Exerion is a great arcade game from Jaleco, released in 1983.

Like so many games in that era, it's based on two 8 bit Z80's clocked at 4 MHz.

The SEGA SG-1000 -cart, mentioned in another writeup is a (not so faithful) conversion of the game, there were also conversions for the NES and MSX systems.

The two weapons in the arcade version are 'slow' double shot bullets (infinite ammo) and a 'very fast' gun (limited charge, recharges with every double shot kill).

The world of Exerion is a pseudo-3D world, the background reacting on your movement. The movement of your ship was made difficult by implementing inertia. Pretty amazing graphics with lots of details. The enemies were quite smart too, sometimes cheating by reappearing from the bottom or the sides of the screen.

It's emulated by MAME, very playable, but the colors are wrong ;-)