After asking two questions that are each missing this tag, and not knowing what to name it, I have brought this to meta. What should the tag used for questions about the instruction sets of processors be named?
In the case of the PDP question, I think instruction-set is an excellent tag since that's what the question is specifically dealing with.
For the Z80 vs 8080 question though, I think the argument could be made for an instruction-set-architecture tag since the question is comparing two separate (albeit similar) instruction sets and "instruction set architecture" refers to the specific design and implementation of a particular instruction set.
In fact, in the CPU design world, there is a team member (or members) responsible for creating the instruction set for a new processor who is referred to as the "instruction set architect".
For this usage I would go for instruction-set for preference.
opcode implies a question about the behaviour or use of a specific instruction rather than the set as a whole.
instruction-set-architecture is a bit wordy and, to me, evokes memories of the Motorola v Intel architecture wars of the 1980s - 90s. Should an instruction set be orthogonal or not. Also CISC v RISC, etc.
cpu-architecture could encompass more than just the instruction set. It could cover all aspects of the design.