of the AL register to contain the correct 1-digit unpacked BCD result.

If the subtraction produced a decimal carry, the AH register is decremented by 1, and the CF and AF flags are set. If no decimal carry occurred, the CF and AF flags are cleared, and the AH register is unchanged. In either case, the AL register is left with its top nibble set to 0.

Operands Bytes Clocks
  1 3 NP


ID unaffected DF unaffected
VIP unaffected IF unaffected
VIF unaffected TF unaffected
AC unaffected SF undefined
VM unaffected ZF undefined
RF unaffected AF sets to 1 if there is a decimal borrow; otherwise, it's cleared to 0
NT unaffected PF undefined
IOPL unaffected CF sets to 1 if there is a decimal borrow; otherwise, it's cleared to 0
OF undefined