Path: utzoo!mnetor!uunet!husc6!bloom-beacon!tut.cis.ohio-state.edu!mailrus!ames!amdahl!rtech!hoptoad!gnu
From: gnu@hoptoad.uucp (John Gilmore)
Newsgroups: comp.sys.mac.programmer
Subject: Re: shutting off the instruction cache on 68020
Message-ID: <4269@hoptoad.uucp>
Date: 29 Mar 88 05:45:52 GMT
References: <5473@spool.cs.wisc.edu>
Organization: Grasshopper Group in San Francisco
Lines: 37

bier@speedy.cs.wisc.edu (George Bier) wrote:
> I teach the machine language, assembly language class at the University
> of Wisconsin.
> The way we teach the class here is to keep as much hidden from them as 
> possible and slowly build up to the advanced material.  The above uses
> only absolute addressing (9f) which is the easiest mode to understand.  Since
> they only know absolute addressing, indexing through the array is
> impossible leading to a self-modifying code solution.

Many thanks for the explanation, Mr. Bier, however, as one of the "hot
shit" programmers who wrote to you, I now feel even more strongly about it.
For an ordinary application to be using self modifying code is one thing,
but you are TEACHING this to unsuspecting students!

Why not start with a program to add add up 10 numbers at fixed
addresses using 10 add instructions?  No big deal.  You can go from
there to "add 10 consecutive numbers at any location in memory" by
using base+offset addressing, and from there to "add any number of
numbers at any location in memory" by introducing looping.  At no
point is it necessary to introduce the idea of self-modifying code,
to convey the basic concepts of machine language programming simply
and one-at-a-time.

You might even take 10 minutes to explain that in most computers, you
are protected from writing on your programs because 999 times out of a
thousand, that was not what you wanted to do; you had a store go wild,
and would be happier if it failed there rather than when it tries to
execute the data later.  Not to mention its making the system more
efficient in sharing program text, and making fast hardware easier to
build (which is why you have to turn off the 68020 cache to get
reliable results when you store in the instruction stream).  There are
many positive sides to the inability to write over instructions, and
I'd much rather that you teach those to people, rather than teaching
them how to circumvent the protection so they can use poor techniques.
-- 
{pyramid,pacbell,amdahl,sun,ihnp4}!hoptoad!gnu			  gnu@toad.com
		"Watch me change my world..." -- Liquid Theatre