Path: utzoo!attcan!utgpu!watmath!att!dptg!rutgers!mailrus!ncar!unmvax!bbx!bbxsda!scott
From: scott@bbxsda.UUCP (Scott Amspoker)
Newsgroups: comp.lang.c
Subject: Re: effect of free()
Message-ID: <138@bbxsda.UUCP>
Date: 21 Sep 89 17:03:12 GMT
References: <319@cubmol.BIO.COLUMBIA.EDU> <3756@buengc.BU.EDU> <16507@watdragon.waterloo.edu> <990@kim.misemi>
Reply-To: scott@bbxsda.UUCP (Scott Amspoker)
Organization: Basis International,  Albuquerque, NM
Lines: 25

In article <990@kim.misemi> kim@kim.misemi (Kim Letkeman) writes:
>
>Can you tell me how you would go about causing an 80286 to trap on
>load of a "bad" value in the ES That's? This register is internal to
>the CPU and I doubt that there are any values that would cause a trap
>to be raised simply by loading them.

When you load a segment register on an 80286/80386 in *protected* mode
the CPU actually goes into an in-memory table an load a segment
descriptor in to an on-chip cache so that subsequent address translation
is reasonably fast.  If the segment register is loaded with a value
that does not have an associated segment descriptor then a trap
occurs.  This normally doesn't cause a problem.  ES is not
preserved across CALLs.  C compilers on the 80286 are not inclined
to load ES just to perform a pointer move or compare because

   1) segment register loads are expensive
   2) there is no segment register compare instruction

In *real* mode (MS-DOS) there is no possibility of a bad address trap.

-- 
Scott Amspoker
Basis International, Albuquerque, NM
(505) 345-5232