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From: toddpw@tybalt.caltech.edu (Todd P. Whitesel)
Newsgroups: comp.arch
Subject: Re: Self-modifying code
Message-ID: <12288@cit-vax.Caltech.Edu>
Date: 18 Oct 89 21:49:20 GMT
References: <480@gp.govt.nz> <6481@pt.cs.cmu.edu> <9175@etana.tut.fi> <1619@atanasoff.cs.iastate.edu> <672@sce.carleton.ca> <218@dg.dg.com>
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Reply-To: toddpw@tybalt.caltech.edu.UUCP (Todd P. Whitesel)
Organization: California Institute of Technology
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This isn't a me too, but it is an early and (was) a widespread piece of code.

The original Apple // DOS 3.3 (and its predecessors) came on a "DOS System
Master" diskette with a bunch of nifty utilities and demos. When you booted a
"Master" diskette (you could make one with a utility on the System Master disk)
it would load in from $1D00-3FFF (i.e. 16K) and figure out how much memory
there was. Then it would copy itself up to the top of RAM, via a relocator that
took about 1/2K on the disk (I don't know exactly how long it was). If you used
the INIT command to format a disk, it would be a "Slave" diskette because the
bootable DOS image was already relocated (INIT saved a copy of the DOS in
to disk when it was finished).

one pirating technique was to remove the top row of DRAMs and boot a master,
forcing it to load in 32K. then you INITed a disk with that DOS. after
replacing the DRAMs you booted your favorite 48K copy-protected game (which
almost always used a heavily-modified copy of DOS) and somehow got it to reboot
the 32K slave disk. the slave thinks you only have 32K so it doesn't overwrite
the copy-protected dos above it in memory, and you could examine the patched
DOS at your leisure or even save it's routines to disk. (we had programs which
used the copy-protected image in 48K and internal "standard routines" to copy
a protected disk's files onto a normal disk, this was called "DeMuffin" which
was a pun on the 13-to-16-sector converted on the system master called Muffin.)

ain't nostalgia great?

Todd Whitesel
toddpw @ tybalt.caltech.edu