Debugging linux at boot phase

I downloaded my kernel image (zImage.elf) to the target using JTAG.
When I run the kernel nothing happens on the serial terminal.
When I stop, the PC is at 0x700. This was probably caused by exception.
How can I debug my kernel at this stage ?
Before this exception, the PC is at the area of 0x400000.
I think this is before the relocation to a virtual address of 0xC000000.
Can the BDI2000 JTAG help ?
I'm now using a Xilinx JTAG. The PPC405 is embedded in a Xilinx virtex 4.
Thanks.