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Added decimal mode for ADC and SBC

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This commit is contained in:
Rob McMullen 2017-12-19 17:25:35 -08:00
parent a25ad398e8
commit 20d6e48c9f
1 changed files with 22 additions and 6 deletions
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26
6502.c
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@ -64,7 +64,13 @@ static inline void take_branch()
static void inst_ADC()
{
uint8_t operand = * read_ptr();
int tmp = A + operand + (SR.bits.carry & 1);
unsigned int tmp = A + operand + (SR.bits.carry & 1);
if (SR.bits.decimal) {
tmp = (A & 0x0f) + (operand & 0x0f) + (SR.bits.carry & 1);
if (tmp >= 10) tmp = (tmp - 10) | 0x10;
tmp += (A & 0xf0) + (operand & 0xf0);
if (tmp > 0x9f) tmp += 0x60;
}
SR.bits.carry = tmp > 0xFF;
SR.bits.overflow = ((A^tmp)&(operand^tmp)&0x80) != 0;
A = tmp & 0xFF;
@ -403,11 +409,21 @@ static void inst_RTS()
static void inst_SBC()
{
uint8_t operand = ~(* read_ptr()); // identical to ACD with the operand inverted
int tmp = A + operand + (SR.bits.carry & 1);
SR.bits.carry = tmp > 0xFF;
SR.bits.overflow = ((A^tmp)&(operand^tmp)&0x80) != 0;
uint8_t operand = * read_ptr();
unsigned int tmp, lo, hi;
tmp = A - operand - 1 + (SR.bits.carry & 1);
SR.bits.overflow = ((A^tmp)&(A^operand)&0x80) != 0;
if (SR.bits.decimal) {
lo = (A & 0x0f) - (operand & 0x0f) - 1 + SR.bits.carry;
hi = (A >> 4) - (operand >> 4);
if (lo & 0x10) lo -= 6, hi--;
if (hi & 0x10) hi -= 6;
A = (hi << 4) | (lo & 0x0f);
}
else {
A = tmp & 0xFF;
}
SR.bits.carry = tmp < 0x100;
N_flag(A);
Z_flag(A);
}