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:
> > David Hess (davidwhess.delete@this.gmail.com) on January 11, 2020 6:06 am wrote:
> > > Brett (ggtgp.delete@this.yahoo.com) on January 10, 2020 9:53 pm wrote:
> > > >
> > > > Did not realize that the idea of a 64k limited chip was dead and segments would not revive it.
> > >
> > > How does this differ for the 8086? Wouldn't this objection apply to it also?
> > >
> >
> >
> > The same objection was applied to both since the beginning.
> >
> > 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.

Yes the Z800 added a MMU to split code and data and give 24 bit segment addressing.

The eZ80 extended the registers to 24 bit flat addressing.
http://www.zilog.com/force_download.php?filepath=YUhSMGNEb3ZMM2QzZHk1NmFXeHZaeTVqYjIwdlpHOWpjeTlWVFRBd056Y3VjR1Jt

The Z380 extended the registers to 32 bit flat addressing.
http://www.zilog.com/force_download.php?filepath=YUhSMGNEb3ZMM2QzZHk1NmFXeHZaeTVqYjIwdlpHOWpjeTl6WlhKcFlXd3Zlak00TUM1d1pHWT0=

The original Z80 only took a year to design and each of these simple extensions would have also taken a year, but Zilog dropped the ball for a decade instead.

These are all successful Z80 chips today still in production, had Zilog done these simple upgrades instead of the aggressive incompatible Z8000 and Z80000 designs Zilog would have had far more success.

Intel invested in the 432 failure, but also developed the over designed and expensive 286 which never had a successful OS. Let’s say the 286 would have faced the cheaper eZ80 or Z380 without the expensive MMU taking half the die. Intel may still have won, but the cheaper Z80 line would dominate POS system terminals, etc.

Instead Zilog bet the company on consecutive Hail Mary passes and failed.
I bet Zilog did this because the 16 bit customers were not interested in the extended Z80, and those that wanted more got sucked into the Z8000 and Z80000 fiascos. So the paper launch Z800 got dropped, fumbled for half a decade.

> 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.