[U-Boot-Users] PPC: How to access RAM location 0x0 (unmapped) ?

Hi; In MIPS I can access RAM localtion 0x0 unmapped with 0x8000_0000 (cached) and 0xA000_0000(uncached). I guess this is not possible in PPC? I ask because I would like to call get_ram_size() during the bootup to determine the size of my SDRAM before it is mapped. Anyway, I need to know the size of SDRAM in order to map it.

Thanks & Regards, TEH