Save/load state

[fengb]

Every other emulator does this. Why can't we?

Questions:

• Which format? This should probably be a compressed binary, but there's still a lot to choose from:

  cpu: [mode, interrupt_master, registers]
  mmu: [0x8000-0xffff, mbc state]
  video: [screen, caches?]
  inputs: *
  timer: *

  • manual
  • json probably not due to needing binary data
  • protobuf
  • cap'n'proto

• What memory needs to be copied? At the minimum registers, MMU, and possibly the screen, but we might need a bunch of other internal states. Should we simply copy everything?

• Should we consider rewind semantics?

[fengb]

Format ideas:

1. Custom binary dump. Let's do a simple "size: u16 - payload: []u8 — size: u16 - payload: []u8" for now. This is intentionally not standardized because Zig doesn't have a prebuilt one and I don't want to build one (yet)
2. Header magic number:
   • 0xDC — busts ascii (high bit) + unicode (0xDC00 - 0xDC7F is illegal as first char for both UTF8 and UTF16)
   • 0x00 — version (increment this when the format changes)
   • 0x46 — F
   • 0x44 — D
   • the following are shamelessly stolen from PNG
   • 0x0D — CR
   • 0x0A — LF (DOS line)
   • 0x1A — DOS EOF
   • 0x0A — LF (Unix line)
3. Cart checksum — copy of cartridge cart[0x134..][0..0x18] (24 bytes)
4. Footer trailing checksum decided this is out of scope:
   • we have some simple redundancies already
   • adds a fair bit of complexity
   • compression will have its own (possibly better) checksum

[fengb]

Note: first pass will not do compression because Zig doesn't have a standard implementation yet.

[fengb]

Implemented via 053e8dc

Still iterating super quickly so maybe we shouldn't expose this to the frontend yet.