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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/references.md
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@ -6,6 +6,8 @@ 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 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.
- [GBATEK BIOS halt functions](https://problemkaputt.de/gbatek-bios-halt-functions.htm), for wait contracts, STOP clock gating, allowed wake sources, and the clock-off IF note.
- [GBATEK BIOS function calling conventions](https://problemkaputt.de/gbatek-bios-functions.htm).