Brian Degenhardt
47614daa67
psxRecClearMem walks down through recBlocks from the block containing the written word to pick up straddlers — blocks that start below it but whose bodies reach into the range being cleared. recBlocks is ordered by startpc, not by end address, so stopping at the first block that ends before the write is unsound: a longer block lower down can jump clean over a short one and still cover the write, and the scan quits before it is ever examined. The straddler is then neither removed from recBlocks nor LUT-reset, so it keeps executing stale code. This is the IOP half of the same blind spot3ac41e05bdfixed on the EE; both stops came from801d71f7f0(2009), which added them to iR3000A and iR5900 in one commit and is still upstream in both. Scan past a non-overlapping neighbour instead of stopping on it, bounded by s_maxBlockBytes — the longest guest extent compiled since the last reset, so the walk still terminates as soon as nothing below could reach. The scan cursor is split from the removal start, so a skipped survivor never becomes the base of the removal range. Unlike the EE, IOP blocks have no 4KB page break (psxRecRecompile ends them only at a branch or an already-compiled head), so the mark has no structural cap beyond the 0xffff-instruction assert. IOP blocks are short in practice; the cost is flat-array reads either way. New gate ClearScansPastNonOverlappingNeighborToStraddlerBelow builds the geometry the bug needs — A long, H compiled mid-A, then B truncated against H's compiled head, which is the only way to get a block that both starts inside A and ends before A does. It pokes a word inside A past B's end and requires A to run the new instruction. Red before this change with the JIT returning the stale 0x11 against the interpreter's 0x22, green after. recompiler_tests 1439/1439.
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