stack A stack is a digital storage space that a digital organism uses for storing numbers. The default virtual CPU and the transsmt virtual CPU have two and four stacks, respectively, used for storing numbers. In the default virtual CPU, the 'push' and 'pop' instruction codes are used to move numbers between the registers and the stack, and the 'swap-stk' instruction toggles the active stack in use. A digital organism can theoretically store an arbitrary amount of data in the stacks, but for practical purposes the maximum stack depth is limited to ten. digital storage space