Brian Degenhardt
7f93a80dd7
Under GSBackThreadMode >= Lockstep roughly half the GS work moves to a second thread, and every surface that reports GS cost -- OSD, PerfLog, the Qt status bar, PINE stats, gsrunner's @HWSTAT@ block -- measured the MTGS thread alone. So the split read as a large GS saving. It is not: on a Rogue Galaxy savestate here, mode 0 costs 15.8% / 2.63 ms and mode 3 costs 17.0% / 2.84 ms plus 14.2% / 2.37 ms on the back thread -- about twice the total GS CPU time, bought to halve the critical path. That is a real trade, but nobody could see it. The back thread registers its own handle at entry, as the SW rasterizer workers do; StopBackThread clears it after the join. Unlike every other handle here it is written by a thread other than the one sampling it, so the handle and its running total sit behind a mutex taken twice a second. Installing a handle rebases the total off it, so the first window after a GSreopen respawn measures the new thread rather than its difference against the retired one's. The figure is omitted, not reported as zero, wherever a back thread does not exist -- otherwise a mode 0 vs mode 3 comparison reads a permanent 0% as meaningful. gsrunner latches the presence flag during the run because DumpStats executes after VMManager::Shutdown, by which point the thread has joined.
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