Myth: The 6502 Cannot Single Cycle!

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Let's figure out if Single Cycling an original 6502 CPU is impossible just because it's an NMOS version with dynamic registers - didn't Steve Wozniak make a circuit for the Apple 1 for exactly that purpose?

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Complete schematic, source code and everything about the 65uino (including the source code used for the programmer):

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[francoisleveille]

I tried to debunk so many myths like that on various Commodore forums and was pelted with insults. I was even expelled from Lemon64. Making videos like yours requires lots of efforts and I didn't have the energy and time.

[blarghblargh]

ohhhh cool! I heard this information about the registers losing data with no clock refresh, and was a bit disappointed. I am interested in running a real 6502 as a hardware unit test and source of truth against an emulator I want to make, so this information should be pretty useful.

[jxtq]

Oh, you mean single step not single cycle. Now I understand.

[benjaminshropshire]

It would be a bit more complex, but I'd think it should be possible to adapt that concept to make a break point debugger as well. E.g. feed all the relevant pins into a RAM chip's address-in pins and use one of the data-out pins as break signal. (Thought at that point, just using another CPU to track things might be easier and cheaper.)

[BEdmonson]

This was a great watch, here's my comment for the algorithm gods! This channel needs more subscribers!

[weirdboyjim]

Interesting, I didn't know that about the early 6502's.

[NorthWay_no]

Slightly confused here; didn't both the Ataris and the C= 64 have special pins to let the gfx chip halt the cpu? Was that working different from what you described here?

[phils_arcade]

This is very interesting, love how simple the final board looks. Can you single step Z80 range of processors as well?

[Bigmalc]

An absolutely enjoyable Video on single stepping a old 6502 CPU, I'm repairing a Microtan 65 Full-System built into a 19" Rack, it mostly works but some of the Eprom's don't seam to be getting selected. I'm in England, what would be the postage cost for the ready built stepper. Thanks

[tigheklory]

This is fun, it would be cool if you used segmented displays for the output!

[phill]

I never realised that people thought you could not do it. Several 80s computers from the 80s used the rdy line to stall the CPU when the graphics chip needed cycles (even when the ram ran twice the speed of the CPU and the graphics chip, with each getting access alternately). Even the Atari 2600 6507 has a rdy pin, but single stepping it will cause the screen to fall apart.

[tschak]

The Atari VCS (2600) WSYNC register in the TIA causes the RDY pin to be deserted, until the next scan line. This is CRITICAL for display timing.

[marcuswilliams]

Thanks, this was a great video. Really, peaked my curiosity. As with the Cycle Board, there are 3 buttons. When this run button is up, is this run at full mode, while pushed is this the debug mode? As per the Cycle/Instr button, when this is pushed down, is this the Single Cycle mode?

[nickolasgaspar]

So the 65C02(1981 - found in Atari 8bit XL/XE and some later 800/400 models)essentially has the halting circuit onboard? Since later Atari 800/400 motherboards could accept the 65C02 does that mean that a similar circuit to yours exists on their earlier motherboards?

[colonthree]

"I adore my Commodore Sixtyfour" uwu

[LordPhobos]

Glad to see this video :)
Many years ago, I wanted to try a homebrew but wanting to play with single/manual clocking, I ended up getting a 65c816 (the 16 bit version of the 6502)

Never even tried it with the original.

[johnrickard]

So that wasn't exactly an intentional feature, but Woz never really cared what features were intentional when he was looking for them.

[johncloar]

Nice project, looks like a lot of fun. Thanks for the video.

[bryede]

4:08 I don't think the C64's 6510 CPU has SYNC capability.

[MCPicoli]

But what happens if the clock speed gets down to zero hertz (regardless of the circuit shown), specially if using the older versions of the processor? I think part of the myth may still be true - not being able to run if single clock cycles are fed to it. In the video the 6502 still receives the clock at full speed, but execution is "gated" by the RDY pin.