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I still think that in IBM's case this depends on what you mean by mainframe. Midrange systems (ie. AS/400) are power boxes and today even sold as such (IBM will sell you the hardware as "IBM Power system" with choice of AIX, OS/400 or Linux as OS). On the other hand I'm almost certain that System z CPUs are microcoded monstrosities that run z/Architecture machine code pretty much natively.

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I've worked on the compilers for Z systems and on the z/PDT emulator that someone else mentioned. The processors they run are indeed monsters. The instruction set is especially interesting as you can see the transition from 24-31-64 bit. Messing around with the SAM (set address mode) instructions was always fun. They also have some interesting interruptable instructions that can move or compare up to 2GB of memory.

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