How to change the clock frequency settings (CPU,DSP,DDR).

Hello All,

I am very new to this beagleborad platform. I am trying to test the
beagle board by lowering the clock frequency.
I.e changing the clock frequency settings of CPU,DSP, DDR of the
Beagle-xm rev B

I am using simple demo linux kernel image(Armstrong) throuh SDCard.
While the target is booting.. I interrupted SCOM and changed the
environment variables for CPU
   setenv mpurate 800

   mw 48004040 0x009ae0c to work at 430Mhz ( to change the DSP )

1) I dont understand how these values are calculated for 430Mhz...
2) and how to change the DDR clock rate.?

Please help me in solving these problems.

Thanks in Advance.