- Disassembler/debugger
- Visualizations for tiles, sprites and palette colours
- Rewind mode
- Levels visualizer for the 4 sound channels
- Plot for channel 4 wave pattern
- A command console with some commands for debugging the emulation. See table below.
- Easy to read code. Highly modularized design and liberal usage of comments makes it a good reference for writing your own emulator.
Note: Any numeric argument can be suffixed with 'h' to indicate a hex number (e.g. 204 -> CCh)
| Command | Syntax | Description |
|---|---|---|
| Open | o|open [file] |
Load a new ROM file. Files can also be dragged and dropped over the window. |
| Run | run|play |
Run the emulation. |
| Pause | pause |
Pause the emulation. |
| Reset | reset |
Reset emulation state as if the ROM was just loaded. |
| Save Game | save |
If the ROM type has external RAM or a real-time clock, their states are saved to disk (automatically happens on shutdown too). |
| Rewind stats | rwstats |
Print some rewind-mode stats to the console |
| Exit | exit |
Close the emulator (can also close the console or viewport windows instead) |
| Print Info | p|print cpu|gpu|rom|timers |
Print the state of one of the components to the console. |
| Stepping | step | stepn n | vblank |
Tick the core once, n times or until a vblank occurs. Emulation must be paused first. |
| Toggle Breakpoint | bp addr 0|1 |
Set or unset (depending on second arg) a breakpoint at the given address. |
| Toggle Memory Breakpoints | wbp|rbp addr [value] |
Pause/break when a write or read occurs at the given address. If a value is given it must match what's being written or read. |
| Delete Breakpoint | bp|wbp|rbp addr del |
Delete the breakpoint so it's no longer being checked. |
| List Breakpoints | bp|wbp|rbp |
|
| Memory Dump | memdump addr n |
Print out a hex dump of n bytes of memory at the given address. |
| Colour Correction | ccm 0|1 |
0: washout colours similar to the physical LCD screen, 1: multiply by 8 (more vibrant colours) |
| Brightness | brightness 1.0- |
Set a brightness factor (colur channels are multiplied by this number) |
| GPU Debug Views | show|hide tiles|palettes|sprites |
Show or hide one of the debug GPU's debug views. |
| Toggle Sound Channels | chan 1|2|3|4 0|1 |
Turn one of the four sound channels on or off. |
| Muting | mute|unmute |
|
| Trace Files | trace 0|1 |
Start or stop appending to a trace file that records the cpu's state (file saved to cwd ). |
| Screenshot | screenshot [filepath] |
Save a 160x144 bmp screenshot to the given path (or save it to cwd with a random name if no path is given). |
A config file named gem.ini is used to persist some user settings. It's created the first time you run the emulator and can then be modified.
| Config Key | Description |
|---|---|
| VSync | Synchronize GPU presents with the display's refresh rate. |
| ResolutionScale | Multiply the window size by an integer to increase its size. |
| *Key | A keyboard key to map the Game Boy button to. See SDL_Keycode. |
| DMGPalette | A '|' delimited list of four colours for the DMG palette (starting with white/colour 0). Each colour can be formated using rgb(x,x,x) or as a 24-bit hex number: FFCFEAh. E.g. DMGPalette=F7FBE1h|BAC964h|438A5Eh|436F8Ah |
To rewind gameplay press and hold the R key then release when you want to stop. Hold down the T key while releasing R to continue from the start of the rewind (i.e. discard the rewind operation).
When enabled this feature will save a snapshot of the last N seconds of game play allowing you to rewind to a previous state.
| Config Key | Description |
|---|---|
| RewindBufferDuration | How many seconds of game play to save to the rewind buffer. |
| RewindFramesBetweenSnapshots | How many frames to skip before taking a rewind snapshot. |
| RewindEnabled | Enable or disable the rewind feature and buffer. |
Note: These need to be passed in when launching gem.exe.
| Arg | Description |
|---|---|
--vsync |
Synchronize GPU presents with the display's refresh rate. |
--no-sound |
APU is not initialized and isn't ticked at all. |
--dmg |
Emulate DMG hardware instead of the CGB. |
--pause |
Pause after loading a ROM file. |
--res-scale=... |
Multiply the window size by an integer to increase its size. |
| Game Boy | Keyboard |
|---|---|
| Arrow buttons | Arrow keys |
| A | A |
| B | S |
| Start | Enter |
| Select | Left shift |
- gem.core: All the hardware emulation logic is in this library. It has no dependencies and is written in portable C++.
- gem.gui: A UI front-end and debugger for the emultation core. The main window uses just SDL2 while the debugger is written with ImGui.
- SDL2
- Dear ImGui
- FFmpeg
- BGB Emulator Debugger - super useful during the development of gem.
- Droid Sans font
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