In message 3E16E5AE.4090002@sjtu.edu.cn you wrote:
Hi all:
<u-boot>/cpu/mpc8xx/start.S
.... relocate_code: mr r1, r3 /* Set new stack pointer */ mr r9, r4 /* Save copy of Global Data pointer */ mr r10, r5 /* Save copy of Destination Address */ .... sub r15, r10, r4
/* First our own GOT */ add r14, r14, r15 /* the the one used by the C code */ add r30, r30, r15 ^^^....
I have followed the code to the end of board_init_r(). I didn't figured out where r30 is used as mentioned.
Which part of this section from the README didn't you understand?
... we are restricted by the relevant (E)ABI specifications for the current architecture, and by GCC's implementation. For PowerPC, the following registers have specific use: R1: stack pointer R2: TOC pointer R3-R4: parameter passing and return values R5-R10: parameter passing R13: small data area pointer R30: GOT pointer R31: frame pointer (U-Boot also uses R14 as internal GOT pointer.)
If you want to know all the ugly details I recommend to read the (E)ABI specifications and the relevant parts of the GCC source code.
Best regards,
Wolfgang Denk