Raspberry Pi Pico 6502 emulator

[jfoucher]

I recently received a few raspberry pi picos!
My original idea was to use one of them as a VGA output for my 6502 computer, but that did not really work out.

So I took a different tack about what to do with them and decided to run a 6502 emulator. The I/O is very simple and goes through the Pico's USB to serial capability.

For the emulation code, I started out with this and added support for 65C02 instructions to be able to run Taliforth on it. You can also choose to run it in 6502 mode by changing one #define statement.

It passes all of Klaus Dorman's 65C02 test suite.

I then added 6522 support from this repository.

And putting it on a breadboard, we get some blinkenlights !

output.gif (277.29 KiB) Viewed 15976 times

While running the tests, I get about 3.1 MHz of emulated speed if I overclock the Pico to 280 Mhz. That seems fairly slow, so I've been looking at Daryl's code for an ARM assembly 6502 emulator. I have absolutely no experience in ARM assembly though, so it will be a while before I can try and rewrite the emulator in ARM assembly that is compatible with the RP2040 microcontroller... If we go by Daryl's result (ARM: 900 MHz -> 6502: 97MHz) I should be able get more than 25 MHz 6502 emulated speed out of the Pico!

Here is the github repo if anyone is interested : https://github.com/jfoucher/pico-6502

[BigEd]

Bravo! (And thanks for the various links - I do like a well-linkified post.)

[dp11]

You might like to check out my 6502 core for the BBC tube interface.

https://github.com/dp111/PicoTube

[jfoucher]

Wow, yeah that looks great dp11, will definitely try to integrate your 6502 core. Thanks !

[BigEd]

Thanks Dominic! I note with amusement that there are vestiges in there of my a6502 emulator, written for a different flavour of ARM and based on Chris Baird's work and, IIRC, Ian Piumarta's and Acorn's ideas.

I mention this partly because searching for a6502 here leads to some previous threads where we discuss performance of emulator code, for example:
emulator performance on embedded cpu
which sets an expectation of a 10x ratio.

It's a good thing to experiment with, emulation of 6502 on ARM, as there's lots of solution space to explore.

[enso]

Thank you. What a great reason to buy a pico!

[jfoucher]

Dominic, I am integrating your 6502 core and trying to get it to run the 6502 tests. However I cannot for the life of me get it to pass the full test suite. It hangs while doing the second BRK test. Is the core supposed to pass a test suite ?
If it does, then do I need anything outside of the core itself to handle the BRK instruction ?

Thanks.

[jfoucher]

I have changed many things on what I do on the pico, it now runs a simplified VIc-20 emulator using Dominic’s 6502 core. The Vic chip is extremely simplified and most registers don’t do anything. The vias are not emulated. All this in the interest of speed and laziness. ATM it runs at around 3-4 MHz

Also the colors and the screen scaling are all wrong but at least it runs basic!

[fredericsegard]

That is a fascinating experiment, jfoucher. 3.1MHz is a decent speed on an emulated microcontroller, I suppose.

[dp11]

Sorry for the delay. I was pretty sure the core passed the tests. I'll take a look next week.

[jfoucher]

dp11 wrote:

Sorry for the delay. I was pretty sure the core passed the tests. I'll take a look next week.

Oh that’s Great, thanks a lot !

[dp11]

Just to check I'm see what you are seeing can you let me know the error output ?

[jfoucher]

Here are the last few lines of output with TRACE_6502 enabled :

Code: Select all

2701 7c f9 02  f2 06 46 ed ff
16ce 08 88 88  f2 06 46 ed ff
16cf 88 88 88  f2 06 46 ed fe
16d0 88 88 28  f2 06 45 6d fe
16d1 88 28 f0  f2 06 44 6d fe
16d2 28 f0 fe  f2 06 43 6d fe
16d3 f0 fe 10  f2 06 43 ed ff
16d5 10 fe 90  f2 06 43 ed ff
16d7 90 fe 50  f2 06 43 ed ff
16d9 50 fe c9  f2 06 43 ed ff
16db c9 f2 d0  f2 06 43 ed ff
16dd d0 fe e0  f2 06 43 6f ff
16df e0 06 d0  f2 06 43 6f ff
16e1 d0 fe c0  f2 06 43 6f ff
16e3 c0 43 d0  f2 06 43 6f ff
16e5 d0 fe ba  f2 06 43 6f ff
16e7 ba e0 ff  f2 06 43 6f ff
16e8 e0 ff d0  f2 ff 43 ed ff
16ea d0 fe a9  f2 ff 43 6f ff
16ec a9 08 8d  f2 ff 43 6f ff
16ee 8d 00 03  08 ff 43 6d ff
16f1 a9 17 8d  08 ff 43 6d ff
16f3 8d 01 03  17 ff 43 6d ff
16f6 a9 05 8d  17 ff 43 6d ff
16f8 8d 00 02  05 ff 43 6d ff
16fb a9 17 8d  05 ff 43 6d ff
16fd 8d 01 02  17 ff 43 6d ff
1700 a2 ff 7c  17 ff 43 6d ff
1702 7c 01 02  17 ff 43 ed ff
1708 ad 02 02  17 ff 43 ed ff
170b c9 0a d0  0a ff 43 6d ff
170d d0 fe a9  0a ff 43 6f ff
170f a9 0b 8d  0a ff 43 6f ff
1711 8d 02 02  0b ff 43 6d ff
1714 a9 00 48  0b ff 43 6d ff
1716 48 a9 42  00 ff 43 6f ff
1717 a9 42 a2  00 ff 43 6f fe
1719 a2 52 a0  42 ff 43 6d fe
171b a0 4b 28  42 52 43 6d fe
171d 28 00 88  42 52 4b 6d fe
171e 00 88 08  42 52 4b 20 ff
2724 08 88 88  42 52 4b 24 fc
2725 88 88 88  42 52 4b 24 fb
2726 88 88 c9  42 52 4a 24 fb
2727 88 c9 bd  42 52 49 24 fb
2728 c9 bd f0  42 52 48 24 fb
272a f0 42 c9  42 52 48 a4 fb
272c c9 42 d0  42 52 48 a4 fb
272e d0 fe e0  42 52 48 27 fb
2730 e0 52 d0  42 52 48 27 fb
2732 d0 fe c0  42 52 48 27 fb
2734 c0 48 d0  42 52 48 27 fb
2736 d0 fe 85  42 52 48 27 fb
2738 85 0a 86  42 52 48 27 fb
273a 86 0b ba  42 52 48 27 fb
273c ba bd 02  42 52 48 27 fb
273d bd 02 01  42 fb 48 a5 fb
2740 c9 30 d0  30 fb 48 25 fb
2742 d0 fe 68  30 fb 48 27 fb
2744 68 c9 34  30 fb 48 27 fb
2745 c9 34 d0  34 fb 48 25 fc
2747 d0 fe ba  34 fb 48 27 fc
2749 ba e0 fc  34 fb 48 27 fc
274a e0 fc d0  34 fc 48 a5 fc
274c d0 fe ad  34 fc 48 27 fc
274e ad ff 01  34 fc 48 27 fc
2751 c9 17 d0  17 fc 48 25 fc
2753 d0 fe ad  17 fc 48 27 fc
2755 ad fe 01  17 fc 48 27 fc
2758 c9 20 d0  20 fc 48 25 fc
275a d0 fe a9  20 fc 48 27 fc
275c a9 ff 48  20 fc 48 27 fc
275e 48 a6 0b  ff fc 48 a5 fc
275f a6 0b e8  ff fc 48 a5 fb
2761 e8 a5 0a  ff 52 48 25 fb
2762 a5 0a 49  ff 53 48 25 fb
2764 49 aa 28  42 53 48 25 fb
2766 28 40 4c  e8 53 48 a5 fb
2767 40 4c 68  e8 53 48 ef fc
1720 08 88 88  e8 53 48 20 ff
1721 88 88 88  e8 53 48 20 fe
1722 88 88 c9  e8 53 47 20 fe
1723 88 c9 e8  e8 53 46 20 fe
1724 c9 e8 d0  e8 53 45 20 fe
1726 d0 fe e0  e8 53 45 23 fe
1728 e0 53 d0  e8 53 45 23 fe
172a d0 fe c0  e8 53 45 23 fe
172c c0 45 d0  e8 53 45 23 fe
172e d0 fe 68  e8 53 45 23 fe
1730 68 c9 30  e8 53 45 23 fe
1731 c9 30 d0  30 53 45 21 ff
1733 d0 fe ba  30 53 45 23 ff
1735 ba e0 ff  30 53 45 23 ff
1736 e0 ff d0  30 ff 45 a1 ff
1738 d0 fe a9  30 ff 45 23 ff
173a a9 ff 48  30 ff 45 23 ff
173c 48 a9 bd  ff ff 45 a1 ff
173d a9 bd a2  ff ff 45 a1 fe
173f a2 ad a0  bd ff 45 a1 fe
1741 a0 b4 28  bd ad 45 a1 fe
1743 28 00 88  bd ad b4 a1 fe
1744 00 88 08  bd ad b4 ef ff
2724 08 88 88  bd ad b4 a7 fc
2725 88 88 88  bd ad b4 a7 fb
2726 88 88 c9  bd ad b3 a5 fb
2727 88 c9 bd  bd ad b2 a5 fb
2728 c9 bd f0  bd ad b1 a5 fb
272a f0 42 c9  bd ad b1 27 fb
276e e0 ad d0  bd ad b1 27 fb
2770 d0 fe c0  bd ad b1 27 fb
2772 c0 b1 d0  bd ad b1 27 fb
2774 d0 fe 85  bd ad b1 27 fb
2776 85 0a 86  bd ad b1 27 fb
2778 86 0b ba  bd ad b1 27 fb
277a ba bd 02  bd ad b1 27 fb
277b bd 02 01  bd fb b1 a5 fb
277e c9 ff d0  ff fb b1 a5 fb
2780 d0 fe 68  ff fb b1 27 fb
2782 68 c9 f7  ff fb b1 27 fb
2783 c9 f7 d0  b7 fb b1 a5 fc
2785 d0 fe ba  b7 fb b1 a4 fc
2785 d0 fe ba  b7 fb b1 a4 fc
.... Forever

Here is the last line

Hope that helps

[hoglet]

Just a guess from quickly looking at the code, but is the BRK instruction indevertantly clearing the V flag?

I think this line needs to preserve the V Flag (bit 6) in rPbyteLo
https://github.com/dp111/PicoTube/blob/ ... mM0.S#L395

This would also effect IRQ and NMI interrupts.

Dave

[jfoucher]

hoglet wrote:

I think this line needs to preserve the V Flag (bit 6) in rPbyteLo

Yes, it probably does. I just tried to modify it but I must be really bad at ARM assembly because everything I tried caused the pico to crash, even things that seem to me should be no-ops...