My first thought was writing an IBM PC-compatible (1981) game as an OS using BIOS interrupts, but it turns out there is very little documentation on 8-bit assembly for the IBM PC BIOS. Instead, I'm aiming to use MS-DOS' QBASIC.
To be honest, writing it in pure assembly wouldn't be too bad, but it's a lot easier with qbasic or turbo pascal 6/7 in that regards. Still would be 8 or 16bit in the correct video mode.
