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gb-rs

Game Boy emulator written in rust. No unsafe code so far.

The CPU passes all the instruction tests as well as all the timing tests (see the "Ressources" section below for the links to the tests).

Saving is implemented, it creates a file with the ".sav" extension in the same directory as the ROM being executed if it supports saving.

Sound is implemented with adaptative resampling to match the sound card sample rate. Note that it might take a few seconds for the algorithm to settle on the correct sample rate so you might get a few dropped sound packets when you start the emulator. Normally once this training time has elapsed there should be no drops.

The sound unit is not completely accurate however, some games have weird glitches in the sounds (The Legend of Zelda in particular). I'm working on that...

Things that remain to be implemented:

• Serial link
• Support for various types of cartridges
• Maybe GameBoy Color support?

The emulator is not optimized at all but thanks to the GB's measly 4Mhz system clock it should run at speed on any half-decent desktop CPU. Don't forget to build with cargo --release do enable the optimizations however.

The display and input are handled through SDL2. That code is modular and abstracted away from the emulator core so it shouldn't be difficult to add support for alternative backends if need be.

The controls are hardcoded in src/ui/sdl2/controller.rs at the moment: you'll have to edit the update_key, update_button and update_axis function if you want to rebind the various keyboard and controller controls.

The defaults are:

GameBoy button	Keyboard	Controller
A	Left Alt	A
B	Left Control	B
Start	Return	Start
Select	Right Shift	Back
Up	Up	DPadUp / LeftY axis
Down	Down	DPadDown / LeftY axis
Left	Left	DPadLeft / LeftX axis
Right	Right	DPadRight / LeftX axis

The Escape key exits the emulator.

By default the emulator is built with the original Gameboy bootrom which scrolls the logo down the screen before actually jumping into the game. By building with the --features sgb_bootrom option you can opt to use the Super Game Boy bootrom instead which is much faster to boot up.

Game Support

Games that have been tested:

• Tetris
• Bombjack
• DrMario
• Kirby's Dream Land
• The Legend of Zelda - Link's Awakening
• Motocross Maniacs
• Wario Land
• Bubble Ghost
• Castelian
• Earthworm Jim
• Super Mario Land 1, 2 and 3
• Pokemon Red/Blue
• Space Invaders
• Bomberman
• Trip World
• F-1 Race

Resources

The Game Boy CPU manual: http://marc.rawer.de/Gameboy/Docs/GBCPUman.pdf

The Game Boy Programming manual: http://www.romhacking.net/documents/544/

Opcode map: http://www.pastraiser.com/cpu/gameboy/gameboy_opcodes.html

Infos about many GB quircks: http://www.devrs.com/gb/files/faqs.html

Some extremely useful accuracy tests made by Shay Green: http://tasvideos.org/EmulatorResources/GBAccuracyTests.html

An introduction to GB emulation in Javascript: http://imrannazar.com/GameBoy-Emulation-in-JavaScript:-The-CPU

There are many small errors or discrepancies in the docs listed above, unfortunately I don't know if/how those docs are maintained so I'm going to list the errors I've found here for now:

• RLCA instruction: various sources disagree on whether the Z flag should be modified by the operation. After some research I'm pretty sure it shouldn't be modified (the Z80 processor for instance does not modify Z).

• HALT instruction: when this instruction is executed while interrupts are disabled in the CPU (for instance by a DI instruction) the CPU will still be awoken by any interrupt enabled in the IE register, however the interrupt handler will not be run and the control will be given back to the code that called the HALT. Be careful that the instruction following an HALT is glitchy but that's well documented.

• ADD SP,N (opcode 0xe8) and LDHL SP,N (opcode 0xf8): the values of the carry and halfcarry are computed on the low 8bits.

• RLCA, RLA, RRCA, RRA instructions: the flags are described wrong in the CPU manual: after the execution C contains the rotated bit while N, H and Z are always 0. The equivalent instructions in the extended CB opcode space however set Z if the result is 0 and that's correctly documented.

• Instruction timings: The GameBoy CPU manual gets all the timings of conditional branches wrong: it doesn't say that the number of cycles taken by the instruction to execute depends on whether or not the branch is taken. More generally, both this manual and the opcode map linked above have some discrepencies when it comes to instruction timings. In the end it's probably safer to use the timings directly from the assembly source for the accuracy tests.

• The actual state machine for the configurable GPU LCD interrupt seems not well described anywhere. I tried to put a lot of comments in my code to describe my approach, however I'm not sure whether it's 100% accurate.

• When there are more than 10 sprites on a line only the 10 first in OAM order are displayed. The sprite's X coordinates and priority don't matter, only the position in OAM.