Demo: https://www.youtube.com/watch?v=f5uygzEmdLw

Hi HN,

I built a RISC-V emulator that implements the RV32IM instruction set and a minimal syscall interface to run DOOM. A few weeks ago, I got my first output with a simple hello world assembly program.

Since then I have been working tirelessly to get DOOM to run.

I needed to figure out how to run C programs first, and came across newlib, which allows the underlying environment to implement the syscall stubs one by one until the programs run.

I have also added ELF loading, but currently only a single `PT_LOAD` segment is supported.

To port DOOM, I used doomgeneric, which was quite convenient to get working once the required stubs were in place.

DOOM renders to a fixed area in memory (0x705FDD = VRAM_START):

  0x7FFFFF  +-------------------------------------+
            |                                     |
            |        QUEUE_SIZE (32 bytes)        |
            |                                     |
  0x7FFFDF  +-------------------------------------+ <-- QUEUE_START
  0x7FFFDE  |          QUEUE_READ_IDX             |
  0x7FFFDD  |          QUEUE_WRITE_IDX            |
            +-------------------------------------+
            |                                     |
            |                                     |
            |       VRAM (1,024,000 bytes)        |
            |                                     |
            |                                     |
  0x705FDD  +-------------------------------------+ <-- STACK_START
            |                Stack                |
            |                  |                  |
            |                  v                  |
            |                                     |
            |                  ^                  |
            |                  |                  |
            |         Program data + Heap         |
            |                                     |
  0x000000  +-------------------------------------+

I made a small linker script so that the entry point of a C program is at _start and virtual address is always 0. That kept the ELF loader code simple.

Inputs are written to the queue by rvcore which are then intercepted by DOOM running inside it.