The pdp-1 has two architectural registers: AC and IO and they're not addressable as memory. The PDP-10 has a similar internal structure but fetches and stores t

The pdp-1 has two architectural registers: AC and IO and they're not addressable as memory. The PDP-10 has a similar internal structure but fetches and stores the accumulator from/to memory* for instructions that use it. * in the PDP-6 it really is external memory, in the later models (aka PDP-10) it was moved into the CPU