3-3-2 Packed BCD Arithmetic

Packed BCD numbers are stored two digits to a byte, in 4-bit groups referred to as nibbles. The ALU is capable of performing only binary addition and subtraction, but by adjusting the sum or difference the correct result in packed BCD format can be obtained. The correction rule for addition is:

If the addition of any two digits results in a binary number between 1010 and 1111, which are not valid BCD digits, or there is a carry into the next digit, then 6 (0110) is to be added to the current digit.

For example:

Essentially, the rule is needed to "skip over" the six bit combinations that are unused by the BCD format whenever such a skip is warranted.

Figure 3-22 Packed BCD adjust instructions.

Name		Mnemonic and Format		Description
Decimal adjust for addition		DAA		(AL)Sum in AL adjusted to packed BCD format
Decimal adjust for subtraction		DAS		(AL)Difference in AL adjusted to packed BCD format
Flags: OF flag is undefined and all other condition flags are affected.
Addressing modes: Operates only on the AL register.

Figure 3-23 Packed BCD addition.

Instruction		Action		Contents of
				AL	CF	AF
MOV AL,BCD1		(AL)34		34	--	--
ADD AL,BCD2		(AL)34+89		BD	0	0
DAA		ADJUST		23	1	*
MOV BCD3,AL		(BCD3)23		23	1	*
MOV AL,BCD1+1		(AL)18		18	1	*
ADC AL,BCD2+1		(AL)18+27+(CF)		40	0	1
DAA		ADJUST		46	0	*
MOV BCD3+1,AL		(BCD3+1)46		46	0	*
				*Not important

Figure 3-24 Packed BCD subtraction.

Instruction		Action		Contents of
				AL	CF	AF
MOV AL,BCD1		(AL)34		34	--	--
SUB AL,BCD2		(AL)34-12		22	0	0
DAS		ADJUST		22	0	*
MOV BCD3,AL		(BCD3)22		22	0	*
MOV AL,BCD1+1		(AL)12		12	0	*
SBB AL,BCD2+1		(AL)12-46-(CF)		CC	1	1
DAS		ADJUST		66	1	*
MOV BCD3+1,AL		(BCD3+1)(AL)		66	1	*
				*Not important
Result = 5622 = -3378 in 10's complement

This is page 31.