AMD Am186 Series, Instruction Set

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Instruction Set 4-8
AAS ASCII Adjust AL After Subtraction AAS
What It Does
AAS converts an 8-bit unsigned binary number that is the difference of two unpacked
decimal (BCD) numbers to its unpacked decimal equivalent.
Syntax
Description
Use AAS only after a SUB or SBB instruction that leaves the byte result in AL. The lower
nibbles of the operands of the SUB or SBB instruction must be in the range 0–9 (BCD).
AAS adjusts AL so that it contains the correct decimal result. If the subtraction produced a
decimal borrow, AAS decrements AH and sets CF and AF. If there is no decimal borrow,
AAS clears CF and AF and leaves AH unchanged. AAS sets the top nibble of AL to 0.
Operation It Performs
Flag Settings After Instruction
Form Opcode Description
Clocks
Am186 Am188
AAS 3F ASCII-adjust AL after subtraction 7 7
AAS
if (((AL = AL & 0x0F) > 9) || (AF == 1))
/* AL is not yet decimal */
/* (note high nibble of AL is cleared either way */
{
/* convert AL to decimal and unpack */
AL = (AL - 6) & 0x0F;
AH = AH - 1;
/* set carry flags */
CF = AF = 1;
}
else
/* clear carry flags */
CF = AF = 0;
15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
Processor Status
Flags Register
reserved
OF DF IF TF SF ZF AF PF CF
? = undefined; – = unchanged
? – – – ? ? res res ? res
AF=1 if carry or borrow to low nibble
AF=0 otherwise CF=1 for carry or borrow to high-order bit
CF=0 otherwise