From owner-freebsd-current@FreeBSD.ORG  Wed Jul  1 16:37:43 2009
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Date: Wed, 01 Jul 2009 18:37:40 +0200 (CEST)
From: Alexander Best <alexbestms@math.uni-muenster.de>
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Subject: Re: building device drivers for FreeBSD 7.2+ /AMD64
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have you tried this article?
http://www.freebsd.org/doc/en_US.ISO8859-1/books/arch-handbook/driverbasics.html

i don't think it's amd64 specific though.

cheers.
alex