mu/subx

The SubX VM

Bytecode interpreter for a subset of the 32-bit x86 ISA.

• Only instructions that operate on the 32-bit E*X registers. (No floating-point yet.)
• Only instructions that assume a flat address space; no instructions that use segment registers.
• No instructions that check the carry or parity flags; arithmetic operations always operate on signed integers (while bitwise operations always operate on unsigned integers)
• Only relative jump instructions (with 8-bit or 16-bit offsets).

These rules yield a clean instruction set. We don't care about running arbitrary binaries, just those generated by our forthcoming Mu compiler.

Targeting a VM enables more comprehensive tests for the compiler, without requiring access to processor/memory state without getting bogged down in details of the ELF format, ABI, STABS debugging format, etc., etc.

Having the VM implement a real (and ubiquitous) instruction set makes it easy to generate native binaries outside of tests.

Just unit tests so far:

./subx test

x86 instruction set resources used in building this:

• Intel programming manual (pdf)
• Concise instruction reference
• Single-page cheatsheet (pdf)