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gameboy-rust/docs/references.md
0xrsydn ab625dc55d fix(cpu): support single-load writeback register aliases 
Allow ARM LDR/LDRB/LDRH/LDRSB/LDRSH to share non-PC base and destination registers with writeback. Keep the loaded value after computing addresses from incoming operands. Preserve PC restrictions, unsupported user transfers, store behavior, and atomic memory diagnostics.

Add original regressions for index modes, offsets, widths, alignment, sign extension, all active banks, timing, I/O/IRQ ordering, and failures. The unchanged pinned public ARM ROM progresses past tests 360 and 361, then fails at test 362 on an unmapped 0x80000000 read; no full-suite pass is claimed.

Verified on Darwin arm64: debug/release workspace and core suites, Python adapter tests, Clippy, rustdoc, native ROM-window checks, and all ten demo smoke tests. Document the new behavior and next memory-access blocker.
2026-10-05 20:15:53 +07:00

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# References
External sources consulted while implementing the emulator. Add new sources here, or in a "References" section of the matching `docs/hardware/` file when a link explains one specific behavior.
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 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).
- [GBATEK BIOS memory-copy services](https://problemkaputt.de/gbatek-bios-memory-copy.htm).
- [GBATEK BIOS rotation/scaling services](https://problemkaputt.de/gbatek-bios-rotation-scaling-functions.htm), for record layouts, angle units, and output strides.
- [VisualBoyAdvance-M BIOS services](https://github.com/visualboyadvance-m/visualboyadvance-m/blob/master/src/core/gba/internal/gbaBios.cpp), reviewed for integer affine rounding, signed matrix intermediates, and SoftReset status handling. Our sine table is generated mathematically.
- [GBATEK BIOS arithmetic services](https://problemkaputt.de/gbatek-bios-arithmetic-functions.htm).
- [GBATEK BIOS decompression services](https://problemkaputt.de/gbatek-bios-decompression-functions.htm), including BitUnPack descriptor fields, differential-filter headers, Huffman tree layout, and width constraints.
- [mGBA BIOS service implementation](https://github.com/mgba-emu/mgba/blob/master/src/gba/bios.c), reviewed for RegisterRamReset register defaults, angle-polynomial coefficients and quadrant conventions, BitUnPack ordering/offsets, and Huffman tree layout and packing.
- [HALTCNT hardware-access tests](https://github.com/mgba-emu/mgba/issues/2309), for BIOS-only writes and halfword access.
- [GBATEK display status and IRQs](https://problemkaputt.de/gbatek-lcd-i-o-interrupts-and-status.htm).
- [GBATEK display dimensions and timings](https://problemkaputt.de/gbatek-lcd-dimensions-and-timings.htm).
- [mGBA display implementation](https://github.com/mgba-emu/mgba/blob/master/src/gba/video.c), used to cross-check hidden-line HBlank IRQs and comparison-write edges.
- [GBATEK affine background registers](https://problemkaputt.de/gbatek-lcd-i-o-bg-rotation-scaling.htm).
- [NanoBoyAdvance background implementation](https://github.com/nba-emu/NanoBoyAdvance/blob/master/src/nba/src/hw/ppu/background.cc), reviewed for internal origins, line-end increments, and affine mosaic behavior.
- [GBATEK text and affine map layouts](https://problemkaputt.de/gbatek-lcd-vram-bg-screen-data-format-bg-map.htm).
- [GBATEK background control](https://problemkaputt.de/gbatek-lcd-i-o-bg-control.htm).
- [GBATEK tile/map memory layout](https://problemkaputt.de/gbatek-lcd-vram-overview.htm).
- [mGBA text-background renderer](https://github.com/mgba-emu/mgba/blob/master/src/gba/renderers/software-mode0.c), reviewed for out-of-range fetch behavior.
- [GBATEK sprite overview and rendering limits](https://problemkaputt.de/gbatek-lcd-obj-overview.htm).
- [mGBA sprite preprocessing](https://github.com/mgba-emu/mgba/blob/master/src/gba/renderers/common.c) and [row preparation](https://github.com/mgba-emu/mgba/blob/master/src/gba/renderers/video-software.c), reviewed for inspection costs and aggregate work limits.
- [GBATEK sprite attributes](https://problemkaputt.de/gbatek-lcd-obj-oam-attributes.htm).
- [GBATEK sprite rotation/scaling parameters](https://problemkaputt.de/gbatek-lcd-obj-oam-rotation-scaling-parameters.htm).
- [GBATEK sprite tile mapping](https://problemkaputt.de/gbatek-lcd-obj-vram-character-tile-mapping.htm).
- [Tonc regular sprites](https://gbadev.net/tonc/regobj.html).
- [mGBA sprite renderer](https://github.com/mgba-emu/mgba/blob/master/src/gba/renderers/software-obj.c), reviewed for tile alignment and wrapping.
- [GBATEK bitmap backgrounds](https://problemkaputt.de/gbatek-lcd-vram-bitmap-bg-modes.htm).
- [GBATEK display control](https://problemkaputt.de/gbatek-lcd-i-o-display-control.htm).
- [GBATEK window registers and priority](https://problemkaputt.de/gbatek-lcd-i-o-window-feature.htm).
- [GBATEK color special effects](https://problemkaputt.de/gbatek-lcd-i-o-color-special-effects.htm).
- [GBATEK mosaic register](https://problemkaputt.de/gbatek-lcd-i-o-mosaic-function.htm).
- [NanoBoyAdvance text sampling](https://github.com/nba-emu/NanoBoyAdvance/blob/master/src/nba/src/hw/ppu/background.inl), reviewed for vertical counter subtraction.
- [NanoBoyAdvance mosaic registers](https://github.com/nba-emu/NanoBoyAdvance/blob/master/src/nba/src/hw/ppu/registers.cc), reviewed for phase-preserving size writes and documented timing uncertainties.
- [Horizontal sprite mosaic hardware findings](https://github.com/mgba-emu/mgba/issues/2933), for latch transitions and transparent metadata.
- [NanoBoyAdvance sprite pipeline](https://github.com/nba-emu/NanoBoyAdvance/blob/master/src/nba/src/hw/ppu/sprite.cc), reviewed for one-row-ahead preparation, mosaic phase, priority updates, and OBJ-window exclusions.
- [NanoBoyAdvance display scheduling](https://github.com/nba-emu/NanoBoyAdvance/blob/master/src/nba/src/hw/ppu/ppu.cc), reviewed for cycle40 sprite initialization, line227 preparation, and vertical window comparisons on hidden lines.
- [NanoBoyAdvance composition](https://github.com/nba-emu/NanoBoyAdvance/blob/master/src/nba/src/hw/ppu/merge.cc), reviewed for palette lookup and horizontal mosaic after sprite preparation.
- [mGBA window renderer](https://github.com/mgba-emu/mgba/blob/master/src/gba/renderers/video-software.c), reviewed for inverted bounds and vertical edge flags.
- [NanoBoyAdvance window implementation](https://github.com/nba-emu/NanoBoyAdvance/blob/master/src/nba/src/hw/ppu/window.cc), reviewed for persistent vertical flags, four-cycle horizontal comparisons, offscreen columns, and end-edge precedence.
- [GBATEK keypad input](https://problemkaputt.de/gbatek-gba-keypad-input.htm), for KEYINPUT/KEYCNT fields and selected-button OR/AND matching.
- [mGBA keypad sampling](https://github.com/mgba-emu/mgba/blob/master/src/gba/gba.c) and [control writes](https://github.com/mgba-emu/mgba/blob/master/src/gba/io.c), reviewed for repeated OR requests, AND snapshot suppression, and newly selected held keys.
- [NanoBoyAdvance keypad implementation](https://github.com/nba-emu/NanoBoyAdvance/blob/master/src/nba/src/hw/keypad/keypad.cc), compared for control access widths; its AND/retrigger behavior differs from the chosen polling model.
- [mGBA keypad timing issue](https://github.com/mgba-emu/mgba/issues/2490), for frontend frame-based input timing limits.
- [Tonc hardware interrupts](https://gbadev.net/tonc/interrupts.html), for keypad source enable and IF/IE bit 12.
- [GBATEK memory mirrors and video byte writes](https://problemkaputt.de/gbatek-gba-unpredictable-things.htm).
- [GBATEK GBA memory map and bus widths](https://problemkaputt.de/gbatek-gba-memory-map.htm).
- [GBATEK GBA DMA transfers](https://problemkaputt.de/gbatek-gba-dma-transfers.htm).
- [mGBA DMA implementation](https://github.com/mgba-emu/mgba/blob/master/src/gba/dma.c), used to cross-check masks, repeat behavior, and Game Pak source increments.
- [GBATEK GBA timers](https://problemkaputt.de/gbatek-gba-timers.htm).
- [GBATEK GBA interrupt control](https://problemkaputt.de/gbatek-gba-interrupt-control.htm).
- [GBATEK processor registers and modes](https://problemkaputt.de/gbatek-arm-cpu-register-set.htm).
- [GBATEK status transfers](https://problemkaputt.de/gbatek-arm-opcodes-psr-transfer-mrs-msr.htm).
- [GBATEK CPU exceptions](https://problemkaputt.de/gbatek-arm-cpu-exceptions.htm).
- [GBATEK ARM data processing](https://problemkaputt.de/gbatek-arm-opcodes-data-processing-alu.htm), including R15/status notes. Its legacy `{P}` description alone does not establish ARM7 behavior.
- [mGBA ARM instruction implementation](https://github.com/mgba-emu/mgba/blob/master/src/arm/isa-arm.c), reviewed for Rd=15 test/compare SPSR restoration, User/System fallback, and no result write/refill. Also reviewed its word/byte/halfword/signed load ordering: writeback precedes the destination result.
- [jsmolka/gba-tests](https://github.com/jsmolka/gba-tests/tree/a7113b67e63f83a9b321696ddd7042ccfad6c881), pinned for independent ARM testing. Reviewed its MIT license, startup, result macros, compare/status tests, and load/writeback alias tests.
- [armwrestler-gba-fixed](https://github.com/destoer/armwrestler-gba-fixed) and [arm7wrestler](https://github.com/Arisotura/arm7wrestler), considered as test sources; not integrated.
- [SingleStepTests ARM7TDMI](https://github.com/SingleStepTests/ARM7TDMI), considered as an experimental, emulator-generated test source; not integrated.
- [GBATEK ARM single data transfers](https://problemkaputt.de/gbatek-arm-opcodes-memory-single-data-transfer-ldr-str-pld.htm), for address calculation, indexing, load extension, and nominal timing.
- [Arm armasm LDR constraints](https://support.arm.com/documentation/dui0801/l/A32-and-T32-Instructions/LDR--register-offset---A32-), reviewed to distinguish portable assembly restrictions from the ARM7 alias behavior tested here.
- [Pinned halfword alias tests](https://github.com/jsmolka/gba-tests/blob/a7113b67e63f83a9b321696ddd7042ccfad6c881/arm/halfword_transfer.asm), for loaded-value precedence in pre/post-indexed `LDRH` aliases.
- [GBATEK memory alignment](https://problemkaputt.de/gbatek-arm-cpu-memory-alignments.htm).
- [GBATEK multiply instructions](https://problemkaputt.de/gbatek-arm-opcodes-multiply-and-multiply-accumulate-mul-mla.htm).
- [GBATEK block transfers](https://www.problemkaputt.de/gbatek-arm-opcodes-memory-block-data-transfer-ldm-stm.htm).
- [GBATEK swaps](https://www.problemkaputt.de/gbatek-arm-opcodes-memory-single-data-swap-swp.htm).
- [GBATEK Thumb register operations](https://problemkaputt.de/gbatek-thumb-opcodes-register-operations-alu-bx.htm).
- [GBATEK Thumb loads/stores](https://problemkaputt.de/gbatek-thumb-opcodes-memory-load-store-ldr-str.htm).
- [GBATEK Thumb address calculation](https://problemkaputt.de/gbatek-thumb-opcodes-memory-addressing-add-pc-sp.htm).
- [GBATEK Thumb stack and multiple transfers](https://problemkaputt.de/gbatek-thumb-opcodes-memory-multiple-load-store-push-pop-and-ldm-stm.htm).
- [GBATEK Thumb jumps and calls](https://problemkaputt.de/gbatek-thumb-opcodes-jumps-and-calls.htm).
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.
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