Brian Degenhardt
9f73c77d59
Saving a state from the pause menu aborted every assert-enabled build. The screenshot the save embeds goes through MTGS::RunOnGSThread, which asserts it is on the CPU thread, and the JNI entry point ran the whole save inline on whatever thread the picker dispatched it from. Parking the VM first, which is what these two entry points did, is not the same guarantee. It stops the EE, but the MTGS ring's write position is single-producer and owned by the CPU thread, and the CPU thread does not stop producing when the VM is paused: its idle loop keeps draining Host::PumpMessagesOnCPUThread() every 16 ms, so any GS-settings apply or window resize queued from the UI pushes to the same ring the save is pushing to. Two producers claiming one slot drops a packet, and a dropped data-packet header leaves the GS thread parsing payload qwords as command tags. So marshal both entry points with a blocking Host::RunOnCPUThread, matching what commitSettings and changeDisc in the same file already do. The park stays: it stops the EE for the inline zip and holds the audio pause the picker is built around. Thread identity is what makes the ring pushes legal. The load path is fixed alongside it. It has the identical violation — Freeze on the way in, plus a recompiler cache reset — and goes unreported only because MTGS::Freeze pushes its packet directly rather than through RunOnGSThread. Its follow-up present moves into the same task, which also stops it racing the resume in the pause guard's destructor.
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