Digital data processing system respoonsive to instructions containing operation code modifiers

A system for modifying the manner in which a processor in a digital computer system responds to operation codes in certain instructions. All instructions to which the system responds have operation code syllables containing an operation code and an operation code modifier. In instructions having certain operation codes, the operation code modifier contains a value which modifies the manner in which the processor responds to the operation code. When the processor receives an instruction having such an operation code, a part of the processor which is responsive to the operation code modifier employs the value in the operation code modifier to modify the interpretation of the instruction by the processor. The manner in which the value is employed depends on the operation code. Several uses of the operation code modifier are disclosed. It is used in instructions which have varying numbers of operands to specify the number of operands; it is further used in a branch instruction to calculate the new program value; it is finally used in instructions in which the operation code specifies a set of related operations to specify one operation in the set.