I have an MVME6100 board with an MPC7457 CPU and 1GB DRAM.
I'm working in VxWorks 6.2.

My default RAM_LOW_ADRS and RAM_HIGH_ADRS are 0x100000
and 0x800000 respectively. LOCAL_MEM_AUTOSIZE is disabled.
LOCAL_MEM_SIZE is set to 0x6000000. No user reserved memory.

I can successfully move RAM_LOW_ADRS and RAM_HIGH_ADRS
to 0x1800000 and 0x1f00000 respectively. However, I can't move them
any higher. In fact, any address at or above 0x2000000 causes the
bootrom to hang (showing NO signs that a boot is in progress).

As far as I can tell, I should be able to move these addresses to
0x2000000 or higher. My sysPhysMemDesc[] doesn't contain any
overlapping memory and I'd expect that would be a kernel boot problem
anyway, right?

Anyone have any ideas what I should be looking for?