C74-6502 Overview

The C74-6502 is a homemade implementation of the venerable MOS 6502 microprocessor. The 6502 was revolutionary in its time, and widely adopted in early home computers and gaming consoles. This is a microprogrammed, discrete-component 6502, a design architecture that was prevalent in 1970s-era mainframe computers and pre-dated modern microprocessor technology.

The C74-6502 is plug-in compatible with a wide variety of 6502 systems, including classic computers from Commodore and Apple, as well as modern systems based around the 65C02 microprocessor still in production today. This CPU runs at speeds of up to 20MHz (vs. 1 MHz for the original MOS 6502).

Why build a 6502?

The MOS 6502 microprocessor is legendary in the industry for having helped launch the personal computer revolution. Its design  included a number of important innovations which delivered ground-breaking performance for the price, and made possible such iconic home computers and gaming systems as the Apple II, the Commodore 64, the VIC 20, the Atari 2600 and the Nintendo Entertainment System (NES). As a result, millions of people’s first experience with a computer of any kind was with one powered by the innovative 6502.

For many enthusiasts, understanding the inner workings of pioneering devices like the 6502 is a long-held dream; and an opportunity to gain a unique perspective into the fundamental principles of CPU design. There is a growing community of enthusiasts around the world who have devoted themselves to making CPUs of various descriptions. The C74-6502 is one such example.

See Examples Of Homemade CPUs

See An Online Model of the 6502

The Build Process

This CPU is a microprogrammed, discrete-component design. In contrast to modern single-chip microprocessors, discrete-component CPUs consist of hundreds of relatively simple individual building-block Integrated Circuits (ICs), laid out on large Printed Circuit Boards (PCBs) and connected together by thousands of individual connections (traces).

The C74-6502 was designed by hobbyists working at home, soldered by hand and built using simple tools and techniques. It consists of some 200 ICs and 4,000 individual solder points laid out on two sister PCBs.

It took two years to complete the design and build the first prototype. See the original build forum thread.

Images From The Build-Process

card a-registers sch p1
Fig. 1 – Sample Schematic
C74-6502 Panel Top V1
Fig. 2 – PCB Layout
Panel Split
Fig. 3 – PCB Panel
Fig. 4 – Populated PCBs
S-PC Patch Detail
Fig. 5 – Layout Patch
CPU Forest
Fig. 6 – First Connections
CPU Debug Setup
Fig. 7 – Debug Setup
LA Setup
Fig. 8 – Taking Measurements
SBC CPU Sandwich
Fig. 9 – CPU Stacked Assembly (with SBC on top)

A big day in the lab — First Life!


After two years of work, finally it was time for the ultimate test of compatibility: would the C74-6502 be able to replace the microprocessor inside a genuine vintage 6502 computer? A 1983 VIC-20 is a fitting machine to use for this test. The VIC-20 is an iconic device — it set the standard for early home computers and was the first computer of any kind to sell over a million units.

Successfully replacing this machine’s MOS 6502 microprocessor would mean that the C74-6502 is functionally equivalent to the original, and a deserving bearer of the 6502 designation. A photo-blog below documents the occasion:

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C74-6502 Ready For Duty
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VIC 20’s MOS 6502 Microprocessor
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C74-6502 Transplant Complete
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Victory! — It Works
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Programming With The C74-6502
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Playing “Frogger” — A Classic
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Testing With Other Games
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C74-6502 Pays Homage To The Original

When I’m 64 …

The Commodore 64 proved to be a next-level challenge for the C74-6502. C64 programmers are notorious for exploiting every nuance of the machine’s hardware in pursuit of new and exciting effects. As a result, many games and demos on this machine depend on subtle details in the CPU’s function and timing. To run the C64 software catalogue, the C74-6502 would need to replicate the operation of the MOS 6510 microprocessor in meticulous detail.

Doing so required extensive testing, and several careful modifications to the prototype boards. Below is one such modification, worked out on a scratch-pad, and used to correct the action of an NMI when interrupting an IRQ already in progress:

Patch: NMI either hijacks an IRQ, or waits up to four cycles

Despite enduring several such patches and fixes, in the end the C74-6502 was able to run a wide variety of C64 games; and the prototype boards performed like a champ!

The C74-6502 ran every game tested!

Over seventy-five games were selected from various “best of” lists as a reasonable test suite of both complex and popular software. See the list of currently verified titles here.

At this point, we can be reasonably confident that the CPU will run most games in the C64 catalogue, although it’s hard to know for sure. Please feel free to suggest any games or demos which might exercise the machine in unique ways. It will be interesting to see whether any compatibility issues remain.