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fix(bios): initialize affine scales before cartridge boot

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0xrsydn 2026-10-06 22:35:26 +07:00
commit 1b39a7f33f
8 changed files with 157 additions and 3 deletions
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docs/hardware/bios.md
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@ -23,6 +23,12 @@ Minimal boot initializes:
- Supervisor stack: `0x03007fe0`.
- BIOS IRQ flags at `0x03007ff8` and the callback pointer at `0x03007ffc`: zero.
- IME: zero; POSTFLG: one.
- BG2/BG3 affine PA and PD: 256 (identity scale), through executed ARM halfword stores.
Fresh device state supplies zero PB, PC, and origins. Raw `Memory::new` defaults remain unchanged.
This prevents bitmap and affine ROMs from sampling one pixel across the screen when they rely on firmware initialization.
Original regressions verify bitmap coordinates, both affine backgrounds, and scanline capture without ROM-side matrix writes.
See the [boot graphics investigation](../research/boot-video.md) for the independent homebrew failure and evidence.
It then enters `0x08000000` in ARM System mode with IRQ/FIQ masks clear.
It does not reproduce Nintendo's logo, cartridge-header checks, RAM clearing, boot delays, or full hardware initialization.