Designing a CPU with only Memory Chips

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Four brown perf board circuits are visible in the foreground, each populated with many large DIP integrated circuits. The boards are connected with grey ribbon cable. Behind the boards a vacuum fluorescent display shows the words “DIY CPU.”

Building a simple 8-bit computer is a great way to understand computing fundamentals, but there’s only so much you can learn by building a system around an existing processor. If you want to learn more, you’ll have to go further and build the CPU yourself, as [MINT] demonstrated with his EPROMINT project (video in Polish, but with English subtitles).

The CPU began when [MINT] began experimenting with uses for his collection of old memory chips, and quickly realized that they could do quite a bit more than store data. After building a development board for single-chip based programmable logic, he decided to build a full CPU out of (E)EPROMs. The resulting circuit spans four large pieces of perfboard, weighs in at over half a kilogram, and took several weeks of soldering to create.

The star of the system is the ALU, which runs an instruction set inspired by the Z80, but with some optimizations and added features. In particular, it has new operations for multiplication, division, bitstream operations, more advanced bit shifting, and a wide range of mathematical functions, including exponents, roots, and trigonometric functions. [MINT] documented all of this in a nicely-formatted offline booklet, available under the project’s GitHub repository. It’s currently only possible to program for the CPU using opcodes or a custom flavor of assembly, but there are plans to write a C compiler for it.

Even without being able to write in a higher-level language than assembly, [MINT] was able to drive a VFD screen with the EPROMINT, which he used to display some clips from The Matrix. This provided an opportunity to demonstrate basic debugging methods, which involved dumping and analyzing the memory contents after a failed program execution.

Using memory chips as programmable logic gates is an interesting hack, and we’ve seen Lisp programs written to make this easier. Of course, this isn’t the first CPU we’ve seen built without any chips intended for logic operations.

Thanks to [Piotr] for the tip!



from Blog – Hackaday https://ift.tt/JzaZl3k

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