pstef
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pcsx2/FPU.cpp maintains FCR31's O and U on every EE FPU arithmetic op: checkOverflow saturates an infinite result to +/-fMax and raises O with the sticky SO, clearFPUFlags takes them back down. The recompiler's fast path maintains neither, so an overflowing MUL.S leaves FCR31 reading 0x01000001 where console says 0x01008011. The interpreter is the side that matches the console, so the recompiler is the side that has to move. kFamCases sweeps the whole class, every arithmetic op that touches O or U, rather than the two rows the capture happens to reach. Five tests go in DISABLED, because the emitter that would have closed them was measured and reverted; what it got wrong and what a redesign owes are in the comment above DISABLED_EnginesAgreeExceptOnTheOverflowFlags. DISABLED_UnderflowFlagsNeedFzOff holds the U half, which needs FZ off and brings the denormal-value question with it. The one cross-engine row left, "NAN math", is not about the missing flags: ADD.S on two raw exp-255 words has the engines computing different things before any flag logic runs, and CHECK_FPU_EXTRA_OVERFLOW aligns it exactly. DISABLED_NanMathOverflowIsAnOperandClampModeDifference attributes it to the operand-clamp mode axis instead of leaving it unexplained in kFcrEngineDivergences.
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