crater
======

**crater** is an emulator for the [Sega Game Gear][game gear], with an included
[Z80][z80] assembler/disassembler, written in C.

[game gear]: https://en.wikipedia.org/wiki/Sega_Game_Gear
[z80]: https://en.wikipedia.org/wiki/Zilog_Z80

Why?
----

While the internet is full of emulators for retro game systems, writing one is
nevertheless a fun learning project.

crater is named after [31 Crateris][crateris], a star that was – for a short
time in 1974 – misidentified as [a moon of Mercury][moon]. Mercury was Sega's
codename for the Game Gear during development.

[crateris]: http://www.astrostudio.org/xhip.php?hip=58587
[moon]: https://en.wikipedia.org/wiki/Mercury%27s_moon

Installing
----------

Only OS X and Linux are tested. You'll need a modern compiler that supports C11
([clang][clang] preferred) and [SDL 2][sdl2]. Using Homebrew, you can
`brew install sdl2`; using apt, you can `apt-get install libsdl2-dev`.

Run `make` to create `./crater`. To build the development version with debug
symbols and extra diagnostic info (they can exist simultaneously), run
`make DEBUG=1`, which creates `./crater-dev`.

crater has a number of test cases. Run the entire suite with `make test`;
individual components can be tested by doing `make test-{component}`, where
`{component}` is one of `cpu`, `vdp`, `psg`, `asm`, `dis`, or `integrate`.

[clang]: http://clang.llvm.org/
[sdl2]: https://www.libsdl.org/

Usage
-----

Running `./crater` without arguments will display a list of ROM images located
in the `roms/` directory, and then ask the user to pick one, or enter their own
ROM path. You can provide a path directly with `./crater path/to/rom`.

Add or symlink ROMs to `roms/` at your leisure. Note that they must end in
`.gg` or `.bin` to be auto-detected.

Add `--fullscreen` (`-f`) to enable fullscreen mode, or `--scale <n>`
(`-s <n>`) to scale the game screen by an integer factor.

Add `--debug` (`-g`) to display detailed information about emulation state
while running, including register values and memory contents. You can also
pause emulation to set breakpoints and change state.

`./crater -h` gives (fairly basic) command-line usage, and `./crater -v` gives
the current version.

### Assembler/Disassembler

crater has built-in support for converting Z80 assembly into ROM images, as
well as attempting the reverse process (i.e., disassembling).

`--assemble <input> [<output>]` (`-a`) converts source code into a `.gg` binary
that can be run by crater. `--disassemble <input> [<output>]` (`-d`) executes
the opposite operation. If no output file is given, crater will use the name of
the input file, with the extension replaced with `.gg` for `-a` and `.asm` for
`-d`. By default, this will never overwrite the original filename; pass
`--overwrite` (`-r`) to let crater do so.

Credits
-------

Special thanks to [SMS Power!][sms_power]'s excellent [development][sms_dev]
section, which has been invaluable in figuring out many of the Game Gear's
details, including ROM header structure and the memory mapping system. Various
source code comments reference their pages.

Also thanks to [Thomas Scherrer's Z80 website][scherrer] for many useful
resources about the Game Gear's CPU, including info about
[undocumented opcodes][undoc_ops] and [flags][undoc_flags]. Finally, credit
goes to [ClrHome][clrhome] for their helpful Z80 [instruction table][clrtab].

[sms_power]: http://www.smspower.org/
[sms_dev]: http://www.smspower.org/Development/Index
[scherrer]: http://z80.info/
[undoc_ops]: http://www.z80.info/z80undoc.htm
[undoc_flags]: http://z80.info/z80sflag.htm
[clrhome]: http://clrhome.org/
[clrtab]: http://clrhome.org/table/