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From: John Levine <johnl@taugh.com>
Newsgroups: comp.arch
Subject: Re: Address bits again, Article on new mainframe use
Date: Thu, 12 Sep 2024 20:15:01 -0000 (UTC)
Organization: Taughannock Networks
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According to Michael S  <already5chosen@yahoo.com>:
>x86 Real mode segmentation is a hack to the address space. 80286
>protected mode segmentation is something else. The only similarity
>between the two is maximal size of segment is the same.

The 386 had 32 bit segments which should have made segmented code practical and efficient
and allowed giant programs with lots of gigabyte segments. But Intel shot themselves in
the foot. One problem was that loading a segment register to switch segments remained
extremely slow so you still needed to write your program to avoid doing so.

The other was that they mapped all the segments into a 32 bit linear address space, and
paged the linear address space.  That meant that the total size of all active segments
had to fit into 4GB, at which point people said fine, whatever, set all the segment
registers to map a single 4GB segment onto the linear address space and used it as
a flat address machine.


-- 
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