OT: how big would it be?

The instructionset differences largely are in the OPR group where some of the microcoded undocumented combinations were actually useful. When viewed from the older members twoward the newest this is not a great problem. the extended arithmetic however there were many flavors that were somewhat compatable. a PDP-5, straight-8, 8I, 8E, 8A and DECMATE (I/II/III?). Each represents a level of difference on the base design. For example the 8A had mechanism in hardware for stacks (actually IOTS so it's retrofitable!). The DECmates represent two different chips the 6100 (base 8Eish) and the 6120 (more 8Aish) both having a control pannel mode unique to them. The commonality is great from the 5 though the DMIII that you can wite program that run on both. This made the PDP-8 series likable (software investment). Allison