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feat(cartridge): add 32 KiB SRAM save memory

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Select it with --save-type sram (SaveDevice::Sram). The array is byte-wide,
starts erased, and repeats every 32 KiB through the 0x0e00xxxx save window.
A CPU byte store is staged and committed when its step completes, so failed
steps roll back and reads later in the step see the byte. Unlike Flash, SRAM
can be read from ROM code. Halfword/word access and DMA stay diagnostic until
the 8-bit bus behavior for those widths is verified.

SRAM shares the save-image API, host save files, and save states. Its device
code is new, so existing save states keep loading.

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 11:51:27 +07:00
commit 32eaffa0fe
15 changed files with 405 additions and 30 deletions
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docs/hardware/cartridge.md
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@ -35,7 +35,7 @@ Reads past supplied cartridge bytes remain explicit memory errors, not open-bus
All three Game Pak ROM windows use the same supplied bytes and existing wait-state rules.
Ordinary cartridge bytes remain read-only. CPU and DMA writes to the `0x08`–`0x0c` ROM windows pay their bus cost and are ignored, as on hardware.
Writes to the `0x0d` window remain `ReadOnly` diagnostics until EEPROM is modeled there. Host `Memory::write*` calls stay strict everywhere.
Optional `--save-type flash64` or `--save-type flash128` enables [nominal Flash programming, erase, and reads](saves.md).
Optional `--save-type flash64` or `--save-type flash128` enables [nominal Flash programming, erase, and reads](saves.md); `--save-type sram` selects [32 KiB SRAM](saves.md#sram).
Without `--save-file PATH`, changes remain volatile. With it, the frontend loads an exact-size image and persists changes on clean exit or when a ROM window ends on a diagnostic.
Missing images start erased. Existing images receive unique backups before replacement; incomplete Flash operations refuse persistence.
Read the [storage safeguards and limits](saves.md#desktop-persistence) before using a valuable save.