Decimal Arithmetic Instructions

Decimal Adjust AL after Addition (daa)

daa

Operation

decimal-adjust AL -> AL

Description

Use daa only after executing the form of an add instruction that stores a two-BCD-digit byte result in the AL register. daa then adjusts AL to a two-digit packed decimal result.

Example

Decimal adjust the two-BCD-digit in the AL register:

daa

Decimal Adjust AL after Subtraction (das)

das

Operation

decimal-adjust AL -> AL

Description

Use das only after executing the form of a sub instruction that stores a two-BCD-digit byte result in the AL register. das then adjusts AL to a two-digit packed decimal result.

Example

Decimal adjust the two-BCD-digit in the AL register:

das

ASCII Adjust after Addition (aaa)

aaa

Operation

ASCII-adjust AL -> AL

Description

You use aaa only after executing the form of an add instruction that stores a two-BCD-digit byte result in the AL register. aaa then adjusts AL to contain the correct decimal result. The top nibble of AL is set to 0. To convert AL to an ASCII result, follow the aaa instruction with:

or %al, 0x30

Table 2-8 shows how aaa handles a carry.

Table 2-8 aaa Handling a Carry

Carry	Action
decimal carry	AH + 1; CF and AF set to 1
no decimal carry	AH unchanged; CF and AF cleared to 0

Example

Adjust the AL register to contain the correct decimal result after an add instruction that stores a two-BCD-digit byte.

aaa

ASCII Adjust after Subtraction (aas)

aas

Operation

ASCII-adjust AL -> AL

Description

Use aas only after executing the form of an add instruction that stores a two-BCD-digit byte result in the AL register. aas then adjusts AL to contain the correct decimal result. The top nibble of AL is set to 0. To convert AL to an ASCII result, follow the aas instruction with:

or %al, 0x30

Table 2-9 shows how aas handles a carry.

Table 2-9 How aas Handles a Carry

Carry	Action
decimal carry	AH - 1; CF and AF set to 1
no decimal carry	AH unchanged; CF and AF cleared to 0

Example

Adjust the AL register to contain the correct decimal result after a sub instruction that stores a two-BCD-digit byte

aas

ASCII Adjust AX after Multiply (aam)

aam

Operation

AL 10 -> AH mod 10 AL -> AL

Description

You use aam only after executing a mul instruction between two BCD digits (unpacked). mul stores the result in the AX register. The result is less than 100 so it can be contained in the AL register (the low byte of the AX register). aam unpacks the AL result by dividing AL by 10, stores the quotient (most-significant digit) in AH, and stores the remainder (least-significant digit) in AL.

Example

Adjust the AL register to contain the correct decimal result after a mul instruction between two BCD digits:

aam

ASCII Adjust AX before Division (aad)

aad

Operation

AL + (AH x 10) -> AL 0 -> AH

Description

aad prepares two unpacked BCD digits for a division operation that yields an unpacked result. The least-significant digit is in AL; the most-significant in AH.

aad prepares the AL and AH registers:

AL + (AH x 10) -> AL
 0 -> AH

AX is then equal to the binary equivalent of the original unpacked two-digit BCD number.

Example

Adjust the AL and AH registers for a division operation by setting the AX register equal to the original unpacked two-digit number:

aad