A digital computer system in which data storage is referred to by a descriptor comprising an object number (AON 27111) denoting a variable-length block of storage, an offset (OFF 27113) indicating how far into that block a desired data item begins, and a length field (LEN 27115) denoting the length of the desired data item. Separate means exist for manipulating each of the three descriptor portions, thus facilitating repetitive operations on related or contiguous operands. Various levels of microcode control are included. Each level of microcode control has its own stack (902-905) facilitating interrupts between levels. Stacks are duplicated in "secure stacks" (504) in memory to protect against loss of state data from the stacks.