I think you imply this, but to make it explicit: IBM mainframes run <i>some</i> instructions in microcode, others are native. The ones that are native are not slowed down by the existence of the microcode engine.<p>IBM analyzed programs-in-use to figure out which instructions bottlenecked real programs. These were the ones prioritized into native hardware. Others, where the bottleneck wasn&#x27;t the central processor--like moving a block of memory from one location to another--were executed in microcode (at least back in the day: I was one of the engineers on the S&#x2F;390 microcode engine.)