Magpie, a 16-bit RISC-like virtual cpu

What is Magpie?

Magpie is my 4th virtual CPU. The first two were avc and avc2, neither of which fared too well. avc2 got further, but I ran out of energy for building an infrastructure soon after making a boot-from-drive system. My third was bcpu, which is much more relevant to Magpie.

A while back, me and some mates in the bucket webring discord server decided to make a toy OS. I then decided I was gonna make a CPU for it to run on. bcpu was heavily based on x86. In retrospect, this wasn’t the smartest choice, because x86 actually isn’t that interesting. You get some GPRs and some overbuilt instructions. Decoding is complicated, register-register operations are complicated, it’s all a bit naff really. I had almost finished the core emulator when I happened upon the wikipedia article on SPARC. This led me down a rabbit hole of RISC architectures, including ARM, MIPS, the original Berkeley RISC, and RISC-V, which is where Magpie began.

Magpie is heavily based on RISC-V and SPARC. It has 16-bit instructions, condition code branching, and a flat register file.

Originally, Magpie had load and branch delay slots. I ended up scrapping this because they made interrupts much harder to manage.

The Magpie emulator is available on github here.

Contents:

• Essentials
• Registers
  • Special registers
  • Flags in detail
• Instructions
  • I format
  • M format
  • J format
  • R format
• Interrupts
  • IRQs
  • NMIs
  • Writing an ISR
• orangejuice
  • Syntax
  • Directives
• emu
  • Memory map
  • Usage

Project status

Currently, the Magpie emulator is finished. The assembler is almost finished.