Hal 21 is a shoot'em up released by SNK in 1985 for Arcades. It can be considered a spiritual predecessor to the Alpha Mission series.
Gameplay is very similar to Namco's Xevious, with a button for firing regular shots and a button for bombing ground targets, controlled by a crosshair target above the player's ship. Items are strewn across the landscape in the form of "E" letters, which give players bonus points or power-ups the ship's weapons. Aside from aerial enemy patterns and ground tanks and turrets, the player also has to deal with obstacles in the form of square structures which appear when the player gets close and are indestructible.
One unique feature from HAL 21 is the bosses, which are composed of multiple small parts. Whenever the player hits a boss part, it flies off from the boss's body before reattaching.