Ditto. The most elegant archtecture ever, IMO. "The Development of the C Language", by Dennis M. Ritchie: https://www.bell-labs.com/usr/dmr/www/chist.html says: The PDP-7, however, did have a few `auto-increment' memory cells, with the property that an indirect memory reference through them incremented the cell. This feature probably suggested such operators to Thompson; the generalization to make them both prefix and postfix was his own. Indeed, the auto-increment cells were not used directly in implementation of the operators, and a stronger motivation for the innovation was probably his observation that the translation of ++x was smaller than that of x=x+1. Note the "probably"; unless Ken remembers, and says something, that's probably the best we are going to get. The PDP-4, -7 and -9 are all the same architecture (very similar to the PDP-1, but simplified a bit), differing only in implementation. (Most PDP-7 code will run on a -9, un-modified.) Basic instructions look like: Instructions had a 4-bit opcode ('000'-'054'), 1 bit of indirect, and 13 bits of address. It was a load-store architecture, with a single accumulator. So, yes, similar to an -8. There are other opcodes for non-memory operations ('074' opcode), and I/O ('070'), using bits in the 'address' field. ('060' opcodes were for the optional EAE.) All of the -4/-7/-9 had the 'auto-increment on locations 010-017' when indirecting through them' feature. Bitsavers has fairly complete docs on them all: http://www.bitsavers.org/pdf/dec/pdp4/ http://www.bitsavers.org/pdf/dec/pdp7/ http://www.bitsavers.org/pdf/dec/pdp9/ Noel