Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!gem.mps.ohio-state.edu!ginosko!uunet!crdgw1!crdos1!davidsen
From: davidsen@crdos1.crd.ge.COM (Wm E Davidsen Jr)
Newsgroups: comp.arch
Subject: Re: Self-modifying code
Summary: It's been done
Message-ID: <1032@crdos1.crd.ge.COM>
Date: 12 Oct 89 15:05:46 GMT
References: <6481@pt.cs.cmu.edu> <9175@etana.tut.fi>
Reply-To: davidsen@crdos1.UUCP (bill davidsen)
Organization: GE Corp R&D Center
Lines: 27

In article <9175@etana.tut.fi>, pl@etana.tut.fi (Lehtinen Pertti) writes:

|  	I've been lately wondering if there is any architecture
|  	with possibility to execute instruction indirectly.

  Among others the GE-600 series, currently marketed as the Honeywell
DPS. It has an execute and execute double (execute two instructions at
location) to use.

  One use of self-modifying code is to get around bad hardware design,
such as making i/o port numbers immediate addresses. If this is the
case, as it has been on a number of machines, you either need to have
one device driver per port or modify the code.

  There was some such limitation on the DPS, and the way I got around
it, rather than write self modifying code, was to build the instruction
I needed on the stack and do a XEC on it. This was useful on the models
which support pure code segments.

  I think the 8080 had i/o port as part of the address of the
instruction, and the Z80 added the ability to have the port address in a
register. Someone will undoubtedly remember it better than I do.
-- 
bill davidsen	(davidsen@crdos1.crd.GE.COM -or- uunet!crdgw1!crdos1!davidsen)
"The world is filled with fools. They blindly follow their so-called
'reason' in the face of the church and common sense. Any fool can see
that the world is flat!" - anon