This talk from Hackover 2025 provides a deep dive into the process of reverse-engineering the Nintendo Game Boy Advance (GBA) to achieve cycle-accurate hardware emulation. The speaker, an emulator developer, outlines the limitations of early emulators and highlights the need for a precise understanding of the hardware's internal operations, particularly the Pixel Processing Unit (PPU). The core of the presentation details a novel method for mapping the PPU's memory access patterns. By writing custom tests to run on the GBA hardware, the speaker measures the exact clock cycles during which the PPU accesses VRAM, OAM, and Palette RAM, stalling the CPU. This data reveals the previously undocumented rendering pipelines for regular backgrounds, affine (rotation/scaling) backgrounds, and sprites. The speaker concludes by demonstrating how implementing these precise findings in an emulator successfully fixes subtle but significant graphical glitches in commercial games, proving the value of detailed hardware analysis for creating truly compatible and accurate emulators.
Unlocking the Game Boy Advance: A Deep Dive into Reverse Engineering for Emulator Accuracy
0:10 Introduction to Handheld Reverse Engineering: The talk focuses on using reverse engineering techniques on the Game Boy Advance (GBA) to gain a deep understanding of its hardware, which is essential for developing highly accurate emulators.
2:35 The Game Boy Advance (GBA): Released in 2001, the GBA is a 32-bit handheld featuring an ARM7TDMI CPU. Its graphics are handled by a 2D Pixel Processing Unit (PPU) that renders graphics using tile-based layers, palettes, and supports effects like rotation and scaling.
4:46 The Evolution of Emulators: The speaker outlines three "waves" of GBA emulators, with each generation improving upon the last by achieving a more accurate simulation of the original hardware's behavior and timing.
5:38 The Need for Emulator Accuracy: Inaccurate emulation can cause games to crash, exhibit graphical or audio glitches, run at the wrong speed (a major issue for speedrunning), or fail to expose bugs in newly developed homebrew software until it is tested on real hardware.
9:17 Reverse Engineering via Hardware Tests: The primary method discussed is writing custom software to run on the GBA. These tests are designed to probe and measure the hardware's behavior under specific conditions, allowing the developer to form and verify hypotheses about its internal functions.
13:12 A Closer Look at the Pixel Processing Unit (PPU): The PPU is responsible for rendering all visuals. It can display up to four background layers and one sprite layer simultaneously, which are composited together based on priority. It operates in various modes that define the properties of these layers (e.g., regular tilemaps, affine tilemaps, or direct bitmaps).
23:36 The Central Mystery: The PPU's Internal Algorithm: While the capabilities of the PPU were well-known, the precise, cycle-by-cycle process it used to fetch data and render pixels was largely undocumented. Discovering this was the speaker's primary goal.
26:20 The Measurement Technique: The speaker developed a technique based on the fact that the PPU has memory priority over the CPU. By precisely timing a VRAM access from the CPU, any delay indicates that the PPU was accessing that memory at the same clock cycle, causing the CPU to wait.
29:23 Achieving Cycle-Accurate Measurement: The technique was refined to measure just one specific clock cycle per frame across thousands of frames. This builds a complete, cycle-accurate map of all PPU memory accesses for a single scanline, revealing its hidden operational patterns.
33:45 Revealing the PPU's Rendering Pipeline: The data shows that for regular backgrounds, the PPU fetches a tilemap entry and then the corresponding 8 pixels. For affine (rotatable/scalable) backgrounds, it must perform separate memory fetches for every single pixel, explaining their higher performance cost and memory bandwidth consumption.
37:54 The Sprite Rendering Process: The analysis also demystifies sprite rendering. The PPU fetches sprite attributes from OAM and pixel data from VRAM one scanline in advance, using a pipelined process to hide the latency of these memory operations.
42:05 Practical Results and Game Fixes: Implementing these newly discovered, cycle-accurate PPU behaviors in the speaker's emulator fixed long-standing graphical glitches. Examples shown include correcting a subtle screen distortion in "Gadget Racers" and fixing a completely broken menu screen in "Madden NFL 06."