OP Code Extender
Original Publication Date: 1981-Jan-01
Included in the Prior Art Database: 2005-Feb-10
Computer systems normally allocate a predetermined number of bits to specify the operation code for instructions. For example, an eight-bit op code field has 256 op code points. When these are used up and expansion is desired, the op code field can be extended and in this instance is extended by placing a branch control word in the primary operation control table. The branch control word specifies a secondary operation control table, and entry into that table is specified by a four-bit extender field contained in the instruction. This arrangement does not require any new instruction formats, and makes use of instructions having previously unused instruction stream bits. Because of this, the new instructions can be handled by existing hardware and microcode using all of the instruction fetch hardware assists.