pstef
83f2510134
The VU's largest value is 0x7FFFFFFF, one binade above FLT_MAX, the same range the EE FPU has. So an exponent-255 word is an ordinary number on the way in and on the way out, and "overflow" starts above it rather than above FLT_MAX. Both engines put the boundary a binade lower. vuDouble() rewrites an exponent-255 operand as 0x7F7FFFFF and VU_MAC_UPDATE() calls every exponent-255 result an overflow; the arm64 COP2 macro emitters clamp the result to +/-FLT_MAX and raise neither O nor U. microVU's per-op operand clamps approximate the same thing from a list of games rather than a rule. 68 rows off an SCPH-90000 through VU0 macro mode, scored per engine and per column, with what each engine cannot yet reproduce recorded per case so a fix trips the test as loudly as a regression. Nothing is fixed here. Two of the rows are structural rather than about range. An overflowed product does not become 0x7FFFFFFF before the accumulate: an addend of -0x7FFFFFFF cannot cancel it. An underflowed product does become zero before it. And the multiplier is the EE's, with the same one-ULP deficit decided by ft's mantissa alone. The harness grows two things the rows need: a VADDA encoder, and an opt-out from Run()'s VU0 JIT-vs-interp auto-diff for tests that score each engine against a hardware capture instead of against the other engine.
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