Intel 64 and IA-32 Architectures Software Developers Manual Volume 1, Basic Architecture

Vol. 1 7-13
PROGRAMMING WITH GENERAL-PURPOSE INSTRUCTIONS
(see Section 4.7, “BCD and Packed BCD Integers”). Adding two packed BCD values
requires two instructions: an ADD instruction followed by a DAA instruction. The ADD
instruction adds (binary addition) the two values and stores the result in the AL
register. The DAA instruction then adjusts the value in the AL register to obtain a
valid, 2-digit, packed BCD value and sets the CF flag if a decimal carry occurred as
the result of the addition.
Likewise, subtracting one packed BCD value from another requires a SUB instruction
followed by a DAS instruction. The SUB instruction subtracts (binary subtraction) one
BCD value from another and stores the result in the AL register. The DAS instruction
then adjusts the value in the AL register to obtain a valid, 2-digit, packed BCD value
and sets the CF flag if a decimal borrow occurred as the result of the subtraction.
7.3.3.2 Unpacked BCD Adjustment Instructions
The AAA (ASCII adjust after addition), AAS (ASCII adjust after subtraction), AAM
(ASCII adjust after multiplication), and AAD (ASCII adjust before division) instruc-
tions adjust the results of arithmetic operations performed in unpacked BCD
values (see Section 4.7, “BCD and Packed BCD Integers”). All these instructions
assume that the value to be adjusted is stored in the AL register or, in one instance,
the AL and AH registers.
The AAA instruction adjusts the contents of the AL register following the addition of
two unpacked BCD values. It converts the binary value in the AL register into a
decimal value and stores the result in the AL register in unpacked BCD format (the
decimal number is stored in the lower 4 bits of the register and the upper 4 bits are
cleared). If a decimal carry occurred as a result of the addition, the CF flag is set and
the contents of the AH register are incremented by 1.
The AAS instruction adjusts the contents of the AL register following the subtraction
of two unpacked BCD values. Here again, a binary value is converted into an
unpacked BCD value. If a borrow was required to complete the decimal subtract, the
CF flag is set and the contents of the AH register are decremented by 1.
The AAM instruction adjusts the contents of the AL register following a multiplication
of two unpacked BCD values. It converts the binary value in the AL register into a
decimal value and stores the least significant digit of the result in the AL register (in
unpacked BCD format) and the most significant digit, if there is one, in the AH
register (also in unpacked BCD format).
The AAD instruction adjusts a two-digit BCD value so that when the value is divided
with the DIV instruction, a valid unpacked BCD result is obtained. The instruction
converts the BCD value in registers AH (most significant digit) and AL (least signifi-
cant digit) into a binary value and stores the result in register AL. When the value in
AL is divided by an unpacked BCD value, the quotient and remainder will be automat-
ically encoded in unpacked BCD format.