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Rework overflow logic, and add missing handling

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This commit is contained in:
Peter Evans 2018-03-30 23:04:21 -05:00
parent 0ccabda71e
commit 6653da3c20
1 changed files with 27 additions and 3 deletions
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30
src/mos6502.arith.c
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@ -23,7 +23,14 @@ DEFINE_INST(adc)
MOS_CARRY_BIT();
vm_8bit result = cpu->A + oper + carry;
MOS_CHECK_NVZ(cpu->A, result);
MOS_CHECK_NZ(result);
cpu->P &= ~MOS_OVERFLOW;
if (((cpu->A ^ oper) & 0x80) &&
((cpu->A ^ result) & 0x80)
) {
cpu->P |= MOS_OVERFLOW;
}
// Carry has different meanings in different contexts... in ADC,
// carry is set if the result requires a ninth bit (the carry bit!)
@ -75,6 +82,13 @@ DEFINE_INST(adc_dec)
// are decimal in hexadecimal form.
vm_8bit result = ((modsum / 10) << 4) | (modsum % 10);
cpu->P &= ~MOS_OVERFLOW;
if (((cpu->A ^ sum) & 0x80) &&
((cpu->A ^ result) & 0x80)
) {
cpu->P |= MOS_OVERFLOW;
}
// As you can see, decimal comports a different meaning for the
// carry bit than its binary version
cpu->P &= ~MOS_CARRY;
@ -242,8 +256,8 @@ DEFINE_INST(sbc)
}
cpu->P &= ~MOS_OVERFLOW;
if ((cpu->A & 0x80) != (oper & 0x80) &&
(cpu->A & 0x80) != (result8 & 0x80)
if (((cpu->A ^ oper) & 0x80) &&
((cpu->A ^ result8) & 0x80)
) {
cpu->P |= MOS_OVERFLOW;
}
@ -298,5 +312,15 @@ DEFINE_INST(sbc_dec)
vm_8bit result = ((diff / 10) << 4) | (diff % 10);
MOS_CHECK_Z(result);
// This is a bit of an odd sequence... but overflow in decimal is a
// bit odd to begin with. I ended up replicating the behavior of
// AppleWin here.
cpu->P &= ~MOS_OVERFLOW;
if (((cpu->A ^ diff) & 0x80) &&
((cpu->A ^ result) & 0x80)
) {
cpu->P |= MOS_OVERFLOW;
}
cpu->A = result;
}