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From: raylau@dasys1.UUCP (Raymond Lau)
Newsgroups: comp.sys.mac
Subject: Re: What is wrong with the Sumacc C compiler
Message-ID: <1939@dasys1.UUCP>
Date: Tue, 10-Nov-87 16:58:39 EST
Article-I.D.: dasys1.1939
Posted: Tue Nov 10 16:58:39 1987
Date-Received: Fri, 13-Nov-87 20:42:39 EST
References: <7486@dartvax.UUCP> <7508@dartvax.UUCP> <21522@ucbvax.BERKELEY.EDU> <2283@watcgl.waterloo.edu>
Organization: The Big Electric Cat
Lines: 36
Keywords: C compiler gripe
Summary: Moving code

In article <2283@watcgl.waterloo.edu>, kdmoen@watcgl.waterloo.edu (Doug Moen) writes:
> oster@dewey.soe.berkeley.edu.UUCP (David Phillip Oster) writes:
> ...concerning why the sumacc compiler is no good...
> >The authors of the compiler seem to be unaware that on a Macintosh
> >executable code can move while the program is running. Unlike all Macintosh
> >compilers, they generate position _dependent_ code, and have a funky
> >loader scheme to resolve non-relocatable references at program load
> >time. Eventually, the code moves, and all that position dependent code
> >points at never-neverland.
> 
> Sorry, but this sounds blatantly impossible.  If the code were to move
> while the program is running, then *all of the return addresses and
> function pointers on the stack would become invalid* and the program
> would crash, regardless of what compiler was used to create the program.
> 
> Now, I can see that maybe desk accessories might move around,
> *but not application programs*.

I think the reference to moveable code is that when a prgm is segmented, each
segment, except the main segment, is relocatable when and if loaded into
memory.  When anything in that segment is called, a jsr is made to the
jump table, which contains a jump to the actual procedure.  When there're}i
no references to procedures in that segment, it may be unloaded, and the
entry in the jump table would be replaced by a jmp to a routine which would
1. load in the segment and
2. finally call the original procedure desired.

I believe that is accurate, if not detailed.  It's been a while since I've
reviewed the info on the jump table...so, no promises.


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