mpc8540 and SDRAM burst

I’m using the mpc8540 processor with QNX 6.3.0 SP2.
It has the main memory in DDR-RAM. In addition there is a small SD-RAM
on the local bus. Accesses to this memory (SD-RAM) is really slow, since
the processor never uses burst accesses. The documentation claims that
the processor always performs burst accesses but it doesn’t (I attached
a scope to the local bus)!
Is there any special functions to use in order to force burst access?
I already tried different settings for the SD-RAM controller: cache
disabled, enabled, witre-through, write back, …

Do I have to solve this issue in QNX or at a lower level (asm instructions)?