Adrian (a.delete@this.acm.org) on September 27, 2021 10:59 pm wrote:
> Adrian (a.delete@this.acm.org) on September 27, 2021 10:40 pm wrote:
> > 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.
> >
> >
>
>
> I want to add that because of the difficulties of using the PDP-11 paging, in simpler
> operating systems, i.e. UNIX, paging was not used at all in user programs.
>
> UNIX processes were limited in size to what was accessible without switching pages, i.e. an
> up to 64 kB text segment and an up to 64 kB sum of the data, bss, stack and heap sizes.
>
> The entire up to 4 MB PDP-11 memory was used only by running concurrently many UNIX processes.
>
> On the other hand on Intel 8086 there was no problem e.g. to use a 512 kB array
> in your program, if you compiled it for the large or compact memory models.

When all this business about PDP and paging was started here I thought the DECsystem-10 was meant, not PDP-11! That had proper virtual memory and ran big time sharing systems. Well big for the time! ;-)