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From: shap@sfsup.UUCP (J.S.Shapiro)
Newsgroups: comp.sys.intel
Subject: Re: comp.sys.intel
Message-ID: <866@sfsup.UUCP>
Date: Tue, 25-Nov-86 03:12:29 EST
Article-I.D.: sfsup.866
Posted: Tue Nov 25 03:12:29 1986
Date-Received: Tue, 25-Nov-86 10:05:13 EST
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Organization: AT&T Information Systems, Summit N.J.
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> OK, all you intel hackers, here's a question for you.  This will tell
> me very quickly if I want to ever deal with segments or not.
> 
> 1) How does virtual to physical (page => frame) mapping occur?  Are there
>    seperate page tables for each segment, or what?  (I know, dumb
>    question).

Right. Dumb question. It does the right thing.

> 
> 2) OK, a better question:  suppose my process forks (spawns a child)
>    and I would like to set things up as follows:
> 
>    o  child gets personal copy of stack.
> 
>    o  child gets shared copy of text.
> 
>    o  child gets gets shared (with parent) copy of data addresses
>       0x01ffff..0xffffff (for example).
> 
>    o  child gets personal copy of data addresses 0x1ffffff and above.
> 
>    In other words, I want to take the section of memory with virtual
>    addresses 0x01ffff to 0xffffff and have that shared.  Can this be
>    __easily__ done with the 386 memory management?  Seems like it 
>    would be a nice thing to support.  Don't forget, there may be 
>    valid data addresses both above and below the segment in question;
>    you can't split it into two, have to go to three chunks.

All easy, though the sharing naturally has to happen in page-sized
increments.

> 
> 3) And yet another nifty question: can I make the new non-shared
>    data segments be copy-on-write?  

Yes, write protect the pages and use software to figure out what to do.
Straightforward. There is also a user-use bit out there to play with in the
PTE.

> 
> 4) How about using a segment to catch stack overflow?  Dynamically altering
>    segment size?  From user mode? Supervisor mode?  After or during
>    allocation?  

There is a stack segment size notion.