<!-- Adopted 2026-09-16 from docs/XBOX360_ARCHITECTURE.md in the project root —
     early notes from before any of Sylpheed's formats were decoded. Only the
     platform facts are kept; the per-format "status" sections and the old
     `just sniff` workflow were out of date and are dropped. Not measured by this
     project. -->

# Xbox 360 Architecture — RE Quick Reference

A condensed reference for reverse engineering Xbox 360 games. The longer
[technical reference report](xbox360-re-technical-reference.md) has the
binary-level detail. Project Sylpheed's own formats are decoded, and each has a
page under [`docs/re/`](../re/INDEX.md).

---

## CPU: IBM Xenon

| Property | Value |
|----------|-------|
| Architecture | PowerPC 2.02 (64-bit ISA, 32-bit addressing in games) |
| Cores | 3 symmetric cores × 2 SMT threads = **6 logical processors** |
| Clock | 3.2 GHz |
| Execution | In-order, dual-issue superscalar (no speculation) |
| L1 I/D cache | 32 KB each, per core |
| L2 cache | 1 MB unified, shared across all 3 cores |
| Vector registers | **VMX-128** — 128 × 128-bit VRs per thread (vs. standard AltiVec's 32) |
| Endianness | **Big-endian** |

**Key for RE:** All multi-byte integer and float values in game binaries are big-endian.

The VMX-128 extension adds dot-product instructions (`vmsum3fp128`/`vmsum4fp128`)
and D3D pack/unpack instructions absent from standard PowerPC. Their semantics are in
the [PowerPC instruction manual](../../tools/ppc-manual/README.md).

---

## GPU: Xenos (ATI R400/R500 derived)

| Property | Value |
|----------|-------|
| Clock | 500 MHz |
| Shader processors | 48 unified (3 SIMD × 16, each with 5-wide VLIW ALU) |
| Peak throughput | ~240 GFLOPS |
| On-chip memory | **10 MB eDRAM** (framebuffer, depth, MSAA — no separate VRAM) |
| System RAM | 512 MB GDDR3 shared with CPU |
| Northbridge | GPU also acts as northbridge / memory controller |

### Command Packets (PM4)

The GPU is driven by a **PM4 command ring buffer** written by the CPU. Key packet types:

| Type | Description |
|------|-------------|
| `DRAW_INDX` | Indexed draw call |
| `SET_CONSTANT` | Write shader constant registers |
| `SET_TEXTURE_CONTROL` | Texture fetch constant (address, format, tiling mode) |
| `LOAD_ALU_CONSTANT` | Load vertex shader constants |

Xenia Canary's `src/xenia/gpu/` implements a full PM4 parser — invaluable for
understanding draw calls and state management.

### Texture Tiling

Xbox 360 textures are stored **tiled** in memory, not linear; the Xenos hardware
de-tiles on fetch. Texture data read from game files must be de-tiled before it is
displayed. Tile size depends on the texture format (DXT1 = 4×4 blocks, etc.).

---

## Executable Format: XEX2

`default.xex` is the main game executable in Microsoft's **Xbox EXE (XEX2)** format.

| Offset | Field | Notes |
|--------|-------|-------|
| 0x00 | Magic | `58455832` = `XEX2` |
| 0x04 | Module flags | Security, format flags |
| 0x08 | Header data offset | Points to optional header list |
| 0x10 | Security info offset | RSA signature, image hash |

The executable contains encrypted, compressed PowerPC code. The full layout is in
[`xex2-format.md`](xex2-format.md). This project loads and disassembles it with
`crates/sylpheed-xex` and `crates/sylpheed-xexdb`.

---

## Platform File Formats

### XPR2 — Texture Container

| Offset | Field | Value |
|--------|-------|-------|
| 0x00 | Magic | `58505232` = `XPR2` |
| 0x04 | Total size | u32 BE |
| 0x08 | Header size | u32 BE — data starts at this offset |
| 0x0C | Object count | u32 BE |

Each texture object contains a D3DFORMAT, width/height/depth/mip levels, and tiled
pixel data.

### XWB / XSB — XACT Audio Banks

Wave banks (`.xwb`) and sound banks (`.xsb`) are part of Microsoft's **XACT audio
system**. Audio data is encoded as **XMA** (Xbox Media Audio), a proprietary
Microsoft codec based on WMA Pro.

---

## Useful External References

- [xboxdevwiki.net](https://xboxdevwiki.net) — Community Xbox 360 hardware documentation
- [Xenia Canary source](https://github.com/xenia-canary/xenia-canary) — GPU/kernel RE reference
- [xdvdfs](https://github.com/antangelo/xdvdfs) — Pure Rust XISO reader (a dependency of `sylpheed-formats`)
- [binrw](https://binrw.rs) — Derive-macro binary format parsing (a dependency of `sylpheed-formats`)
- [Free60 Project](https://free60project.github.io/wiki/) — Original Xbox 360 open-source hardware documentation