BCD Support about olcnes HOT 8 OPEN

uint8_t olc6502::ADC()
{
	fetch();

	temp = (uint16_t)a + (uint16_t)fetched + (uint16_t)GetFlag(C);

	SetFlag(Z, !(temp & 0x00FF));

	if (GetFlag(D))
	{
		if ((((uint16_t)a & 0x0F) + ((uint16_t)fetched & 0x0F) + (uint16_t)GetFlag(C)) > 9) temp += 6;
		if (temp > 0x99) temp += 96;
		SetFlag(C, temp > 0x99);
	}
	else SetFlag(C, temp > 0xFF);

	SetFlag(N, temp & 0x80);
	SetFlag(V, !(((uint16_t)a ^ (uint16_t)fetched) & 0x0080) && (((uint16_t)a ^ temp) & 0x0080));
	a = temp & 0x00FF;
	return 1;
}


uint8_t olc6502::SBC()
{
	fetch();

	temp = (uint16_t)a - (uint16_t)fetched - (uint16_t)!GetFlag(C);

	SetFlag(N, temp & 0x0080);
	SetFlag(Z, !(temp & 0x00FF));
	SetFlag(V, (((uint16_t)a ^ temp) & 0x0080) && (((uint16_t)a ^ (uint16_t)fetched) & 0x0080));

	if (GetFlag(D))
	{
		if ( (((uint16_t)a & 0x0F) - (uint16_t)!GetFlag(C)) < ((uint16_t)fetched & 0x0F) ) temp -= 6;
		if (temp > 0x99) temp -= 0x60;
	}

	SetFlag(C, temp < 0x100);
	a = temp & 0xFF;
	return 1;
}

from olcnes.

@brccabral That's expected behavior, as detailed in https://www.qmtpro.com/~nes/misc/nestest.txt

The nestest.nes cart checks to make sure the decimal flag is never set, so the above code would have to only be enabled via a future emulator option for non-NES architectures.

from olcnes.

@rmcuenen I don't know any game that uses the decimal mode. Also it was removed from late chips to save money on royalties, licenses and such. And, IIRC, @OneLoneCoder will not be implementing this

from olcnes.

@rmcuenen Neither the Ricoh 2A03 or the 2A07 (PAL) implemented BCD. There are no games that use it, because it isn't available on any NES or Famicom.

from olcnes.

Atari 2600 games definitely used it! If you plan on making this as a more common 650x emulator.

from olcnes.

I tested the proposed code above, but the ADC() implementation breaks "nestest.nes" Immediate tests.

from olcnes.

That is because the 'nestest.nes' does not support/expect BCD mode.

https://github.com/christopherpow/nes-test-roms/blob/master/other/nestest.txt
01Fh - ADC # failure (decimal mode was turned on)

If you want to test BCD for the 6502 you can use BCDTEST_beeb:

• load the contents of BCDTEST_beeb at 0x2900;
• write JSR 2900h to address 0x200;
• put an RTS at 0xffee, but also make sure you can trap that address;
• set the program counter to 0x200 and keep running until it is at 0x203;
• then test that the value at 0x84 is 0. If it is something else, that indicates failure.

from olcnes.

I see, thanks guys.

from olcnes.