JoltCore Spark 16 (JC16)

This is a fun little project I did in 2 weeks on vacation, however it has now been expanded with many more hours invested into it. It is an extension of my original 8-bit CPU which is now 16 bits, has 8 general purpose 16 bit registers, I/O ports, up to 128KiB RAM and ROM, basic maths and logical instructions, a fairly mature assembler, basic and performant emulator written in C++ and more.

Important

The entire CPU is a custom design, from bit shifter to ISA.

In the winter of 2024 and 2025, it was improved further by adding some optimizations in the context of a school project mentored by Steve Furber, co-developer of the original ARM architecture.

This design is going to be thoroughly documented. You can read the docs here.

The project also features an own instruction set architecture. Key features are:

• support for 8-bit immediates in every ALU instruction:

  add r0, r1, #3
  lshift r2, #2, r3
  

• encapsulated operations, so you can calculate two things at once and apply modifications to your ALU operands:

  sub r4 (add r2 r3) r1
  nand r5 r1 (not r0)
  

• 3-bit immediates in all of said encapsulated operations:

  nor r1 (sub r7 #5) r6
  

• fixed instruction length of 24 bits
• LDI command to immediately load any arbitrary 16-bit value

The assembler tries to make coding as comfortable as possible:

• performs preprocessing and optimization
• it uses a custom assembly language that supports...
  • register notation with either digits (r0, r2, ...) or letters (ra, rc, ...)
  • named jumps and code blocks
  • pre-processor directives such as define statements which allow you to use named registers and constants
  • comments
• detailed errors (with line numbers and descriptions) are thrown once problems are detected

You can find the assembler in the respective folder, asm. It is written in Python. To use an example program, assemble it and copy the entire content of the .hex file, then paste it into the ROM inside of Logisim Evolution. The source for these programs is inside of the asm/programs folder.

A command to assemble into all possible output formats, using debug output and optimizations could look like this:

./jcasm ./asm/programs/fibonacci.asm -o ./asm/build/fibonacci.out -dO -f bBrx

The emulator can be found in emu. It is written in C++. As of right now, only files exported from the assembler in the raw ROM format can be used. This is how it can be used:

./jcemu ./asm/build/fibonacci.out

Note

You need Logisim Evolution to use this project. It is an open source logic simulator available here: https://github.com/logisim-evolution/logisim-evolution

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Have fun with it!