OPERAND SPECIFIER PROCESSING BY GROUPING SIMILIAR SPECIFIER TYPES TOGETHER AND PROVIDING A GENERAL ROUTINE FOR EACH

A method of specifying the operands for a microcoded CPU employs a combination of a set of microinstruction routines for generic operand modes, along with hardware primitives for selecting various specific types of operand treatment. Decoding of a machine-level instruction produces an entry point for the microstore, selecting one of the set of generic operand modes. Also, decoding of the instruction produces control bits that are used directly to select the specific operand type or used by the hardware primitives. In this way, branching is avoided in the microinstruction sequences used for operand specifying, but yet the amount of microcode needed is a minimum.