Brian Degenhardt
80e4d09b99
The kick's arming window exists to answer one question -- has this game read
back recently enough to be worth kicking for -- so that titles which never read
back see zero change. It counted render passes, and 128 passes means completely
different things in different titles: about three frames of OutRun 2006, but
only about three quarters of a Rogue Galaxy frame. So RG armed the window at its
one readback per frame, spent it partway through, and then ran the rest of every
frame with the kick silently switched off. Nothing asked for that; it fell out of
the unit.
Count the window in frames since the last readback instead, which is the unit the
comment already claimed ("~a few frames' worth of render passes") and the unit the
decision is actually about. The cadence stays in render passes, where a uniform
interval is what you want. The never-read-back guarantee is unchanged and still
carried by the ~0u sentinel.
Measured on M2/Honeykrisp, 60-90 frames per dump, gsrunner without -perf: total
GPU stall (readback wait plus command-buffer activate stall) is unmoved --
Rogue Galaxy 554ms before and 558ms after, OutRun 2006 320ms and 319ms, both
inside run-to-run spread. Shadow of the Colossus and Black, which never read
back, take zero kicks before and after. So this is not a speed change here; it
removes a scene-dependent cliff that a device where the kick matters more could
land on.
While measuring, the threshold's cost model turned out to be badly wrong, so
correct the comment. "RPs-per-frame / threshold extra submits" predicts ~14
kicks/frame for Rogue Galaxy; the real figure is 2, because the fence gate -- not
the threshold -- is what binds. With three command buffers only two submissions
can be in flight, and ~3300 of ~3400 offers to kick find the next command buffer
still executing. Sweeping the threshold 8->16 measured -2% stall on Rogue Galaxy
and +12% on OutRun 2006, so it is left alone.
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