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From: m0154@tnc.UUCP (GUY GARNETT)
Newsgroups: comp.sys.amiga.programmer
Subject: Re: Starting another copy of your own code (was Re: Help! (AmigaDOS CreateProc/LoadSeg)
Message-ID: <752@tnc.UUCP>
Date: 12 Mar 91 21:28:44 GMT
References: <63329@eerie.acsu.Buffalo.EDU> <19578@cbmvax.commodore.com> <06417.AA06417@babylon.rmt.sub.org> <18cb4d63.ARN0b56@swinjm.UUCP> <1991Mar9.170859.4810@Sandelman.OCUnix.On.Ca> <06493.AA06493@babylon.rmt.sub.org>
Reply-To: m0154@tnc.UUCP (GUY GARNETT)
Organization: The Next Challenge, Fairfax, Va.
Lines: 25


I don't think that the examples described previously are
self-modifying code (at least, not from the point of view of a 680x0
family processor).  The reason: Self modifying code (the kind which
causes programs to break) is where some values got into the code
cache, and then were changed (by whatever agency method; its not
important).  When the program goes back to re-execute the instruction
(its probably inside some kind of loop), if the cache hits (probably)
then we execute the old (before change) instruction (on a miss,
everything works fine).  In the example code from AmigaMail, the
'code' is a single instruction which is built into a data structure. 
When it is branched to, the code cache will miss (that location has
never been executed before; yes, the value is in the DATA chache, but
the two are separate entities).

Therefore, the method should be safe, as long as each separate task to
be launched has its own CodeHdr.  If you try to re-use a CodeHdr (by
stuffing a new value in CodeHdr.Function) then it *IS* self-modifying
code, and may fail.  The method is also always safe if you invalidate
the cache right after you put a value in CodeHdr.Function; this will
work no matter how many times you try to re-use the structure (of
course, trying to re-use CodeHdr will probably have other side-effects
as well).

Wildstar