only half of physical memory showing up

[Hey Tom I Changed My Name! <kunze@CS.ColoState.EDU>]

i just installed debian on a new machine and only half of the memory is
showing up (64 out of 128 megs) in linux. i have the following:

pototo distribution (the rescue disk for slink would not boot)
athlon 550mhz processor
gigabyte 200mhz athlon motherboard
1 x 128mb SDRAM DIMM

the bios shows that all the memory is there when it boots, but i only have
64 megabytes available in linux. does anyone have any ideas what the
problem is or what i can do to fix it?

thanks is advance for any help

matt kunze