[CentOS] x86_64 and memory reservation

Hi,

I have an Athlon 64 2800+ with an ASUS K8V-MX mATX motherboard.
2x512 MB DDR400 Ram.

Now the problem: 8MB of RAM is used by the shared memory of the video 
card, so I would expect 1016MB RAM to be available for Linux (that's how 
much the BIOS shows).
Indeed if I go into memtest86+ v1.50 then I see:
in e820-Std mode: 1015MB ram, with 432K reserved
in e820-All mode: 1016MB ram, with 0K reserved

This seems OK and everything works as expected.

However when I boot CentOS 4.2 x86_64 (latest kernel) I end up with _way_ 
less available RAM - 21.5 MB is listed as reserved by the kernel - why???

To quote from dmesg:

Memory: 1017796k/1040000k available (2398k kernel code, 21460k reserved, 
1303k data, 164k init)

This seems to make little sense... is this reserved area actually used by 
the kernel for something? or is it just sitting idly waiting for a better 
tomorrow?

Cheers,
MaZe.