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Only set negative if operand has it set

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That is, don't consider A & operand for negative.
This commit is contained in:
Peter Evans 2018-01-21 16:12:03 -06:00
parent ecd8a7974a
commit 5d8403aaa7
1 changed files with 12 additions and 4 deletions
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16
src/mos6502.bits.c
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@ -51,10 +51,18 @@ DEFINE_INST(asl)
*/
DEFINE_INST(bit)
{
// We're just relying on the modify_status function to do negative
// and zero; we also need to do overflow, but overflow is checked in
// a slightly different way with BIT...
mos6502_modify_status(cpu, MOS_NZ, oper, cpu->A & oper);
// Zero is set if the accumulator AND the operand results in zero.
cpu->P &= ~MOS_ZERO;
if (!(cpu->A & oper)) {
cpu->P |= MOS_ZERO;
}
// But negative is set not by any operation on the accumulator; it
// is, rather, set by evaluating the operand itself.
cpu->P &= ~MOS_NEGATIVE;
if (oper & 0x80) {
cpu->P |= MOS_NEGATIVE;
}
// Normally, overflow is handled by checking if bit 7 flipped from 0
// to 1 or vice versa, and that's done by comparing the result to