DOS Defender is an x86 real mode DOS game for Ludum Dare #31 (December, 2014). Since few DOS machines still exist, the target platform is actually DOSBox, though it should work to some extent on any DOS system. The game can be played with a mouse but it's intended to be played with a joystick/gamepad.

• How to build DOS COM files with GCC
• Random DOS Game Show Special: DOS Defender (2014)
• The New Retro Show plays DOS Defender

Just point DOSBox at the COM binary.

$ dosbox DOSDEF.COM

There are no external assets, so mounting isn't necessary.

All you need is a GCC compiler that can target i386. In theory, it doesn't matter if it's for Linux, Windows, etc., nor is there a need for something like DJGPP. The compiled code will be packaged as a COM file using a linker script (e.g. --oformat=binary). Unfortunately, there's a long-standing bug in MinGW that prevents this from working correctly without objdump.

The final game is a 10kB executable. To run, all it needs is 64kB of RAM and 64kB of video memory. With some tweaking, it could even run without a need for any OS.

DOS Defender is written in GCC's dialect of C and probably can't be built with any other compiler. The host platform is treated like an embedded system, so there's no intention of actually building the game from within DOS itself. Compilation is done as a single translation unit to allow the optimizer full reign in minimizing the code size (COM files are limited to 64kB).