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Re: accessing video cards

   Date: Wed, 6 Mar 91 10:27:50 PST
   From: allenb@cs.uoregon.edu
   To: info-macl@cambridge.apple.com
   Subject: accessing video cards
   I am trying to write directly into video memory from lisp.  When I
   get the baseaddr from the gdevice record it returns a large negative
   number rather than a pointer.  I presume that this is because Lisp
   does not recognize 32 bit addresses.  Does anyone know how to coerce 
   this number into an address that will allow me to do block moves to
   the video card?
Fixnums in MCL 1.3 are the only Lisp objects with the sign bit set.
Hence, an address in the upper half of the Mac's address space appears
to Lisp to be a fixnum.  If you ignore this fact and use the apparent
fixnum as an address all should be well.  Very little, if any, type
checking is done on arguments to traps or %GET-PTR and friends.