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Change carry flag strategy according to docs

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This commit is contained in:
Peter Evans 2018-01-21 12:47:36 -06:00
parent 5f93bdae25
commit f9a95827f5
1 changed files with 20 additions and 3 deletions
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23
src/mos6502.arith.c
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@ -32,7 +32,14 @@ DEFINE_INST(adc)
*/
DEFINE_INST(cmp)
{
mos6502_modify_status(cpu, MOS_NZC, cpu->A, cpu->A - oper);
mos6502_modify_status(cpu, MOS_NZ, cpu->A, cpu->A - oper);
// Carry works slightly different with the cmp-style instructions;
// it's set if A >= oper.
cpu->P &= ~MOS_CARRY;
if (cpu->A >= oper) {
cpu->P |= MOS_CARRY;
}
}
/*
@ -41,7 +48,12 @@ DEFINE_INST(cmp)
*/
DEFINE_INST(cpx)
{
mos6502_modify_status(cpu, MOS_NZC, cpu->X, cpu->X - oper);
mos6502_modify_status(cpu, MOS_NZ, cpu->X, cpu->X - oper);
cpu->P &= ~MOS_CARRY;
if (cpu->X >= oper) {
cpu->P |= MOS_CARRY;
}
}
/*
@ -50,7 +62,12 @@ DEFINE_INST(cpx)
*/
DEFINE_INST(cpy)
{
mos6502_modify_status(cpu, MOS_NZC, cpu->Y, cpu->Y - oper);
mos6502_modify_status(cpu, MOS_NZ, cpu->Y, cpu->Y - oper);
cpu->P &= ~MOS_CARRY;
if (cpu->Y >= oper) {
cpu->P |= MOS_CARRY;
}
}
/*