Save State Format (v2)

This documents the chunked save-state format introduced in Snes::saveState/loadState (state file version 2).

Goals

• Forward/backward compatible: per-file header + per-chunk versioning.
• Corruption-resilient: CRC on each chunk; size caps to avoid runaway allocations.
• Explicit layout: avoid raw struct dumps to sidestep padding/endianness issues.

File Structure

uint32  magic   = 'YAZE' (0x59415A45)
uint32  version = 2
repeat until EOF:
  Chunk {
    uint32 tag      // ASCII packed: 'SNES', 'CPU ', 'PPU ', 'APU '
    uint32 version  // per-chunk version; currently 1 for all
    uint32 size     // payload size in bytes (capped at 16 MiB)
    uint32 crc32    // CRC-32 of payload
    uint8  payload[size]
  }

Chunk Payloads (v1)

• SNES: Core machine state (WRAM, timers, IRQ/NMI flags, latched ports, timers, etc.).
• CPU: CPU registers/flags + breakpoint list (capped to 1024 entries).
• PPU: PPU registers, VRAM/CGRAM/OAM, layer/window/bg structs written field-by-field.
• APU: APU registers, ports, timers, and 64K ARAM (capped) plus DSP/SPC700 state.

Compatibility

• Legacy v1 flat saves (no magic) are still loadable: the loader falls back if the magic/version header is missing. They do not carry CRCs and remain best-effort only.
• Host endianness: serialization assumes little-endian hosts; load/save will fail fast otherwise.

Validation & Errors

• Size guard: any chunk size > 16 MiB is rejected.
• CRC guard: mismatched CRC rejects the load to avoid partial/dirty state.
• Missing required chunks (SNES, CPU, PPU, APU) rejects the load.
• Streams are checked for fail() after every read/write; callers receive absl::Status.

Extending

• Add a new chunk tag and bump its per-chunk version only. Keep file version stable unless the top-level format changes.
• Keep payloads explicit (no raw struct dumps). Write scalars/arrays with defined width and order.
• If you add new fields to an existing chunk, prefer:
  1. Extending the payload and bumping that chunk’s version.
  2. Keeping old fields first so older loaders can short-circuit safely.

Conventions

• Tags use little-endian ASCII packing: ‘'SNES’->0x53454E53.
• CRC usesrender::CalculateCRC32`.
• Max buffer cap mirrors the largest expected subsystem payload (WRAM/ARAM).