Available memory

I’m using Linux (64 bits) with the last OpenCL driver&sdk.
CL_DEVICE_MAX_MEM_ALLOC_SIZE is just 1/4 of the total memory of my laptop (128 of 512MB with a 8600M GS). This is conform to the official OpenCL specification, but it’s very frustrating to just have access to 128MB since I have 512MB.
Is there any reason to this restriction ?
Thanks and sorry for my weak English…