David Hess (davidwhess.delete@this.gmail.com) on September 27, 2021 11:35 am wrote:
> blaine (myname.delete@this.acm.org) on September 26, 2021 11:19 pm wrote:
> >
> > As someone who was there: Morse was under severe constraints in design of the 8086.
> > I would not read anything into segments but as an expedient under constraints.
>
> I will add as an experienced 8080 and Z80 programmer at the time, the 8086 segmenting scheme was seen as a great
> improvement over contemporary developing solutions. Its limitations only became a problem later when single
> data structures exceeded the size of the entire 64 kilobyte address range of the previous generation of processors,
> and the alternatives (Zilog, others), except for a single larger address range (68000), were worse.
>


The 1978 Intel 8086 segmenting scheme was a great improvement not only over the existing microprocesors with a 16-bit address space, but also over what was available for the most popular minicomputers, i.e. DEC PDP-11.

DEC PDP-11 extended its 16-bit address space to a 22-bit address space (i.e. 4 MB) using a very primitive paging scheme.

Paging in PDP-11 and similar minicomputers had nothing to do with paging in VAX, 80386 and similar CPUs, where it was used for dynamic relocation, memory protection and virtual memory.


Paging in PDP-11 was primarily only a method for address space extension, which was far more inconvenient to use than the segments of 8086, which could start at any 16-byte aligned address in the memory and which could be loaded together with in-segment pointers with a single instruction (JMP FAR, LDS, LES).

In PDP-11 you could access simultaneously the equivalent of two 64 kB segments, a data segment and a code segment, but whenever you needed to access data or code from another part of the memory switching the right pages was a much more tedious and convoluted process than using far pointers with 8086.

Also, the fact that 8086 had 4 simultaneously accessible segments instead of only 2 reduced a lot the need of switching segments in comparison with PDP-11.