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feat(cartridge): add 512-byte and 8 KiB serial EEPROM saves

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Select them with --save-type eeprom512 or eeprom8k. The part follows GBATEK's
protocol on Game Pak data bit 0: DMA3 sends one bit per halfword, '11' +
address + '0' requests a 64-bit block that reads back after four dummy bits,
and '10' + address + data + '0' writes one. A write reports busy for a nominal
108,368 cycles before the block is stored. 8 KiB parts decode the low 10
address bits. ROMs above 16 MiB map EEPROM only at 0x0dffff00..0x0dffffff.

Bits shift out only inside machine steps, so failed steps roll back. CPU
stores, byte/word accesses, CPU reads during a read-out, and malformed or
overlapping requests are diagnostics. Blocks are stored first-bit-as-MSB,
matching common .sav layouts, and work with save files and save states.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HhfHfHEuzv2AUTd6erW1gU
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0xrsydn 2026-10-14 12:06:17 +07:00
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docs/references.md
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@ -6,6 +6,7 @@ Hardware references used for the core:
- [GBATEK ARM instruction cycle times](https://problemkaputt.de/gbatek-arm-cpu-instruction-cycle-times.htm).
- [GBATEK GBA system control and WAITCNT](https://problemkaputt.de/gbatek-gba-system-control.htm), including HALTCNT power-mode selection.
- [GBATEK GBA cart backup EEPROM](https://problemkaputt.de/gbatek-gba-cart-backup-eeprom.htm), for the 0x0d mapping by ROM size, request and read-out bitstreams, bit-0 bus, DMA3-only transfers, 10 decoded address bits, and the ~108,368-cycle write.
- [GBATEK GBA unpredictable things](https://problemkaputt.de/gbatek-gba-unpredictable-things.htm): with no cartridge, the ROM area reads as `Address/2 AND FFFFh`, because the Game Pak shares its data lines with the low address lines. The emulator applies the same undriven-bus pattern beyond the ROM chip.
- [mGBA `src/gba/memory.c`](https://github.com/mgba-emu/mgba/blob/master/src/gba/memory.c): out-of-bounds ROM loads return `(address >> 1) & 0xFFFF` per halfword. mGBA applies it from the image size; this emulator keeps bytes up to the power-of-two chip boundary diagnostic.
- [GBATEK BIOS reset functions](https://problemkaputt.de/gbatek-bios-reset-functions.htm), for SoftReset state and restart selection, plus RegisterRamReset flags, RAM bounds, and forced blank.