Re: alt-Speculation
As much as I love the PDP-11 as an architecture, it would still have run out of steam in the late '80s. The problem was the memory model, and the mixed-endian nature of the system.
Without further architectural evolution (which was the VAX-11 in 1978), the PDP-11 was limited to 64KB processes (unless you used overlays) mapped into an overall 22-bit (4MB) maximum address space.
Don't get me wrong. It was a magic architecture, and because of the orthogonality of the ISA, I used to be able to decode PDP-11 machine code directly from octal dumps on paper. But it was a '70s architecture, not an '80s one.
The '80s should have belonged to Motorola 68000, NS16032 or 32032 (a very nice instruction set), or possibly ARM, running UNIX derivatives.
Just imagine if the IBM PC had had a 68000 with enough of a cut-down UNIX back in 1982. As soon as hard-disks became available (PC-XT time scales), we would have had multi-tasking full UNIX systems on the desktop, a bit like the AT&T 3B1.
PDP-11s survive (even to the current day and into the future according to a recent El-Reg article) because they are fine industrial controllers for systems that do not need large amounts of code to perform their function.