porter wrote:No, it was 16bit but could address 256Mb of memory with 18 address lines. A 6502 has 16 address lines but that doesn't make it a 16bit computer. If you went by that argument an 8086 was a 19bit computer and an 8088 was a 20bit computer.
Not to pointlessly nitpick a really old statement in a zombie thread, but... sure, why not, I'll nitpick an old statement in a zombie thread.
I think Porter is confusing the PDP-7 with the PDP-11. The PDP-7 was a genuinely old-fashioned 18 bit machine, descended from the PDP-1 architecture. (It was basically a cheaper and faster PDP-4, which in some respects was a *simplified* PDP-1) What made it "old fashioned" was that it had no general-purpose registers and a relatively tiny instruction set with 16 opcodes and only a 13 bit "direct" addressing range. It had some "interesting" features compared to similar machines of the age which made it better suited to real time/multitasking than other more batch-oriented "small computers", but it's really a different animal from just about any microprocessor-based computer.
The PDP-11, on the other hand, was a solidly 16-bit machine. They all had 16 bit direct addressing, but "real" UNIX was limited to the PDP-11/45 and larger systems which had a segment-based MMU which expanded physical addressing to 18 to 22 bits. (A "Mini-UNIX" was available for the smaller models, but that lacked any form of memory protection.)
In some respects the closest microcomputer analog to the fully UNIX-capable PDP-11 models is the Intel 80286. The addressing model is similar and the 80286 has a capable-enough MMU to fully allow for virtual memory and swapping. (Keep in mind the reason that Bill Gates called the 80286 "brain-damaged" was that it didn't include a facility for virtualizing "real mode", and thus couldn't easily multitask existing DOS programs. If DOS is off the table it's a quite capable CPU by... late 1970's standards.)