Digging into the history briefly, patchable microcode seems to have existed in commercially released computers as far back as the 1960s. "...CCROS [microcode]

Digging into the history briefly, patchable microcode seems to have existed in commercially released computers as far back as the 1960s. "...CCROS [microcode] cards on the [IBM System/360] Model 30 could be replaced in the field. This made design modifications easier to accomplish, as well as providing for field diagnosis. ... The later System/360 Model 25 and models of the later System/370 series were configured with at least one portion of the control store being read-write for loading microcode patches and microdiagnostics." https://people.cs.clemson.edu/~mark/uprog.html https://people.cs.clemson.edu/~mark/uprog.html