Thought you folks might be interested in this, since the Dreamcast has an
SH4 processor.

http://www.inode.at/hitmen/hitdc/dc1.htm

... appears to show how a German hacker developed a hardware hack for
the Dreamcast which allows him to boot his own code on the machine, as
well as run a debugger on a PC connected via parallel port.
Hope this is useful to you: I personally would love to see a UNIX running
on Dreamcast...

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                 "(The Mk 2 only reached prototype stage
           and was abandoned due to a design fault in the hull)"
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