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Also check the N flag (i.e. set it to zero)

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Peter Evans 2018-02-15 13:01:58 -06:00
parent e415b3e490
commit 067c0cea2f
1 changed files with 5 additions and 2 deletions
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7
src/mos6502.bits.c
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@ -97,8 +97,11 @@ DEFINE_INST(lsr)
{ {
SET_RESULT(oper >> 1); SET_RESULT(oper >> 1);
// Set ZERO if it should apply // Set ZERO if it should apply; also inspect the NEGATIVE flag, but
mos6502_modify_status(cpu, MOS_ZERO, oper, result); // note that doing so will have the effect of _always_ unsetting the
// N flag, because literally any byte value shifted right will
// immediately lose the sign bit and be non-negative.
mos6502_modify_status(cpu, MOS_NZ, oper, result);
// However, we handle carry a bit differently here. The carry bit // However, we handle carry a bit differently here. The carry bit
// should be 1 if oper & 0x1; that is, when we shift right, we want // should be 1 if oper & 0x1; that is, when we shift right, we want