Brian Degenhardt
0b9e9cdfcb
The per-lane colour offsets were packed with the signed saturating pack while both code generators used the unsigned one. The mask above the pack has already put every lane in 0..65535, which makes the unsigned pack the identity and makes the signed pack flatten everything from 32768 up to 32767. A descending gouraud gradient is how a lane gets there: its offset is negative, the mask turns it into a large positive, and the pack saturates it. Every pixel of the group then carries that instead of its own colour, for the whole scanline. The mask and the unsigned pack were introduced together to fix exactly this, in "GS/SW: Mask color gradients to prevent incorrect clamping"; a later refactor that rewrote the same lines to change how the shift table is loaded retyped the tail back to the signed pack. The generators were not part of that refactor, which is why only the C++ path regressed and why nothing noticed. Where the path is reachable, measured rather than argued: with the rasteriser JIT on, a probe at the top of the C++ setup never fires across corpus replays that generate tens of kilobytes of scanline code apiece. It is entered only when there is no code memory to compile into at all, and that same condition turns off the EE, IOP and VU recompilers, so it is not a configuration anyone plays in. What it is, is the path a measurement runs under -- the only way to ask what the renderer computes without a JIT in the way, and so the arbiter of a generated-code question. It was about to arbitrate one, and would have lied: the gs-shade console capture re-run under it differed from the generated arm in 42,240 bytes, concentrated in exactly the gouraud colour it was to be asked about. It is now byte-identical, and the generated arm is byte-identical to before the change, so nothing a shipping build renders moves. The new suite runs both paths over the same spans and compares the setup state and the stored pixels, so the next divergence anywhere in the scanline fails loudly instead of waiting for a capture to find it.
ARMSX2 — Native ARM64 JIT Fork of PCSX2
ARMSX2 is a free and open-source PlayStation 2 (PS2) emulator based on PCSX2. Its purpose is to emulate the PS2's hardware, using a combination of MIPS CPU Interpreters, Recompilers and a Virtual Machine which manages hardware states and PS2 system memory. This allows you to play PS2 games on your phone, PC, or gaming handheld, with many additional features and benefits.
Thank You
The ARMSX2 team is eternally indebted to the PCSX2 project it is based on. We are so fortunate to build on their 20 years of hardcore development.
About This Fork
The upstream PCSX2 project ships an ARM64 interpreter build for ARM, but its high-performance JIT recompilers (EE, IOP, VU0, VU1, and vtlb fast memory) are x86-64 only.
This fork exists to close that gap. The goal is to preserve the correctness features of 20 years of PCSX2 development, while generating the fastest native ARM performance possible.
Current status:
- ✅ EE (Emotion Engine) recompiler — integer, float, MMI, COP0/COP1/COP2, branches, load/store
- ✅ IOP (I/O Processor / R3000A) recompiler — full integer, load/store, branches, coprocessors
- ✅ VU (Vector Unit) recompiler — microVU skeleton + Upper FMAC vector ISA complete; Lower ISA and runtime complete
- ✅ vtlb fast memory
- ✅ Native ARM64 binary builds and boots the PS2 BIOS
- ✅ 2D games are already playable
- ✅ 3D games run
Why LLMs / AI Were Used
A word on methodology:
The x86-64 JIT code in upstream ARMSX2 is already proven correct — it has run thousands of PS2 titles for years. The challenge in this port is not emulator design or JIT theory; it is mechanical translation of a large, well-understood x86-64 assembly codebase into equivalent ARM64 assembly (via VIXL) while preserving the exact same register-allocation contracts, block lifecycle, and recompiler semantics.
Large language models (LLMs) were used as an accelerant for this translation work — pattern-matching x86 JIT boilerplate to ARM64 equivalents, scaffolding emit routines, and keeping the porting velocity high. The JIT logic (block compiler, dispatcher, analysis passes, flag pipelines, clamping rules, Tri-Ace hacks, etc.) is taken directly from the upstream x86 implementation and validated against it. Nothing was hallucinated from scratch.
In other words: the hard engineering was done by the PCSX2 team over two decades. The hard typing — translating ~50k lines of x86 emitter code into ARM64 — is what AI helped compress.
System Requirements
ARMSX2 targets ARM64 across desktop (macOS, Windows, Linux) and mobile (Android, iOS/iPadOS), all from the single shared core. Our setup documentation page contains additional details on software and hardware requirements.
Please note that a BIOS dump from a legitimately-owned PS2 console is required to use the emulator. For more information, visit this page.
Building
Check out our github actions for the latest build recipe