Writing directly to memory

Hey all,

I am attempting to utilise movitz to learn about low level programming and
hopefully contribute a few drivers.  One of the things that I am trying to do
is play around with some VGA programming, mainly because graphics programming
gives instant satisfaction.

I have successfully changed video mode to 640x480x16bpp by manipulation of the
vga register sets, and movitz continues to run (i.e. doesn't crash), as I can
switch back. :)

However, I am now stuck when it comes to writing directly to memory.  I want to
write to the vga memory area (as reported by vga-memory-map) but I couldn't
work out how to do it.  I attempted using the form (setf (io-port #xa000
:unsigned-byte16) #x0000), but this failed to write to memory (as reported by a
subsequent io-port read call)

Anyone able to give me any hints?

Martin