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From: bombadil@diku.dk (Kristian Nielsen)
Newsgroups: comp.sys.amiga.programmer
Subject: Re: Self modifying code
Message-ID: <1991May29.134419.4980@odin.diku.dk>
Date: 29 May 91 13:44:19 GMT
References: <5005@orbit.cts.com> <1991May28.120630.10150@cs.umu.se>
Sender: bombadil@freja.diku.dk
Organization: Department of Computer Science, U of Copenhagen
Lines: 38

dvljrt@cs.umu.se (Joakim Rosqvist) writes:

>Ok, I have heard before that you should not use self modifying code.
>Can anybody give a good reason for this, besides it being bad programming
>pratice. 
>I just got the RKM manuals and they only stated "for compatibilty with
>010/020/030 etc. don't use selfmodifying code"
>Could it be that data written to an address that is also in cachememory
>will update the memory, but not the cache?

  Yes, this is the problem - I believe on the '040 its even worse, since a
write might only go to the (data) cache and not be written to memory until
later.

>Consider for instance that I would like to write a fast completely
>unwinded linedrawing routine and wrote code for drawing one pixel.
>Then when the program start, I would allocate memory for 320 such routines
>and putting 320 copies of my routine there. Would this be considered as
>selfmodifying code, and therefore forbidden?

  Yes, this, too will break on 680x0 with cache.

>If so, compare it to when the OS is loading a program that contains
>non-PC relative references to itself which will have to be rewritten
>depending on the address the code was loaded to.

  Yes, the OS has to do this, so it has to know how to flush the cache -
actually the loading of the code itself could be termed selfmodifying. I
think 2.0 includes a function like LoadSeg() that 'loads' code from a
SegList already in memory. And before asking about being 1.3-compatible:
don't bother (at least when 2.0 becomes avaiable). Hopefully (IMHO), in 6
months it will be impossible to buy an 1.3 Amiga.

>/$DR.HEX$

>--------------------------------------------
>Email to:  dvljrt@cs.umu.se
>2 Windows on the screen?  Oh no, not me.  I just Logout & Go.