NIOS II - Instantiating array on SDRAM

Hi group,

I am trying to develop a digital camera around the NIOS II using the Stratix development board. I need to instantiate a large buffer (2MW) in the SDRAM. In my code I instantiated my array like this:

alt_u16 Image_Buffer[0x2000000] __attribute__ ((section (".sdram"));

The SOPC builder has an SDRAM of 4MX32bits, so I should have enough space to instantiae the array. When I am trying to compile I am getting a messege that my SDRAM is full unless I am reducing the array size to like 20000. All of my program and data are stored in different memories (ext sram and internal sram). Questions:

1. What is the limitations of the array size?
2. Am I doing the right thing in instantiating the array like this?

I will appreciate any help, Zohar