I386 segmentation highlights

By: Adrian (a.delete@this.acm.org), October 4, 2021 9:53 am
Room: Moderated Discussions
sr (nobody.delete@this.nowhere.com) on October 4, 2021 7:53 am wrote:
>
> But I386 can also invert segment addressing and I believe that it's also it's default behavior for stack.
> So in special case of one stack and one data segment stack addressing starts from top of memory(growing
> down) and is using inverted offset( so max offset is still at top of memory and is also segment max


The offset was not inverted, only the meaning of the segment limit.

Quoting Intel:

"The meaning of the segment limit is reversed for an expand-down segment. While an offset in the range from 0 to the segment limit is valid for other kinds of segments (outside this range a general-protection exception is generated), in an expand-down segment these offsets are the ones which generate exceptions. The valid offsets in an expand-down segment are those which generate exceptions in the other kinds of segments."


For stack segments this was OK, because the SP was initialized with the end of the segment, then it was decremented towards the segment limit at each stack allocation.

If the stack segment was marked as expand-down, its size could be increased whenever necessary without affecting the already allocated stack or the segments placed lower in the memory.








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    Where do you see this information? (NT)anon22021/09/09 02:45 AM
      Where do you see this information?sr2021/09/11 01:40 AM
        Where do you see this information?anon22021/09/11 01:53 AM
          Where do you see this information?sr2021/09/11 02:08 AM
            Thank you (NT)anon22021/09/11 04:31 PM
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