As an off-and-on side project, I’m working on a GameBoy emulator. I definitely still have a long way to go (still working on the CPU!) but I wanted to share a detail I felt deserved a more in-depth explanation: the DAA (“Decimal Adjust Accumulator”) instruction. This instruction is notorious for being a bit difficult to wrap your head around, but there is seemingly very little written about it.
In principle, this instruction is quite simple; if we perform an arithmetic instruction with two binary coded decimal (BCD) numbers, we should be able to get an output that is also in BCD.

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The DAA instruction is used in GameBoy programming to perform arithmetic with Binary Coded Decimal (BCD) numbers. BCD is a decimal number whose digits are encoded in binary. The DAA instruction corrects the result of an arithmetic operation to ensure it remains in BCD form. It applies rules such as adding a value of 6 when the unit digit is greater than 9, and adding a value of 60 when the result exceeds 99.

Demystifying the GameBoy/SM83’s DAA Instruction