NX2navi/docs/architecture.md

Architecture

How the Switch 2 build of Ocarina of Time (game version 1.1.1) becomes a native Windows x86-64 program. Code taken from other projects, with its license, is listed in THIRD_PARTY_NOTICES.md.

Pipeline

1. Input

• The ExeFS and RomFS of the user's own dump. Everything extracted or generated stays under build/, generated/ or the user's own folders, never in the repository (tools/check_content.py).

2. CPU: lift ARM64 to C

• Modules: main, sdk and the subsdk* modules. rtld is dropped; the host links the modules.
• A hand-written decoder (recomp/lifter.py) and NEON/FP lowering (recomp/simd_c.py, runtime/simd.h) emit one C function per guest function, direct calls for BL and an address-keyed table for indirect branches.
  • The generator runs in parallel processes, with bounded memory per worker.
  • Jump tables are resolved by a symbolic pass of our own (recomp/jumptables.py).
  • LDXP/STXP (128-bit exclusive pairs) are supported.
  • NZCV and the six most used general registers of each function live in C locals instead of the Cpu (recomp/flags.py, recomp/regs.py): registers are stored back where they changed, before calls, SVCs and exits, and reloaded after them. A call reloads only X0-X18 and X30, as AAPCS64 has the callee preserve X19-X29 and SP; gen.py --no-callee-saved-trust reloads those too.
• Memory: guest address equals host address, all below 2^39 (IPC encodes 39-bit addresses). The heap, stacks and the alias region are reserved with MEM_WRITE_WATCH, so the graphics back end can ask which pages the CPU wrote since the last upload (runtime/memory.c).
• Indirect branches go through rt_dispatch (runtime/dispatch.c), which looks the target up in the address table and tail-calls it.
• Testing: a differential harness checks every lifted instruction class against Unicorn (test-only, never linked into the game).

3. Host

• The SDK boundary is at the nn::* function level: threads and synchronisation, IPC, memory, fs/RomFS, time, account, applet, hid, audio renderer and effects run as host code. Only audren:u and sm: are real IPC services.
• Window, keyboard and mouse: Win32 (gfx_window_win.c, input_win.c). Gamepads and audio output: SDL3 (every connected gamepad is open; the game reads the one pressed last, input.c). Voice: libopus. Movies: a host VP9 decoder over libvpx replaces the SDK's movie::Decoder (runtime/ov_movie.c).
• Settings and menu: every key with its default is in one table (cfg.c), read from nx2navi.ini and NX2NAVI_* variables and written back when the in-game menu changes it. The menu and the performance overlay are Dear ImGui, drawn into the swapchain image (runtime/ui/); their text is in ui_text.c.
• OS calls the host makes go through runtime/platform/ (platform.h, win32.c).

Threads and cores

Every guest thread is a host thread. The game assumes Horizon's scheduler: three application cores, a thread runs only on a core in its affinity mask, and a higher-priority thread takes a core from a lower-priority one. The game keeps per-core data without locks and relies on that, so the host models it (runtime/kernel.c):

• Three virtual cores. A guest thread runs guest code only while it holds a core; the others wait in a priority-ordered list.
• Preemption. When a waiting thread has a better priority than a holder on a core it may use, exactly one holder is flagged. The holder checks the flag at its next SVC and at every indirect call (rt_dispatch), checks again under the kernel lock that a better thread still waits, and only then hands its core over.
• Asynchronous preemption (on unless NX2NAVI_ASYNC_PREEMPT=0): a waiter that has spun 250 us suspends the holder and, if its host rip is in lifted code at host depth 0, moves it to kern_ap_stub, which saves the volatile registers and gives the core up there. A holder of a guest mutex counts at the priority of its best waiter, and one inside an SDK spin lock is not preempted.
• Host work gives the core back. Queue submits, presents, GPU waits, shader translation, nn::fs calls and NVN memory pool setup run without a core (NVN_BLOCKING, kern_core_away).
• Spin waits. Loops that only another thread can end send their back-edges through rt_spin_gate, which pauses the core once per pass and hands it to a waiter after 50 us of polling (NX2NAVI_SPIN, NX2NAVI_SPIN_HANDOFF_US).

4. Graphics

• NVN 2 front end (nvn_api_*.c, nvn_obj.h, texture layout and deswizzle, the Switch 2 GOB layout). The game records command buffers through it; nothing reaches the GPU until the game submits.
• Vulkan 1.3 back end. At QueueSubmitCommands the recorded NVN commands are replayed into Vulkan command buffers on the submitting thread (vk_exec*.c); a recorder thread makes the vkCmd* calls from a ring of packets (vk_rec.c).
• Memory pools are imported as host memory (VK_EXT_external_memory_host). On a discrete GPU, pools bound as buffers are mirrored in device-local memory and the pages the CPU wrote (write watch) plus the ranges the game flushes are uploaded before a replay (vk_memory.c); without the mirrors the GPU reads them over PCIe and Kokiri Forest runs at 7 fps. GPU-rendered textures are written back when the CPU reads them.
• Uniform snapshot. The game rewrites per-object uniform blocks soon after it submits the frame that reads them; at the start of a submission's replay every uniform range it binds from CPU-written pools is copied, and the draws bind the copy (vk_usnap.c).
• Driver data. The NVN driver keeps a per-stage constant bank (c0). The back end fills it as the driver does: storage and uniform tables, push constants and, for NVN 2 compute, the dispatch's group counts in bytes 0-0xb.
• Storage buffers the shaders reach through pointer tables in memory are bound at the address the table holds (GfxShaderRes.ptr, vk_exec_draw.c ptr_addr).
• Bindless textures: the texture and sampler pools are descriptor arrays kept in step with the game's pool writes (vk_exec_draw.c pool_sync); slots that resolve alike share one lookup.
• Shaders: Ampere (SM86) code is lowered to Maxwell (tools/xlate/AmpereLowering.cs) and translated to SPIR-V by the pinned shader translator (tools/vendor/ryujinx, 1.3.2) with our patches (tools/xlate/patches/), built with .NET 10 NativeAOT as nx2navi_xlate.exe and run as separate processes. Translations are cached on disk; shaders and pipelines seen in play are prepared at the next boot. The game waits for it (runtime/boot.c boot_wait): about half a second when warm; after a translator, GPU or driver change a "Preparing shaders" screen shows the progress (ui/menu.cpp menu_boot_frame). With the graphics pipeline library the boot builds recorded pipelines in record order for at most 20 s after the shaders are ready and leaves the rest to the pipeline workers below normal priority; a draw that meets one before it is built gets a fast-linked pipeline (vk_pipeline.c PREWARM_PIPE_MS). NX2NAVI_PREWARM_FULL=1, and drivers without the library, wait for every pipeline. NX2NAVI_PREWARM_WAIT_MS caps the whole wait, after which the rest finishes in the background.
• Mesh and task shaders run as VK_EXT_mesh_shader; indirect mesh draws read NVN's {count, first} record through a ring of Vulkan {x, y, z} records filled on the GPU.
• Pipelines: VK_EXT_graphics_pipeline_library where the driver has it: stage libraries compiled ahead, a fast link at a new state, an optimised pipeline replacing it from a background thread. A draw whose pipeline is not ready is skipped when one of its pass's targets was rendered in the previous frame, and waits otherwise. Compute pipelines are compiled outside the pipeline lock, and one compile serves every thread that needs it.
• The game's tensor-core upscaler is replaced by a decode-and-scale pass (vk_tonemap.c); vk_fsr.c runs AMD FSR 1 (EASU, RCAS) on the present when the window is larger than the picture.
• ASTC textures are uploaded as they are where the device samples ASTC, and decoded by a compute shader everywhere else (vk_astc.c); under 20 GiB of video memory the decoder encodes them to BC3.
• Occlusion queries never block the CPU; a result that is not ready counts as visible.
• Allocation: Vulkan Memory Allocator. Debugging: Vulkan validation layers, per-pass frame dumps (NX2NAVI_DUMP, vk_dump.c), a sampling profiler (NX2NAVI_PROFILE, tools/prof_report.py) and a flight recorder (NX2NAVI_FLIGHT). RenderDoc cannot capture, because it hides VK_EXT_external_memory_host from the application.

Maxwell behaviour the back end has to match: maxwell.md.

5. Toolchain

• Generated C: MSYS2 UCRT64 Clang + LLD. The generated code uses GNU extensions (__int128, musttail, vector types) that cl.exe cannot compile.
• Host code: the same Clang, C11.
• Builds write a Ninja file and run MSYS2 Ninja. Every object is hashed before linking, because a faulty RAM module once flipped bits in stored objects.