Memory subsystem

摘要

A memory subsystem including a read-only memory (ROM), a random access read/write memory (RAM) and a selection system for selecting the output of one of the memories for use by downstream circuitry. The selection of the output is based on input address signals so that the contents of the RAM can substitute for the contents of selected locations in the ROM. If a substitution is to be made, an entry is made in a content addressable memory, which stores addresses for which the RAM output is to be substituted for ROM output. A test system is provided to verify the contents of the content addressable memory.