Q & A

The purpose of this site is to share information about the C74-6502. If you have a question, chances are the answer will help others as well. Please ask below. We’ll be glad to help.

Here are FAQs so far:

How did you manage the 20MHz clock-rate?

Reaching a 20MHz clock-rate required two-things: a 50 nanosecond signal-path through the CPU, and pre-fetching microcode to eliminate the ROM wait-time. Details of the critical-path analysis can be found here. A description of the microcode pre-fetch logic is here. Some notes on fine-tuning the critical path are here.

Will you be offering the C74-6502 for sale?

There are no plans to offer kits or assembled units for sale. The sole objective of this effort is to learn, share the knowledge, and hopefully inspire others to do the same. That said, you will find all the information necessary to build a C74-6502 for yourself here, including PCB Gerber Files and Bills of Material to order parts.

When can we expect the sections describing the CPU internals to be posted?

Updated Internals pages have now been posted. Please send us a note to let us know what you think, or just to say hello. All comments and questions welcome.

Will you be following this up with an “accelerator” version of the CPU?

There has been some interest in using the C74-6502 to accelerate vintage 6502 systems. While the C74-6502 may be used for that purpose, it would require additional engineering to interface a faster external CPU to an existing computer. We are not currently undertaking that effort. Please send us a note if you are interested in pursuing work in this area. We would be delighted to collaborate.

Why do this?

As a programmer on the Commodore 64 years ago, I often wondered at the magic within the machine. The C64 Programmer’s Guide offered a tantalizing glimpse of it — a fold-out schematic of the internal circuitry attached on the final page. I could see some familiar characters there: 6510, VIC, SID, CIA. But what did it all mean? And what in the world was really going on inside those chips? It is this enduring curiosity that inspired the C74 project.

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