In message 3E1575C3.30905@sjtu.edu.cn you wrote:

since after boot all the registers of mpc860( im using mpc860) is initialized by u-boot

U-Boot does not initialize _all_ registers, only those it really needs to.

but bdm4gdb also initialize the register,

BDm4GDB also initializes only the registers you tell it in the init script.

so how to use bdm4gdb to debug the u-boot??

Modify your init script for BDM4GDB to initialize only those registers you really need, and/or those that don't cause conflicts with U-Boot. For a start, just initialize SYPCR (to turn off the watchdog), and nothing else.

Best regards, and a Happy New Year!

Wolfgang Denk