Microprocessor having external control store

Abstract From the Canadian patent

A central processing unit (CPU) 10 comprises an external control memory for storing microinstructions which correspond to macroinstructions read from a system memory. The microinstructions are 56 bits in length and are read in 28-bit segments. CPU 10 also comprises an internal memory management unit (MMU) 18 which comprises a plurality of address translation entry (ATE) registers four of which are permanent and sixteen of which are temporary in that the storage of a new translation entry occurs in a least recently used temporary translation entry register. CPU 10 also comprises a plurality of status register bits some of, which are settable only by predefined microinstructions. All of the …