Degrades starch to alpha-, beta-, and gamma-cyclodextrins, as well as linear sugars. Cyclizes part of a (1->4)-alpha-D-glucan chain by formation of a (1->4)-alpha-D-glucosidic bond. Binds 2 calcium ions per subunit. Belongs to the glycosyl hydrolase 13 family. Was originally thought to be an alpha-amylase.