0xrsydn/gameboy-rust
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fix(memory): return the Game Pak address pattern beyond the ROM chip

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CPU and DMA reads past the supplied image returned Unmapped and stopped the
machine; a second game's boot code reads 0x09fe20f8 past a 16 MiB image. The
Game Pak shares data lines with the low address lines, so where no chip drives
the bus a read returns the halfword address, (A >> 1) & 0xffff, per lane
(GBATEK; mGBA uses the same pattern).

The chip is the image size rounded up to a power of two. Bytes between a
trimmed image's end and that boundary are unknown and stay diagnostic, as do
host reads and instruction fetches. Two truncation fixtures move into that gap.

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 11:58:36 +07:00
commit 4e4dde742d
11 changed files with 120 additions and 11 deletions
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docs/architecture.md
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@ -51,6 +51,7 @@ The repository is a Cargo workspace with three crates:
| `crates/gba-core/src/cartridge.rs` | Explicit GPIO/RTC selection, ROM read overlay, pin directions, caller-clock routing, and bounded diagnostics |
| `crates/gba-core/src/cartridge/flash.rs` | Explicit save devices, Macronix IDs/banks, delayed program/erase, bounded DQ7 polling, and staged mutations |
| `crates/gba-core/src/cartridge/sram.rs` | 32 KiB byte-wide SRAM, mirrored offsets, one staged byte per step, and its save-state section |
| `crates/gba-core/tests/rom_bus.rs` | Game Pak reads beyond the ROM chip in every window, trimmed-image gaps, strict host/fetch reads, and DMA sources |
| `crates/gba-core/tests/sram.rs` | SRAM selection, mirroring, byte stores/loads from ROM code, SWPB, width/DMA diagnostics, and save states |
| `crates/gba-core/tests/flash.rs` | Erased/supplied images, chip IDs, all bank addresses, invalid commands, reset limits, DMA rejection, and RTC independence |
| `crates/gba-core/tests/flash_write.rs` | Program values, sector/chip scopes, delays, polling, cancellation, bit transitions, and HALT/STOP |