Bell & Newell's out of print book about computer architecture runs through both architectures (as well as many others). Unfortunately, I don't have a copy so I can't be more specific. I can't even find the photocopies I made of this and other information (I have a couple of large three-ring binders chock full o' architecture descriptions ranging from EDSAC to modern stuff; but my office has moved a couple of times since then, so I've misplaced them). No. The PDP-4 was designed to be able to use PDP-1 memory and I/O devices (IIRC), but the instruction set is different. The instruction set of the PDP-4 is essentially that used by the PDP-7, PDP-9, and PDP-15; the Bell&Newell book (IIR the source C) claimed the primary difference between the PDP-4 and the PDP-7 was that the PDP-7 used ASCII I/O devices while the PDP-4 used a six bit code. The true mystery is, of course, the PDP-3. It was allegedly a 36-bit version of the PDP-1. I've seen one source that had a drawing of the word format for the PDP-3, but no other information (the source was an "encyclopedia of computers" circa 1960 that was composed of two-page descriptions of each machine which appeared to be taken from the marketing literature for the machine. The PDP-3 was listed, a word format was given, and a couple of trivial benchmarkish statistics; time to perform an add, IIRC. I have a copy of the blurb in the aforementioned three-ring binders which I can't find). Rumor has it that a DEC customer built a PDP-3, but DEC never did. Roger Ivie ivie(a)cc.usu.edu