Instruction

SCAMP CPU

SCAMP is my homebrew 16-bit CPU. It stands for something like "Simple Computing and Arithmetic Microcoded Processor".

The CPU is very primitive. It doesn't support interrupts, and has no MMU, and no concept of privilege levels. It currently runs at 1 MHz, above that the CompactFlash interface falls over. It runs a homebrew operating system, with a homebrew programming language and compiler, and can self-host the kernel and all of the system utilities. The environment tastes a bit like an early Unix, but works like CP/M. I used the computer to complete almost all of Advent of Code 2021.

My Advent of Code solutions are here: https://github.com/jes/aoc2021

I have some blog posts here: https://incoherency.co.uk/blog/tags/cpu.html and a web-based emulator available here: https://incoherency.co.uk/scamp/

Here are some pictures:

And here's a video with some explanation and an example session on real hardware:

Architecture

It is a 16-bit CPU. The bus is 16-bit, registers are 16-bit, addresses are 16-bit, and memory contents are 16-bit. The upper 8 bits of an instruction select the opcode, and the lower 8 bits are available for small immediate values. There is no support for: cache, interrupts, virtual memory, DMA, privilege rings, floating point, and ~anything else that is not strictly necessary.

Here is a diagram of the architecture:

For more information, see doc/UCODE.md and doc/ISA.md.

Try it out

The easiest way to try it out is to use the web-based emulator at https://incoherency.co.uk/scamp/

If you want to run it locally, first you'll need to build everything. Try make -j in the root directory of this repository. It probably won't work on the first attempt because it works with multiple different Makefiles that have annoying dependencies. Just run make -j twice or three times. If that doesn't do the trick, run make in sys/ first, and then run it in .. Once you get to the point that it is only complaining about emcc, you're done, unless you also want to build the web-based emulator.

Having built everything, you can go into kernel/ and start it in the emulator with ./run:

~/scamp-cpu/kernel $ ./run
make: 'os.disk' is up to date.
SCAMP boot...
.......................................................OK
starting kernel...
loading init...
      ____   ____    _    __  __ ____   __
     / ___| / ___|  / \  |  \/  |  _ \ / /__  ___
     \___ \| |     / _ \ | |\/| | |_) / / _ \/ __|
      ___) | |___ / ___ \| |  | |  __/ / (_) \__ \
     |____/ \____/_/   \_\_|  |_|_| /_/ \___/|___/

$

If you know how to use Unix you'll probably understand how to use it. If you know how to use CP/M you'll probably understand how it works.

If you want, you can write SLANG programs using kilo and compile them with slc. You've probably never written SLANG before. You can learn how to use it by reading programs under /src (or sys/ in the repo). It's mostly like C but with fewer features and less safety.

How it's made

I did these tasks, but working on several steps at a time (i.e. I didn't wait until after I had built the entire computer before thinking about microcode or the operating system):

1. Create the CPU in Verilog, with a testbench for each part.
2. Replace the raw Verilog with Verilog that only uses 74xx-compatible primitives (e.g. https://github.com/TimRudy/ice-chips-verilog), but still passes the testbenches.
3. Convert the 74xx-Verilog into KiCad schematics (by hand).
4. Order the PCBs and components, solder up the PCBs, build a card cage, build a wooden case.
5. Settle on the instruction set and write the microcode (see doc/table.html, available online at https://incoherency.co.uk/interest/table.html).
6. Write a bootloader, kernel, shell, text editor, compiler, and various system utilities.

Next steps

I suspect the computer will never be "finished", but some things I still want to work on are:

Hardware

• build a hardware multiplication card (and maybe division?)
• build a hardware RNG

Low-level software

• make library blob self-hosting (compile within SCAMP/os)
• write an strace-like debugging tool
• write a DDT-like debugging tool
• better facility for memory profiling (mainly to stop kilo and asm from running out of memory on large inputs)
• atexit()
• static initialisation of globals from constants (e.g. var x = 5 at global scope should not take any code to initialise)
• "const" variables (e.g. const x = 5 should lead to x being replaced with the constant value 5 where possible)

Other software

• write a big-fixed-point arithmetic library
• fixed-point mathematics: pow,tan,asin,acos,atan,atan2
• write a FORTH implementation (should it run on bare metal or under SCAMP/os?)
• write a Z-machine interpreter and port some text adventures
• write some games (pong? breakout? tetris?)

If you happen to want to work on any of the above, I'd be happy to help you!

Resources

I thoroughly recommend the Nand2Tetris course. https://nand2tetris.org/

If you want to do the exercises from Nand2Tetris without learning what a hardware-description language is, and without going through all the lectures, you can play https://nandgame.com/

Ben Eater's videos on 8-bit CPU design are excellent and heavily influenced the design of my CPU.

I found the YouTube playlist for Nicolas Laurent's compiler class quite helpful in writing the parser for the compiler.

The Build Your Own Text Editor is a fantastic tutorial that walks you through implementing antirez's kilo editor.

Balazs Markus's 8250 UART example is helpful.

Daniel Tufvesson has a post that is a good starting point for understanding the 8-bit CompactFlash interface.

PickledDog has a description of an improvement to the RC2014 CompactFlash card that might be helpful. Also Bill Shen's comment on the RC2014 mailing list.

Michał Cierniak has a good blog post about CompactFlash + 8255 interface card with lots of detail on how his CompactFlash interface works.

James Sharman is building an interesting 8-bit pipelined CPU, and has good videos with lots of explanation on how it works.

SCAMP is featured on the Homebrew CPU ring which also features many other interesting homebrew CPU designs.

Contact

You can email me on james@incoherency.co.uk or read my blog: [https://incoherency.co.uk/].