Abstract: A microprogrammable computer system including a central processing unit having a main memory storing data and a microprogrammed control memory consisting of a plurality of read-only memory chips containing microprograms, provides for various variable field addressing, multi-way branching, push-down stack and byte string operations that share common instructions and hardware. Using the system, a contiguous set of bits may be accessed or stored in the main memory without regard to memory word boundaries, the number of bits in the set, or the location of the bits in a memory word. The variable field addressing features are based upon the use of two types of instruction words, viz., field addressing instructions and field descriptors, which extract the appropriate bits from main memory and process them according to the instructions specified.