David Hess (davidwhess.delete@this.gmail.com) on September 28, 2021 12:50 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.
> >
> > ...
> >
> > 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).
> >
> > ...
>
> It was the same for microcomputers. Before the 8086, system, or if you were lucky CPU, specific memory bank switching
> was used somewhat like EMS on the PC later. Segment registers on the 8086 were a huge improvement.
>
> Of course 68000 had none of these problems but it was more expensive and
> Motorola was more difficult to deal with, at least in my experience.
>

Don't talk to me about unlucky low power processors... Very long time ago I worked on one of the first GSM mobile phone to be commercialised, it had a 68hc11 and 8096 processors, and paging memory was done on the 8096 with standard I/O ports... The processor did not know we changed memory page, the DOS debugger did know, and a single step instruction to switch (code) bank took multiple minutes unloading all previous symbols and reloading the new page symbols...
The second most interesting thing of that setup was to invert the bits on the serial line, b7 becomes b0, b6 becomes b1, b5 becomes b2...