RISC-V Emulator (RV32IM + DOOM)

A high-performance RISC-V RV32IM emulator written from scratch in C++ capable of running DOOM via doomgeneric with SDL2 graphics and keyboard input.

Features

Components
RV32I Base Integer ISA (40 instructions)
RV32M Extension (MUL, DIV, REM)
ELF32 Loader (.text, .data, .bss, multi-segment)
Dynamic Heap Allocation (sys_brk)
Host File I/O Syscalls (open, read, write, lseek, close, fstat)
Custom Display & Input ECALLs (0x100–0x103)
SDL2 Interactive Video (320×200 ARGB8888) & Input
DOOM Execution (doomgeneric + doom1.wad)

Architecture

MyRiskVemulator/
├── src/
│   ├── types.h              Fixed-width integer aliases
│   ├── cpu.h                CPU state, opcode enum, instruction struct
│   ├── cpu.cpp              Instruction execution (RV32IM + Syscalls + Custom ECALLs)
│   ├── decode.h             Binary instruction decoding
│   ├── memory.h             Byte-addressable 64MB memory with bounds checking
│   ├── elf.h                ELF loader declaration
│   ├── elf.cpp              ELF32 PT_LOAD segment loader & heapBase setup
│   ├── debug.h              Register, memory, and instruction printing
│   ├── platform.h           Host platform interface (display, input, timer)
│   ├── platform_sdl.cpp     SDL2 frontend implementation
│   ├── platform_stub.cpp    Headless stub frontend
│   └── main.cpp             Entry point, stack argument setup, demo loop
├── dg_riscv.c               doomgeneric RISC-V guest platform layer
├── crt0.S                   RISC-V startup code for guest binaries
├── linker.ld                Linker script (entry at 0x10000)
└── Makefile                 Build system for rvemu and doom.elf

Quick Start

1. Requirements

• C++17 Compiler (g++ / clang++)
• SDL2 (libsdl2-dev on Linux, mingw-w64-x86_64-SDL2 on Windows/MSYS2)
• RISC-V Cross Compiler (riscv32-unknown-elf-gcc or riscv64-unknown-elf-gcc)

2. Building the Emulator and DOOM

cd MyRiskVemulator

# Build the host emulator (rvemu)
make rvemu

# (Optional) Build headless emulator without SDL2
make rvemu_headless

# Cross-compile guest DOOM binary (requires doomgeneric/ clone)
make doom.elf

3. Running DOOM

cd MyRiskVemulator
./rvemu doom.elf doom1.wad

Controls:

• Move / Turn: Arrow Keys
• Fire: Ctrl
• Use / Open Doors: Space
• Speed: Shift
• Menu / Back: Escape
• Select: Enter

4. Running Built-in Demo

Run without arguments to execute the 6-instruction demo:

./rvemu

Supported Syscalls & ECALLs

Linux / Newlib Syscalls

Number	Name	Description
56 / 1024	sys_openat / sys_open	Open host files (e.g. WAD files)
57 / 1026	sys_close	Close file descriptor
62 / 1029	sys_lseek	Reposition file read/write offset
63 / 1030	sys_read	Read data from file / stdin
64 / 1031	sys_write	Write data to stdout / stderr / file
80 / 1028	sys_fstat	Query file status and size
93	sys_exit	Clean exit
169	sys_gettimeofday	High-resolution wall-clock time
214	sys_brk	Dynamic heap expansion
222	sys_mmap	Memory mapping

Custom Game ECALLs

ECALL	Name	Arguments	Description
0x100	DRAW_FRAME	a0=fb_ptr, a1=width, a2=height	Blit 32-bit pixel buffer to host window
0x101	GET_KEY	None (returns packed key in a0)	Poll keyboard events
0x102	GET_TICKS	None (returns ms in a0)	Monotonic milliseconds since boot
0x103	SLEEP	a0=ms	Sleep / yield CPU

References

• RISC-V ISA Specification
• RISC-V Instruction Set Reference
• doomgeneric
• rvcore
• Writing a RISC-V Emulator