1. In a computer system having storage for storing executable and non-directly executable instructions and operands, said directly executable instructions including for every non-directly executable instruction a predetermined directly executable instruction having bit fields for performing instruction fetch phase and operand address calculation for an associated non-directly executable instruction, and a central processing unit for fetching and executing instructions from storage, the improvement comprising: identifying means within said central processing unit for identifying non-directly executable instructions fetched from storage, means responsive to said identifying means indentifying a non-directly executable instruction for retrieving from storage an associated predetermined directly executable instruction having bit fields for performing instruction fetch phase and operand address calculation of said identified non-directly executable instruction, and means responsive to bits in said bit fields of said fetched predetermined directly executable instruction to re-fetch said identified non-directly executable instruction, to compute operand addresses for operands in said non-directly executable instruction as specified in said predetermined directly executable instruction and to store said computed operand addresses to facilitate retrieval of said operands.