Double Dragon, from Japan Technos Corporation (apparently rebadged by Taito in the USA), is a standard two-player cooperative (simultaneous) 2-way scrolling (in some places, 4-way scrolling) side-on beat'em up game. The object of the game is to rescue your girlfriend; if both players survive to the end of the game, then before the game will end, one player has to beat up the other, which is a nice touch of realism.
The game runs on two 6809 processors and a Hitachi 63701. Music is provided by a Yamaha YM2151 FM synthesizer and a YM3012. At the time of writing (1998) there were several people working on emulators, including "Calb" of SNES emulator fame, but there are technical problems with emulating the game. The 63701 is an MC6301 code-compatible microcontroller in a very large package with lots of pins (apparently, it has 56-odd I/O lines), and 8K of internal PROM. Emulating the game may require reading out this PROM area, and I'm still not sure if it's even possible. Since my original review, I know that cloned Double Dragon ROMs have been emulated, and I think that's as close as we're likely to get.
Double Dragon uses a horizontal low-res monitor, one or two 8-way joysticks with three buttons, and it has a standard JAMMA pinout. DIP switch settings are as below:
DIP Switch Bank A
DIP Switch Bank B