Q: logic don't know my memory?

Hello, Sorry for my english... I'm not very good...

so, I have a Imac I5 (2,5ghz) with 12 Go of DDRRAM 1333... my Imac know my memory...

I use a RME fireface 800, and, an usb master keyboard

When I use Logic9 , I use Ezdrummer and other virtual instrument... and some plugin effect...

I open the monitor activity, and i know that i can't use more 4,2go of my DDRRAM...?

When i load more than 3,2 go of sample... logic say, "not enought memory..."??

My Imac know my 12 go of DDRRAM

can you help me please...

thanks, Alexis