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Rewrite SBC logic

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The carry handling was incorrect; carry is set if we went below zero,
but bit 7 is not enough to determine that. It follows that negative
handling needed to be modified accordingly. Overflow handling was also
incorrect.
This commit is contained in:
Peter Evans 2018-03-30 20:07:24 -05:00
parent 07a8a385f5
commit 0ccabda71e
1 changed files with 16 additions and 4 deletions
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20
src/mos6502.arith.c
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@ -225,18 +225,30 @@ DEFINE_INST(sbc)
MOS_CARRY_BIT();
vm_8bit result = cpu->A - oper - (carry ? 0 : 1);
MOS_CHECK_NVZ(cpu->A, result);
int result32 = cpu->A - oper - (carry ? 0 : 1);
vm_8bit result8 = cpu->A - oper - (carry ? 0 : 1);
MOS_CHECK_Z(result8);
// Carry is handled slightly differently in SBC; it's set if the
// value is non-negative, and unset if negative. (It's essentially a
// mirror of the N flag in that sense.)
cpu->P |= MOS_CARRY;
if (result >= 0x80) {
cpu->P &= ~MOS_NEGATIVE;
if (result32 < 0) {
cpu->P &= ~MOS_CARRY;
cpu->P |= MOS_NEGATIVE;
}
cpu->A = result;
cpu->P &= ~MOS_OVERFLOW;
if ((cpu->A & 0x80) != (oper & 0x80) &&
(cpu->A & 0x80) != (result8 & 0x80)
) {
cpu->P |= MOS_OVERFLOW;
}
cpu->A = result8;
}
/*