fix(bios): initialize affine scales before cartridge boot

docs/architecture.md

@@ -76,6 +76,7 @@ The repository is a Cargo workspace with three crates:
 | `crates/gba-core/src/lib.rs` | Core modules |
 | `crates/gba-demos/src/lib.rs` | Demo modules, original instruction/exception bytes, and raw input-test ROM |
 | `crates/gba-core/src/bios.rs` | Original ARM BIOS image builder, minimal boot, IRQ dispatch, waits, and memory services |
+| `crates/gba-core/tests/bios/boot_video.rs` | Firmware identity scales, bitmap/affine startup rendering, capture, and raw-memory isolation |
 | `crates/gba-core/src/bios/reset.rs` | Emitted ARM SoftReset and selective RegisterRamReset; CPU banks, RAM clearing, and supported device-register resets |
 | `crates/gba-core/src/bios/arithmetic.rs` | Emitted ARM division and integer-square-root routines |
 | `crates/gba-core/src/bios/affine.rs` | Emitted ARM background/sprite matrix services, range checks, and generated sine table |

docs/hardware/bios.md

@@ -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.

docs/references.md

@@ -231,6 +231,13 @@ Shared timer prescaler phase:
 
 See [shared divider scope and tests](research/timer-prescaler.md). The correction does not implement delayed register writes or startup edges.
 
+General gameplay and boot graphics:
+
+- [ZeroDayArcade Pong at c784b603](https://github.com/ZeroDayArcade/Pong-Homebrew-GBA/tree/c784b6036a4f188c50932b411e98126bfcbd07d6), for a licensed independent homebrew, source-defined menu/input behavior, and bitmap startup requirements.
+- [mGBA I/O initialization at 3a5e34be](https://github.com/mgba-emu/mgba/blob/3a5e34be33dc7f8f707e5bc9db69e8a430046f21/src/gba/io.c), cross-checked with its BIOS reset implementation for identity affine scales.
+- [gbadoc register summary](https://gbadev.net/gbadoc/registers.html), for affine coefficient meanings. See [boot graphics evidence](research/boot-video.md).
+- [tolik518/GBA_Pong](https://github.com/tolik518/GBA_Pong/tree/422e22a5d1f7d11710e03ffc19d5bff6ac95240f): a future homebrew candidate found during research. It has not been built or tested here.
+
 Use [GBATEK](https://problemkaputt.de/gbatek.htm) and ARM7TDMI documentation for further hardware work.
 Only use game ROMs that you may lawfully use. Do not commit game ROMs, BIOS files, or game assets.
 The ignore file excludes `roms/`, common game ROM extensions, and save files.

docs/research/boot-video.md

@@ -0,0 +1,44 @@
+# BIOS boot graphics initialization
+
+## Observed failure
+
+The MIT-licensed ZeroDayArcade Pong homebrew booted and serviced VBlank interrupts without a CPU diagnostic.
+Version 2 ROM scenarios then found a black captured screen despite running menu and gameplay logic.
+The same checks passed after firmware initialized the affine scales. The ROM bytes and assertions did not change.
+
+The source selects Mode 3 and BG2 but does not write an affine matrix.
+Our raw I/O state starts with zero coefficients. The original boot sequence previously left those values unchanged.
+Zero PA and PD make the renderer sample the same source pixel across the screen.
+This is a firmware initialization gap, not a reason to bypass affine rendering for bitmap modes.
+
+## Correction and independent regressions
+
+The original BIOS now executes ARM stores of 256 to BG2PA, BG2PD, BG3PA, and BG3PD before entering the cartridge.
+Fresh device state already supplies zero PB, PC, and origins.
+The change does not alter `Memory::new`, external firmware, or SoftReset behavior.
+It does not claim complete hardware initialization or exact startup timing.
+
+Original test programs select a video mode without writing matrices.
+Separate horizontal and vertical pixels establish the expected identity transform for bitmap graphics and both affine backgrounds.
+A captured frame checks the timed rendering path. A raw-memory control retains the zero-matrix behavior.
+The bitmap and affine startup tests failed before the correction and pass afterward.
+
+## Compatibility evidence
+
+Pong revision `c784b6036a4f188c50932b411e98126bfcbd07d6` passes bounded menu, settings, return, match-start, ball-motion, and paddle press/release checks.
+Debug and release scenario reports match on Darwin arm64.
+A bounded native Pong window run and the original native-window smoke test complete successfully.
+The automated input checks use the core input interface; they do not establish manual keyboard playthrough or full-game correctness.
+
+The pinned public ARM, Thumb, memory, and BIOS checkpoints still pass in both profiles.
+Additional boot instructions intentionally change startup step/cycle counters and internal firmware addresses.
+No public ROM, game asset, or third-party implementation source is included in this repository.
+
+## References
+
+- [Pinned Pong source and license](https://github.com/ZeroDayArcade/Pong-Homebrew-GBA/tree/c784b6036a4f188c50932b411e98126bfcbd07d6): `main.c` selects Mode 3; `graphics.h` defines rectangles and text positions.
+- [Pinned mGBA I/O initialization](https://github.com/mgba-emu/mgba/blob/3a5e34be33dc7f8f707e5bc9db69e8a430046f21/src/gba/io.c): `GBAIOInit` supplies identity PA/PD values.
+- [Pinned mGBA RegisterRamReset](https://github.com/mgba-emu/mgba/blob/3a5e34be33dc7f8f707e5bc9db69e8a430046f21/src/gba/bios.c): independent cross-check of affine reset values, not copied implementation.
+- [gbadoc register summary](https://gbadev.net/gbadoc/registers.html) and [GBATEK](https://mgba-emu.github.io/gbatek/): affine coefficient roles and register addresses.
+
+These sources support functional initialization. They are not a hardware trace of Nintendo firmware startup.

docs/status.md

@@ -76,6 +76,7 @@ Update this file when a feature lands or a limit is removed.
 - 256 KiB external work RAM and 32 KiB internal work RAM, including mirrors.
 - Read-only cartridge bytes in the three GBA cartridge windows.
 - Raw ROM-file loading with original BIOS boot, terminal diagnostics, and final CPU/step reports.
+- Firmware boot initializes BG2/BG3 identity scales for bitmap and affine programs that do not program their own matrices.
 - ROM windows with scanline capture, keyboard input, focus-loss release, bounded execution slices, and interactive keypad wake from STOP.
 - Optional ROM-window frame limits and an original input-test ROM generator.
 - Headless JSON ROM suites with PC or VBlank completion, register/CPSR/memory/pixel assertions, scheduled buttons, bounded budgets, and structured failure reports.