Quake on an FPGA (MRISC32 CPU) [video]

Original Article ↗ Hacker News Discussion ↗

Quake has been ported to run on a custom MRISC32 softcore CPU implemented in an FPGA, impressing many with how smoothly a 1990s 3D game can perform on fully homebrew hardware, toolchain and ISA. Commenters use it as a springboard to explore dreams of “Quake-specific” hardware, the limits of FPGAs for general-purpose computing, and the role of open-source tools like Yosys and F4PGA in making such projects more accessible. The thread also dives deep into CPU microarchitecture — from vector-first designs to modern x86 and ARM pipelines — to examine how instruction sets, branch prediction, and code density affect performance on projects like this.

Project Overview & Clarifications

  • The demo is Quake’s original software renderer running on a custom 32‑bit MRISC32 softcore CPU in an FPGA, not a pure “all-logic” Quake implementation.
  • The author uses Quake as a realistic benchmark to tune cache, branch predictor, and overall CPU design.
  • On a Cyclone V board it runs around 40 FPS at 320×180 upscaled to 1080p, with a proper FPU and ~110 MHz clock (timing margins are “overclocked” beyond formal closure).

Hardware vs. Software Implementations of Classic Games

  • Several commenters fantasize about Quake-specific hardware implemented entirely in FPGA logic, running at extreme frame rates.
  • Others note that full hardware implementations of large, general-purpose games are unrealistic; softcores plus accelerators or custom instructions are more plausible.
  • Historical context: early arcade games and some consoles used mostly discrete or analog hardware; fully digital GPU-style architectures became dominant much later.

FPGA Tooling and Ecosystem

  • Tooling is widely criticized (e.g., vendor IDEs); seen as a key barrier to broader FPGA use.
  • Open-source efforts mentioned: Yosys, F4PGA/SymbiFlow, Verilator. These are promising but considered less mature than mainstream compiler stacks.

Custom GPUs and Related Projects

  • Several related FPGA graphics projects are cited:
    • A Quake level viewer and tiny Quake renderer on very small FPGAs.
    • A mid‑90s‑style custom FPGA GPU that runs Quake via a Vulkan‑like API.
    • A “Doom chip” and other tiny GPU experiments.
  • There is interest in combining custom CPUs and GPUs on the same FPGA or via PCIe, though hardware and board design complexity are noted.

MRISC32 ISA, Vectors, and Microarchitecture

  • MRISC32 is a “vector‑first” RISC-style ISA with simple hardware vectors that work well for Quake/Doom rasterization loops.
  • Current vector use is hand‑written assembly; compiler auto‑vectorization support is still missing.
  • Discussion dives into pipeline design, branch prediction strategies, and how branch delay slots compare to modern predictors.

RISC vs. CISC and GBOoO Debate

  • Long subthread debates:
    • How modern x86 cores internally translate complex instructions into micro-ops.
    • Whether they should be seen as “load/store RISC under the hood” or as a separate class (“great big out‑of‑order” designs).
    • Tradeoffs in code density, uop cache size, instruction decoding complexity, and power efficiency across x86, AArch64, and RISC‑V.
  • Consensus: front-end complexity and uop caches are a major “x86 tax,” but all high‑performance cores, regardless of ISA, now converge on similar large out‑of‑order microarchitectures.

Display, Resolution, and Retro Aesthetics

  • Some prefer low‑resolution Quake on CRTs, arguing that analog smoothing and brain “fill‑in” made games feel better than today’s sharp LCD scaling.
  • The demo renders at low res, then scales with nearest neighbor to 1080p, producing very sharp pixels; opinions differ on whether additional smoothing would be desirable.

Practical Getting-Started Advice

  • For FPGA newcomers, advice is to:
    • Buy a cheap FPGA dev board and start experimenting.
    • Use Verilator for simulation once designs grow.
    • Browse community resources (e.g., FPGA-focused forums/subreddits) for board and tutorial recommendations.

Miscellaneous

  • Some side discussion on blocking intrusive “Sign in with Google” popups; uBlock Origin with “annoyances” filters is reported to help.