That's a good explanation of the transformer read-only storage (TROS) used by the Model 20 and Model 40. Microcode storage gets even more complicated, as IBM us

That's a good explanation of the transformer read-only storage (TROS) used by the Model 20 and Model 40. Microcode storage gets even more complicated, as IBM use different microcode storage techniques on different 360 models, for a combination of technical and political reasons. The fundamental problem was how to implement tens of kilobytes of ROM in a way that was affordable and could be updated in the field. The Model 30 used CCROS (Card Capacitor Read-Only Store). Like TROS, this used a mylar sheet with holes punched in it. However, in this case, the holes were capacitively sensed; an air compressor forced the sense lines against the card. The card was the same size as a punch card, so you could punch new microcode with a keypunch. The Model 50 and Model 65 used BCROS (Balanced Capacitor Read Only Storage). This was kind of like CCROS, but faster. It used 8 inch by 18 inch mylar sheets with an etched copper pattern like a PCB. Each sheet held 35,200 bits and the system used 16 sheets. The Model 25 stored microcode in the regular core memory, a rather boring solution. For faster performance, the high-end System/360 machines had hardwired control rather than microcode.