Re: Address bits again, Article on new mainframe use

On Thu, 12 Sep 2024 20:15:01 +0000, John Levine wrote:

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.

Intel's only other choice was to use more than 32-bits as the segment
base address and they had run out of bits.

>