Like many write-ups of the 6x00, it totally ignores the peripheral processor (PP) subsystem, which was both simple and innovative. The PP's were 10 independent processors, 12 bits wide, but with an 18 bit accumulator so that they could compute central memory addresses. Each had 4K of 12 bit words that stored both driver code and was used as I/O buffer. But.... there really were not 10, there was one copy of the PPU compute hardware, and 10 copies of the state that all the CDC old-timers referred to as "the PP barrel". It was just a big circular shift register, so the 10 PP's were actually just 10-way hardware multi-threaded. Yes, hardware multi-threading dates from 1957.

There famously were not interrupts in the classical sense in the 6x00, but PP's could read and scribble anywhere in central memory, and they could compute an address and jam it into the CPU's PC. So in effect infinitely flexible vectored interrupts.

4Kx12 is not a lot of program space, so most I/O drivers consisted of one or more PP overlays that would be loaded depending on which I/O device was in use.

If I recall correctly, the operator console required a PP full time -- the console being a couple of CRT's with shared X & Y DACs, and independent Z DACs, so they used vector fonts for everything. A second PP was full time dedicated to scheduling the other 8, at least in the common operating systems. (There were a bunch of operating systems... but I won't get into that.)

Also... somebody (maybe Seymour himself?) worked out a 12 word boot loader... and PP0 had a switch panel of 144 toggle switches arranged in a 12x12 matrix. You could toggle in the bootloader once, and leave it forever. At boot time those 12 words were loaded into PP0 core.