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Make SBC somewhat less broken.

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This commit is contained in:
Radosław Kujawa 2017-02-01 22:15:16 +01:00
parent 436810b79f
commit 8e732f3b94
1 changed files with 8 additions and 5 deletions
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13
src/emulation.c
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@ -800,7 +800,8 @@ emul_sbc(rk65c02emu_t *e, void *id, instruction_t *i)
arg = instruction_data_read_1(e, (instrdef_t *) id, i);
res = e->regs.A - arg;
if (e->regs.P & P_CARRY)
/* if the carry flag is NOT set then "borrow" */
if (!(e->regs.P & P_CARRY))
res--;
if ((e->regs.A ^ res) & (arg ^ res) & 0x80)
@ -808,11 +809,13 @@ emul_sbc(rk65c02emu_t *e, void *id, instruction_t *i)
else
e->regs.P &= ~P_SIGN_OVERFLOW;
/* if the result does not fit into 8 bits then set carry */
if (res > 0xFF)
e->regs.P |= P_CARRY;
else
printf("res is %x\n", res);
/* if the result does not fit into 8 bits then clear carry */
if (res & 0x8000)
e->regs.P &= ~P_CARRY;
else
e->regs.P |= P_CARRY;
/* squash the result into accumulator's 8 bits, lol */
e->regs.A = (uint8_t) res;