256MByte boundary not supported ?

The reason for this limitation is due to the width of the immediate address that can be stored in an instruction word. The largest immediate value is 26 bits wide so that gets you 64MegaWords (32 bit instructions) of address span which equates to 256MB. This only affects the instruction master so if you just need a lot of data storage and can put code elsewhere then you will not hit the 256MB barrier.

Note this is an address span and not simply a memory size thing. For example if you put two 16MB SDRAMs used to store code in your system and space them apart by a 256MB address span you'll run into this as well.