[help] kernel boot parameter "mem=? disparity

[Jun Hu]

Hi Guys:
My machine has 8G memory, when I use kernel boot parameter mem=5G , it only display ¡°4084 M¡± using ¡°free ¨Cm ¡°. 
where the ¡°5120-4084 = 1036M ¡° memory run?

# /bin/uname -a
Linux dom0-93 3.0.13-0.27-default #1 SMP Wed Feb 15 13:33:49 UTC 2012 (d73692b) x86_64 x86_64 x86_64 GNU/Linux

dom0-93:~ # free -m
total used free shared buffers cached
Mem: 4084 259 3824 0 6 84
-/+ buffers/cache: 169 3915
Swap: 1916 0 1916
dom0-93:~ # cat /proc/cmdline 
root=/dev/disk/by-id/cciss-3600508b100104839535656314933001b-part1 resume=/dev/disk/by-id/cciss-3600508b100104839535656314933001b-part2 splash=silent crashkernel=256M-:128M vga=0x317 mem=5120M

why ? 
details info ,please see the attachment.
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