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Simplify sa1 division

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Removes the branch and the double modulo

I have verified that it gives the same results for all 2^32 pairs of inputs
This commit is contained in:
Alcaro 2022-11-23 20:47:04 +01:00 committed by Screwtapello
parent 717606dda9
commit 43e42b2dca
1 changed files with 6 additions and 3 deletions
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9
bsnes/sfc/coprocessor/sa1/io.cpp
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@ -431,14 +431,17 @@ auto SA1::writeIOSA1(uint address, uint8 data) -> void {
mmio.mr = (uint32)((int16)mmio.ma * (int16)mmio.mb);
mmio.mb = 0;
} else {
//unsigned division
//signed division
if(mmio.mb == 0) {
mmio.mr = 0;
} else {
int16 dividend = mmio.ma;
uint16 divisor = mmio.mb;
uint16 remainder = dividend >= 0 ? uint16(dividend % divisor) : uint16((dividend % divisor + divisor) % divisor);
uint16 quotient = (dividend - remainder) / divisor;
//sa1 division rounds toward negative infinity, but C division rounds toward zero
//adding divisor*65536 ensures it rounds down
uint32 dividend_ext = dividend + (uint32)divisor*65536;
uint16 remainder = dividend_ext % divisor;
uint16 quotient = dividend_ext / divisor - 65536;
mmio.mr = remainder << 16 | quotient;
}
mmio.ma = 0;