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Invalid money division discrepancy: Difference between revisions
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===Explanation===
There is no BCD instruction for halving (or shifting) a number in hardware; the game must manually perform long division.<ref>https://github.com/pret/pokered/blob/master/engine/math/bcd.asm</ref> Long division naturally has subtractions inside.
In order to perform BCD addition/subtraction, the code first performs regular addition/subtraction in hexadecimal, then relies on the '''DAA''' (Decimal Adjust register A) hardware instruction. This instruction automatically adds/subtracts hex:6 or hex:60 from a byte if the previous addition/subtraction resulted in an invalid digit or involved a carry/borrow. For example:
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* hex:10 minus hex:1 normally yields hex:F, but it will also report a borrow. The DAA instruction may ''either'' detect the borrow or notice the invalid digit, and subtract another hex:6 to yield hex:9.
Note that in the last case, there is technically no need to look for invalid digits when subtracting, because the only way to get them is through a borrow.
Finally, the
* In order to perform hex:1000 minus hex:222, the code must first subtract the rightmost bytes (hex:00 minus hex:22 yielding hex:DE). DAA then turns this into hex:78, then it reports a carry/borrow.
* The code then subtracts the leftmost bytes (hex:10 minus hex:2 yielding hex:E), then 1 is automatically subtracted due to the above "subtract with carry" reporting a borrow, then DAA turns the hex:D into hex:7.
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