I would probably check to see how good the emulation is of the instructions with https://github.com/Klaus2m5/6502_65C02_functional_tests and my cycle timing checker which includes undocumented instructions : https://github.com/dp111/6502Timing

I've added vectors to the beginning of the code to make it easier to port the test to another system. Now you just need to patch the vectors with putchar, start timer and read timer, instead of building the code.

I've created a 6502 /65C02/65C12 instruction timing checking test suite.

It first of all targeted BBC micro and Master machines ( which have been checked against real hardware). I've extended it to support C64 , but this hasn't yet been checked against real hardware. It passes in VICE , but not in ...

Please do play with my stuff. But I will warn you that it is heavily optimisied for for the tube. If you spot any performance improvements do let me know. Decimal adc has the feeling of a few extra cycles could be removed, but I haven't put any time into it.

I should say pizero non jitter is about 290MHz.the code is on GitHub search for PiTubeDirect. I have the last few bit to do to the jitter. Soon it should be merged into master.

You can't buy the Pizero Silcon unless you talk to broadcom directly and want to buy a lot of them AFAIK.

My Rp2040 6502 core referred to earlier has the aim to be fast but not cycle accurate. On average the 6502 clock rate is 1/4 of the arm M0 clock rate. My pi jit 6502 core is 1/2.5 of the arm clock for a pizero, 1/1.4 for a pi3b and 1 to 1 ish for a pi4. Self modifying code reduces these figures. The ...

Hoglet has kindly tested this and fixed a few issues. I have also fixed my build, but haven't tested it yet.

I've committed a fix. I haven't managed to test it as I have broken my build else where.

Yep that's broken, I'll fix it. Thanks for the report

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

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

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

https://github.com/dp111/PicoTube

You can move stx up in the code then peep hole can spot tax stx and just do sta.

Yes it does add jitter, it does save a few cycles .

All the above I think leads to now removing the zp sample location which make the average case even quicker

NMI_Handler:
PHA

; load sample into SID
@sample:
LDA $FFFF ; self-mod pointer to sample - overwrite this operand during init

; alternate high/low nybble
ASL flag
BCC @writeSID
INC ...