Re: what's a segment, 80286 protected mode

According to George Neuner  <gneuner2@comcast.net>:

The bad taste of segments is from exposure to Intel's half-assed
implementation which exposed the segment selector as part of the
address.
>
Segments /should/ have been implemented similar to the way paging is
done: the program using flat 32-bit addresses and the MMU (SMU?)
consulting some kind of segment "database" [using the term loosely].

The whole point of a segmented architecture is that the segments are visible and
meaningful. You put a thing (for some definition of thing) in a segment to
control access to the thing. So if it's an array, all of the address
calculations are relative to the segment and out of bounds references fail
because they point to a non-existent part of the segment. Similiarly if it's
code, a jump outside the segment's boundaries fails.

Muitics and the Burroughs machines had (still have, I suppose for emulated
Burroughs) visible segments and programmers liked them just fine. The problems
were that the segment sizes were too small as memories got bigger, and that they
weren't byte addressed which these days is practically mandatory.  The 286 added
additional flaws that there weren't enough segment registers and segment loads
were very slow.

What you're describing is multi-level page tables. Every virtual memory system
has them. Sometimes the operating systems make the higher level tables visible
to applications, sometimes they don't. For example, in IBM mainframes the second
level page table entries, which they call segments, can be shared between
applications.




--
Regards,
John Levine, johnl@taugh.com, Primary Perpetrator of "The Internet for Dummies",
Please consider the environment before reading this e-mail. https://jl.ly