A microcode patching method and apparatus provides for fetching of microcode from an external source which, under appropriate conditions, replaces direct reading of microcode from a microcode ROM. In a decoder having a capability to concurrently dispatch up to four instructions and each dispatch pathway having two alternative decoding pathways including a fastpath pathway and a microcode ROM pathway, a technique and apparatus for patching the microcode ROM is described. This technique and apparatus provides that execution codes from the microcode ROM are selectively replaced, during decoding, by codes taken from an external source, such as from an external memory via a byte queue.