Boot

What it is

Software built for another processor needs that processor, or an emulator someone else wrote and nobody on the team can read.

The processor passes riscv-tests, compiled from the suite’s own source and run in the page, and the page names the hardware each test it leaves out would need.

The parts

A processor in plain JavaScript

The processor runs the base integer instructions of RISC-V, multiply and divide, and atomics, with machine mode, user mode, interrupts and traps. It runs in a Web Worker, so the page stays responsive, in one loop that allocates no memory per instruction.

A small machine around it

Around the processor is RAM, a serial port for the terminal, a timer and a register that powers off and restarts. Boot writes a device tree that describes the machine to Linux, and a few instructions of firmware jump to the kernel.

Time counted in instructions

The timer moves one tick for a fixed number of instructions. When the kernel has nothing to do, it waits for the next tick, and the page sleeps until then instead of running in a loop.

One instruction at a time

The inspector runs a small C program or the kernel one instruction at a time. It shows the registers, disassembles the next instructions the way llvm-objdump does, and counts every instruction by kind.