AMD Am186 Series, Instruction Set

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Instruction Set 4-45
DAS Decimal Adjust AL After Subtraction DAS
What It Does
DAS converts an 8-bit unsigned binary number that is the difference of two single-byte
packed decimal (BCD) numbers to its packed decimal equivalent.
Syntax
Description
Execute DAS only after a SUB or SBB instruction that leaves a two-BCD-digit byte result
in the AL register. The SUB or SBB operands should consist of two packed BCD digits.
DAS adjusts the AL register to contain the correct packed two-digit decimal result.
Operation It Performs
Form Opcode Description
Clocks
Am186 Am188
DAS 2F Decimal-adjust AL after subtraction 4 4
DAS
if (((AL & 0x0F) > 9) || (AF == 1))
/* low nibble of AL is not yet in BCD format */
{
/* convert low nibble of AL to decimal */
AL = AL - 6;
/* set auxiliary (decimal) carry flag */
AF = 1;
}
else
/* clear auxiliary (decimal) carry flag */
AF = 0;
if ((AL > 0x9F) || (CF == 1))
/* high nibble of AL is not yet in BCD format */
{
/* convert high nibble of AL to decimal */
AL = AL - 0x60;
/* set carry flag */
CF = 1;
}
else
/* clear carry flag */
CF = 0;