Digittack is a game based upon the Sharp Wondertopia game calculator (that is, a calculator that shipped with a built-in game), manufactured in 1982.
Digits will appear and increase in value. Shoot them before they reach critical mass. Once they reach 9, they will be at critical mass and you have a short time to shoot them. Also, beware the squares, for which you have less time to shoot.
Beware if you miss, for once you miss 3 times, you will lose a life. You have 3 lives per level.