Problem using cacheDmaMalloc

Hello,
I am trying to use cacheDmaMalloc to allocate a size of about 0x40000
in my memory. However, it is returning Null.

I am using VxWorks 6.6, and the processor is PowerPC 750fx. There are
BAT entries as well - previously, the whole 256 MB or RAM was
addressed using BAT. I cut this value into half and placed the
remaining 128 MB in the sysPhysMemDesc. However, when I do this, the
maximum size I can get is 0x5000 (that means, if I try
cacheDmaMalloc(0x5001) it won't work, however cacheDmaMalloc(0x5000)
works). I have attempted to remove the whole 256 MB from RAM and
replace it with the 2nd BAT entry, however when I do this, the system
refuses to boot.

Can anyone guide me into how to resolve this problem?

Thanks.
.

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