On 2025-08-09 5:42 a.m., Peter Corlett via cctalk wrote:
I have no love for the x86 architecture, but note that the 80286 did succeed
at what we can safely assume was the design brief: improve on the 8086, and
add a protected mode. That it's not a very efficient protected mode is an
rather unfortunate design mis-step which still dogs x86 to this day, but if
you pretend that feature doesn't exist and leave the CPU in real mode than
you get a very welcome performance boost.
I would perhaps go further and note that it delivered a technically useful
protected mode in 1982, before my personal favourite architecture of that
era, m68k. The MC68451 external MMU *added* base-and-bounds segmentation to
m68k which seems to date from 1983 (in as much as the oldest datasheet is
from April 1983) and of course ideally also needed to be paired with at
least the 68010 rather than the 68000 which couldn't restart instructions.
That we forget the MC68451 exists shows what a dead-end base-and-bounds was
even back then.
Wikipedia claims the following, which explains Gates' attitude: 'Bill Gates
referred to the 80286 as a "brain-damaged" chip, because it cannot use
virtual machines to multitask multiple MS-DOS applications[22] with an
operating system like Microsoft Windows.' In other words, he didn't like it
because it didn't provide a feature that he wanted for Microsoft. Well,
tough, you should have been working with Intel instead of just freeloading.
I mean, look at the complexity with base-and-bounds, rings, and all that
other arcane stuff which looks like mainframes rather than microprocessors.
Protected mode was clearly designed with IBM rather than Microsoft in mind.
The 8086 and 8088 was stopgap measure, while Intel worked on the BIG
cpu, iAPX 432. I suspect that chip design had major impact on how
segments and segment registers where thought of in computer architecture
up to the 286.
Ben.