Re: Making C compiler generate obfuscated code

On Sat, 18 Dec 2010 06:23:36 +0000 (UTC), glen herrmannsfeldt
<gah@ugcs.caltech.edu> wrote:


>Along the line of such tricks, there are stories of Apple II ROM
>cloners claiming that so many such tricks were used that the Apple
>implementation was the only one possible in the available number of
>bytes. Then, since it was the only one possible, it should not
>protected against cloning.


I've heard these claims about the Apple II ROM for thirty years and I
think they are bogus. I learned assembly programming on a IIe and it
was impossible to get much done without a good understanding of the
ROM routines. Many subroutines had multiple entry points and many
were routinely reused from different contexts, but I don't believe the
Apple's ROM was any more complex than any other computer ROM of that
era (or indeed any 8-bit device today).


The major stumbling block to cloning the Apple IIe without licensing
the ROM code was Applesoft FP BASIC, which was implemented using a
bytecoded 16-bit virtual machine interpreter called Sweet16.
Approximately 5KB of Sweet16 code was included in the 16KB Apple IIe
ROM. The cloners couldn't copy the Sweet16 code, nor could they
figure out how to fit a non-infringing work-alike BASIC into the same
address space.


There were a few other issues with the ROM code for the custom disk
controller and video (in particular the hi-res graphics IIRC). Here
the problem was not having Wozniak's hardware ... work-alike code for
OTS video display and disk controllers couldn't be fit into the same
address space.


There eventually were a number of legal Apple IIe and //c clones, but,
AFAIK, all of them had much larger ROMs than the machine they copied
and had to use bank-switching and address shadowing to present a
compatible memory map for existing Apple programs.




YMMV but I don't consider implementing a virtual machine to be a
"trick". Wozniak created Sweet16 to make programming easier ... it
ultimately saved some ROM space, but it was never intended to deceive
anyone ... the Sweet16 interpreter and the fact that it was used was
documented in the Apple ][ Integer BASIC manual. I don't recall if
Sweet16 ever was mentioned in the FP BASIC manual, but FP BASIC was an
extension of Integer BASIC and so, ISTM, anyone interested should have
inferred that Sweet16 had been used in FP BASIC's implementation as
well.


George

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