Show HN: I built a RISC-V emulator that runs DOOM

44 points by Flex247A a day ago · 4 comments · 2 min read

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  +-------------------------------------+

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