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Fix: Correct FILD emulation on platforms without long double #5760

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Fix: Correct FILD emulation on platforms without long double #5760

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30 changes: 24 additions & 6 deletions src/fpu/fpu_instructions.h
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Original file line number Diff line number Diff line change
Expand Up @@ -196,12 +196,30 @@ static void FPU_FLD_I64(PhysPt addr,Bitu store_to) {
FPU_Reg blah;
blah.l.lower = mem_readd(addr);
blah.l.upper = (int32_t)mem_readd(addr+4);
fpu.regs[store_to].d = static_cast<double>(blah.ll);
// store the signed 64-bit integer in the 80-bit format mantissa with faked exponent.
// this is needed for DOS and Windows games that use the Pentium fast memcpy trick, using FLD/FST to copy 64 bits at a time.
// I wonder if that trick is what helped spur Intel to make the MMX extensions :)
fpu.regs_80[store_to].raw.l = (uint64_t)blah.ll;
fpu.regs_80[store_to].raw.h = ((blah.ll/*sign bit*/ >> (uint64_t)63) ? 0x8000u : 0x0000u) + FPU_Reg_80_exponent_bias + 63u; // FIXME: Verify this is correct!

// First, get a correct double representation.
double val_d = static_cast<double>(blah.ll);
fpu.regs[store_to].d = val_d;

// Now, create a correct 80-bit representation from the double.
// This logic is adapted from FPU_ST80.
FPU_Reg temp_reg;
temp_reg.d = val_d;

int64_t sign80 = (temp_reg.ll & ULONGTYPE(0x8000000000000000)) ? 1 : 0;
int64_t exp80 = temp_reg.ll & LONGTYPE(0x7ff0000000000000);
int64_t exp80final = (exp80 >> 52);
int64_t mant80 = temp_reg.ll & LONGTYPE(0x000fffffffffffff);
int64_t mant80final = (mant80 << 11);

if (val_d != 0.0) { // Zero is a special case
mant80final |= (int64_t)ULONGTYPE(0x8000000000000000); // Set the explicit integer bit
exp80final += (BIAS80 - BIAS64);
}

fpu.regs_80[store_to].raw.h = (static_cast<int16_t>(sign80) << 15) | static_cast<int16_t>(exp80final);
fpu.regs_80[store_to].raw.l = mant80final;

fpu.use80[store_to] = true;
}

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