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Rewrite SBC logic
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.
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@ -225,18 +225,30 @@ DEFINE_INST(sbc)
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MOS_CARRY_BIT();
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vm_8bit result = cpu->A - oper - (carry ? 0 : 1);
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MOS_CHECK_NVZ(cpu->A, result);
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int result32 = cpu->A - oper - (carry ? 0 : 1);
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vm_8bit result8 = cpu->A - oper - (carry ? 0 : 1);
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MOS_CHECK_Z(result8);
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// Carry is handled slightly differently in SBC; it's set if the
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// value is non-negative, and unset if negative. (It's essentially a
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// mirror of the N flag in that sense.)
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cpu->P |= MOS_CARRY;
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if (result >= 0x80) {
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cpu->P &= ~MOS_NEGATIVE;
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if (result32 < 0) {
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cpu->P &= ~MOS_CARRY;
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cpu->P |= MOS_NEGATIVE;
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}
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cpu->A = result;
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cpu->P &= ~MOS_OVERFLOW;
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if ((cpu->A & 0x80) != (oper & 0x80) &&
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(cpu->A & 0x80) != (result8 & 0x80)
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) {
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cpu->P |= MOS_OVERFLOW;
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}
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cpu->A = result8;
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}
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/*
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