1. An instruction system comprising: an instruction root having an operation selection field for selecting an operation to be performed by said data processor; and an instruction prefix having a field selected from the group consisting of: a conditional execution field for selecting a condition under which a data processor will perform said selected operation; an operand length modification field for modifying the selected operation so as to be performed on an operand having a different length; an instruction group field for selecting a length of an instruction group; and a prefix length selection field for selecting a length of said instruction prefix.