Dave5

The first RISC processor, although it didn’t use that name, was the IBM 801. There is a sense in which people today don’t count it since it was not a microprocessor. That was because it was developed in the 1970s, long before integrated circuits were large enough to fit a whole microprocessor. Don’t forget that even the 1983 Arm 1 filled a whole die without room for an on-chip cache.

The IBM 801 had all the main features of a RISC architecture: a number of processor registers used for all computation, no complex addressing modes, one load, and one store instruction. It was initially 24-bit, but soon that was upgraded to 32-bits.

https://community.cadence.com/cadence_blogs_8/b/breakfast-bytes/posts/riskhist

… the major design philosophy at the time was to use a large amount of instructions to do specific tasks within the processor. When designing the special-purpose phone switch processor, IBM removed many of these instructions and then, after the project was cancelled, performed some testing on the incomplete platform to see how it performed as a general-purpose computer. They found that by eliminating all but a few instructions and running those without a microcode layer, the processor performance gains were much more than they would have expected at up to three times as fast for comparable hardware.