Path: utzoo!attcan!uunet!clyde.concordia.ca!news-server.csri.toronto.edu!utgpu!watserv1!sunee!gpsteffl
From: gpsteffl@sunee.waterloo.edu (Glenn Patrick Steffler)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Whats wrong with self Modifying Code?
Keywords: religion, gurus, whats up?
Message-ID: <1990Jul9.163607.18336@sunee.waterloo.edu>
Date: 9 Jul 90 16:36:07 GMT
References: <1990Jul6.201328.24660@csmil.umich.edu> <1990Jul6.201743.24777@csmil.umich.edu> <138523@sun.Eng.Sun.COM>
Organization: Gerbils On Speed Inc.
Lines: 50

I have been waiting for people to show all of the great ways to avoid
self modifying code.  But the examples have been contrived and not the
least instructive.

Lets take a real world example:

A spread sheet program which must perform several thousand iterations
of a formula while recalculating.  The formula can be "compiled" to
the stack, and run such that the execution time is considerably less
than if the formula had been interpreted each time.

A video device driver or some such that uses raster ops to write values
to the video display.  (Given the Amiga has a blitter with this ability
I do not ask for confirmation of the relevance of this arguement)  The
driver can "compile" a raster operation fill algorithm into some small
code segment and run it.  This is indeed self modifying code, but is 
almost essential for speed, because the user hates to wait for screen
refresh.

Anyway, that was just some fodder for you guys.  I submit this for
rationalization or destruction.

In article <138523@sun.Eng.Sun.COM> cmcmanis@stpeter.Eng.Sun.COM (Chuck McManis) writes:
>There isn't anything "wrong" with it per se as long as the system
>supports it. When asked in context to the Amiga there are a couple
>of issues :
>
>Issue : How do you get reliable self modifying code execution
>	when there is an instruction cache present ? Let's take
[...]
>they should be sure to spell out clearly on either the package or
>in a README file that the code will not work reliably on a 68020 or
>68030 system. 

>--Chuck McManis						    Sun Microsystems
>uucp: {anywhere}!sun!cmcmanis   BIX: <none>   Internet: cmcmanis@Eng.Sun.COM
>These opinions are my own and no one elses, but you knew that didn't you.
>"I tell you this parrot is bleeding deceased!"

I agree this is a difficult thing to do, but aliasing the address of 
the self modifying code with several other addresses would make that
point moot.

Later

-- 
Co-Op Scum - U of Loo '91             "Bo doesn't know software" - George Brett

"Just got paid today, got myself a pocket full-o change" - ZZ top
                       Glenn Patrick Steffler