Bizzar memory problem can someone help me diagnose the problem

[WINTERLORD]

ok my machine is a core i7 920 gigabyte mb. i am running a 64bit OS (windows 7 x64) however the computer does see all 6gb of memory but says only 4gb is usuable. whats even more bizzare is that windows see's there is 6gb ram but bios or that black scvreen when you first boot only see's 4gb? how is this possable that windows see's more then bios and in fact there is 6gb ddr3 1600 memory installed.

please help figure this problem out or diagnose it

 

[o1die]

You may check your bios for "memory mapping" and be sure it's enabled. Other bios options for ram include "unganged".