pstef
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Exponent 255 is an ordinary exponent on this FPU: 0x7F800000 is 2^128, 0x7FFFFFFF is the largest number the machine has, and there is no Inf and no NaN. That is one binade above IEEE single, so fpuDouble() folded every such operand down to +/-0x7F7FFFFF on the way in and clampToEeRange() folded a host infinity back to the same word on the way out. ADD.S, SUB.S, MUL.S, their A-forms and the four multiply-accumulates now read their operands through eeToDouble() and round once through a new eeRoundToSingle(). Why the double arithmetic in between is exact, and how the top binade is rounded with no host single to round it in, is at eeGuardedAddSub, MUL_S and eeRoundToSingle. The multiplier's one-ULP deficit had to move across with it or it would have been lost here: it sat on top of fpuDouble() and read the operands as clamped, which changed ft's mantissa and so changed its own predicate. eeMulRound applies it to what eeRoundToSingle produces instead, and the six multiply forms reach it through eeMulProduct's replacement, eeMulAccumulate. One behavior changed beyond the range: an overflowing product now ends a MADD/MSUB, value as well as flag, where the value path used to carry on accumulating and land a binade away. The fast path is untouched. It computes in host singles and cannot hold these values at all; the warning against changing posFmax to suit it is at the top of ee_fpu_overflow_console_conformance_tests.cpp. DIV and RSQRT still read operands through fpuDouble and still saturate through checkDivideByZero's posFmax. A quotient of two singles is not exact in a double, so the eeToDouble route would double-round under the divide unit's round-to-nearest; they need the normalise-and-reassemble treatment the next commit gives them.
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