Explain the DAA instruction algorithm

[Rangi42]

Fixes #1562

Refer to bloglien and SameBoy for DAA explanations. (I followed SameBoy's algorithm here; it's more verbose but also more obvious in the add/subtract distinction.)

[nummacway]

I propose adding that A and the value added to/substracted from it must have been BCD for doing the previous instruction.

Decimal Adjust Accumulator to restore a [or: the] correct BCD representation [of the value in register A] after adding or substracting a BCD value to/from a BCD value in A [or: addition or substraction of two BCD values].

Explicitly naming additions and substractions also excludes CP, which is listed as an arithmethic instruction by the same document.

[Rangi42]

There is no such requirement to use DAA with BCD values, nor to use it only after ADD/SUB/ADC/SBC instructions. That's the obvious/intended use case, but there are others.

[ISSOtm]

Thank you for this write-up!

Resolved

[ISSOtm]

Thank you!