Z80 assembly: BCD numbers and DAA

Add packed decimal digits. Use DAA to adjust the binary result into BCD form.

DAA

BCD means Binary-Coded Decimal. Each decimal digit uses four bits. For example, 45 decimal is 45h in packed BCD. Normal binary addition can make invalid BCD digits. DAA means Decimal Adjust Accumulator. It adjusts A after an ADD or SUB operation.

DAA takes 4 T-states. It uses the N, H and C flags from the previous arithmetic instruction. It then updates S, Z, H, P/V and C. N keeps its previous value.

Run speed

What happened

Program listing

Address	Machine code	Label	Instruction	Comment

The first calculation adds packed BCD 45h and 38h. The second subtracts 38h from 45h. Step through each arithmetic instruction and DAA.

Registers and flags

Flags

DAA reads H, N and C from the previous operation. It adjusts the low digit by 6 when needed and the high digit by 60h when needed. This page ignores undocumented X and Y bits.

Program bytes

Execution trace

#	Address	Instruction	Result	T-states

BCD and binary values differ

Bits	As binary	As packed BCD
45h	69 decimal	45 decimal
38h	56 decimal	38 decimal
7Dh	125 decimal	Invalid BCD digits
83h	131 decimal	83 decimal

45h + 38h first produces binary 7Dh. DAA adjusts it to 83h, the packed BCD form of decimal 83.

DAA is used after an 8-bit ADD or SUB operation on packed BCD values. The program does not validate that the input digits are valid BCD.

Mnemonic glossary

Program mnemonics and directives

Name	Stands for	Meaning here
LD	Load	Loads the first packed BCD number into A.
ADD	Add	Adds packed BCD values as binary.
SUB	Subtract	Subtracts packed BCD values as binary.
DAA	Decimal Adjust Accumulator	Adjusts the result to packed BCD.
HALT	Halt	Stops after both examples.
ORG	Origin	Assembler directive. Sets the code address.