Brian Degenhardt
a3bf73bf7a
FAST_UNALIGNED was defined only inside the ARCH_X86 arm, where it records that AVX-and-later cores stopped punishing unaligned vector loads. On ARM64 the macro was therefore undefined, which the preprocessor reads as zero, so every arm64 build compiled the texture-upload path as though the punishment existed. It never did. LDR Q and LD1 take any address, and GSVector4i's load template ignores its own `aligned` parameter and emits the same instruction either way. So the callers were paying for a distinction with no machine behind it: WriteImage tests the source address and the pitch on every call to choose between three template instantiations of WriteImageBlock and WriteImageColumn that, for 8- and 4-bit columns, compile to identical code. For 32- and 16-bit columns the unaligned arm is not identical, but it is the worse one — eight combining 64-bit loads instead of four 128-bit loads and a swizzle. Defining it collapses all of that. GSLocalMemoryMultiISA.cpp.o goes from 80,368 to 62,184 bytes of .text and from 58 emitted functions to 32, which is what an I-cache on a handheld cares about. Only GSBlock.h and GSLocalMemoryMultiISA.cpp read the macro, so nothing else moves. The retained load strategy is not new code: whenever an upload happened to land 32-byte aligned, arm64 already ran exactly this sequence. What goes away is the arm that only ever ran when it did not.
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