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Use 9-bit rotation, not 8-bit

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This commit is contained in:
Peter Evans 2018-02-15 19:32:01 -06:00
parent 067c0cea2f
commit 058b45e7d1
1 changed files with 12 additions and 2 deletions
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14
src/mos6502.bits.c
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@ -141,13 +141,19 @@ DEFINE_INST(rol)
MOS_CARRY_BIT(); MOS_CARRY_BIT();
SET_RESULT(oper << 1); SET_RESULT(oper << 1);
// Rotations are effectively _9-bit_. So we aren't rotating bit 7
// into bit 0; we're rotating bit 7 into the carry bit, and we're
// rotating the _previous value of the carry bit_ into bit 0.
if (cpu->P & MOS_CARRY) {
result |= 0x1;
}
if (oper & 0x80) { if (oper & 0x80) {
carry = 1; carry = 1;
} }
cpu->P &= ~MOS_CARRY; cpu->P &= ~MOS_CARRY;
if (carry) { if (carry) {
result |= 0x01;
cpu->P |= MOS_CARRY; cpu->P |= MOS_CARRY;
} }
@ -169,13 +175,17 @@ DEFINE_INST(ror)
MOS_CARRY_BIT(); MOS_CARRY_BIT();
SET_RESULT(oper >> 1); SET_RESULT(oper >> 1);
// See the code for ROL for my note on 9-bit rotation (vs. 8-bit).
if (cpu->P & MOS_CARRY) {
result |= 0x80;
}
if (oper & 0x01) { if (oper & 0x01) {
carry = 1; carry = 1;
} }
cpu->P &= ~MOS_CARRY; cpu->P &= ~MOS_CARRY;
if (carry) { if (carry) {
result |= 0x80;
cpu->P |= MOS_CARRY; cpu->P |= MOS_CARRY;
} }