Adrian (a.delete@this.acm.org) on January 11, 2020 11:38 pm wrote:
> rwessel (robertwessel.delete@this.yahoo.com) on January 11, 2020 12:26 pm wrote:
> > Adrian (a.delete@this.acm.org) on January 11, 2020 6:29 am wrote:
> > >
> > > However, 8086 was introduced one year earlier, when the only alternative that would not be
> > > at least one order of magnitude more expensive would have been something like DEC PDP-11.
> > >
> > > The method used by DEC PDP-11 to extend the address space from 16 bit to 22 bit was
> > > much less convenient than the segmentation of 8086 or of 80286, so at introduction
> > > nobody criticized 8086 for its segmentation, because it was progress anyway.
> >
> >
> > One thing to remember is that for small programs (by x86 standards, large for 8080), segmentation
> > as implemented in x86-16 provided a substantial increase in memory available for applications,
> > at near zero cost in complexity. Consider that on 8080, the OS, BIOS, memory mapped I/O, the
> > application and its data all had to fit in 64KB. With the trivial x86-16 small model, the application
> > had a full 64KB for code and another 64KB for data. In many applications this doubled the possible
> > amount of code and tripled the size of the dataset being processed.
> >
> > In many ways it was not until 16-bit protected mode that segmentation became a huge pain.
> > Even there a large part of the problem was the inadequate number of segment registers, the
> > slowness of reloading them, and the loss of a writable CS compared to real mode (which is really
> > the same as the first issue). I'm not suggesting we go back to that, but segmentation in 16-bit
> > protected mode could have been much less painful with some fairly minor changes.
>
>
> 64 k for code plus another 64 k for data was already standard in PDP-11
> and most other 16-bit minicomputers, before 8086 was introduced.
>
> However, in 1978 the 4 segment registers of 1978 and the relatively easy use of far pointers
> were a significant improvement over what was provided by the MMUs of 16-bit minicomputers,
> except that 8086 lacked memory protection, which was rectified only in 80286, at a time
> when segmentation was already considered obsolete, due to MC68000.
>
> Of course DEC VAX had already been introduced before 8086, but
> that was in a completely different class of price and size.


True, but the 8086 wasn't really playing in that market, rather the comparison would be to the 8-bitters, especially the 8080 family.