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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/usability-roadmap.md
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@ -65,7 +65,7 @@ A reproducible source build or an accessible upstream ROM is needed before recor
Next implementation steps:
1. Visible startup/menu progress is established: attract mode reaches the start menu and Birch dialogue, and sparse taps reach the in-game start menu by 12,000 frames ([probe](research/emerald-visible-progress.md)).
The first demonstrated second-game failure is a boot-init halfword store to ROM mirror `0x09fe2ffe`. ROM-window writes are now ignored; reads past the image end remain diagnostics.
The first demonstrated second-game failure is a boot-init halfword store to ROM mirror `0x09fe2ffe`. ROM-window writes are now ignored, and reads beyond the ROM chip return the Game Pak address pattern.
Use original regressions for any hardware correction; do not invent connected serial devices.
2. Validate an external game's completed save/restart/load cycle using a disposable `--save-file` path.
Refine nominal Flash timing and polling only with independent evidence and original regressions.