memory corruption after MMU turned on

[Lei Sun]

Hi all:
   I am trying to bring up customized board based on MPC8270, the
kernel version is 2.4.30, it works fine on one of the Eval board
(PQ2FADS-VR, which use MPC8275).
   The kernel hangs after uncompressing is done, further debug shows
that it hangs right after "rfi " instruction of the turn_on_mmu:
function in head.S

 turn_on_mmu:
    240     mfmsr   r0
    241     ori r0,r0,MSR_DR|MSR_IR
    242     mtspr   SRR1,r0
    243     lis r0,start_here at h
    244     ori r0,r0,start_here at l
    245     mtspr   SRR0,r0
    246     SYNC
    247     RFI             /* enables MMU */

After i step over the RFI instruction, the PC points to C000XXX , but
all memory content is 0 by then.
my BATU0 is 0xC000FFE0  BATL0 is 0x00000002, I have 128MB RAM. The
u-boot works fine, without MMU turned on. We do have some hardware
problem, such that i have to lower the bus frequency to make the SDRAM
stable. What physical parameter could cause the board not working with
MMU on?

Any suggestion?

Thanks
lei