Pickle RISC 16bit CPU

... so I would like to build a computer, this time a little closer to "from scratch".

Goals

• Fun!
• Aiming for a poorly defined balance between "easy to build" and "not too slow"
• Can run a multitasking OS
• Serve web site about itself (like Magic-1 does)
  • SLIP over UART?
  • W5500 module?
• Serial console interface
  • No graphics output
• Run at ~2MHz clock
  • 4MHz as a stretch goal :)
• Most instruction single cycle

State

• Instruction set
  • Needs expanding into microcode
  • Needs to be tweaked according to what's needed in SW, limitations in HW design
• HW design
  • Slightly more than a fuzzy idea of how stuff should should go together
  • (OUTDATED) The block diagram shows most of the general idea
  • There is a design document that contains some (incomplete) information
• SW
  • Skeleton of a toolchain written in Rust (toolchain-rs/)
    • Non-functioning assembler
    • Start of an emulator
      • Emulation works per instruction, but behavior is built up from microcode at compile time.
  • Lisp interpreter is planned as a shell
    • Probably loosely based on Make a Lisp
    • Compiler as a stretch goal

Next steps

This is the high level to do list, roughly in the order of decreasing priorities. Long term plans (and hopes and dreams) go to the notes directory.

• Figure out details of microcode
• Write microcode for instructions
• Finalize assembler
• Emulator
• Some kind of integration tests for emulator <-> assembler
• Lisp interpreter
• Figure out ALU design
• Figure out MMU design
• Figure out interrupt handling
  • Rough idea and the instructions needed to support it is already done
• Start building the hardware in some kind of HDL

Links

Inspiration

• Magic-1: http://www.homebrewcpu.com/
  • Showed me that there is so much magic in this topic.
  • I would like my machine to look like Magic-1 and do the stuff that Magic-1 does.
• Ben Eater's breadboard computer: https://www.youtube.com/playlist?list=PLowKtXNTBypGqImE405J2565dvjafglHU
  • Gave me the idea that I could actually build something like this.
• James Sharman's JAM-1: https://www.youtube.com/c/weirdboyjim/videos
  • Different goals, but just plain awesome :)

Instruction set

• ARM Thumb Instruction set
• AVR instruction set

Peripherials

• Interfacing SDHC card to 6502: https://github.com/gfoot/sdcard6502

The rest

• MUPS/16, a nice 16bit CPU with similar goals/design: http://mups16.net/pages/overview.html#overview
• RISC-V: https://riscv.org/wp-content/uploads/2017/05/riscv-spec-v2.2.pdf
• Simple almost C-like language: https://incoherency.co.uk/blog/stories/slang.html
• Logisim evolution
• Nandgame ALU https://nandgame.com/

  The ALU (Arithmetic/Logic Unit) performs one or more operations on two input values X and Y.

  The six flags select what operations to perform. Each flag trigger an operation when the flag is 1: zx Use 0 for X nx Invert X zy Use 0 for Y ny Invert Y f selects an operation: 0: output is X AND Y 1: output is X + Y no Invert output

  The flags can be combined and the specified order is significant.

  For example, if both zx and nx is 1, then X is inverted 0.