[U-Boot-Users] another question of relocate_code

Wolfgang Denk wd at denx.de
Sat Jan 4 14:57:48 CET 2003

In message <3E16E5AE.4090002 at 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

Software Engineering:  Embedded and Realtime Systems,  Embedded Linux
Phone: (+49)-8142-4596-87  Fax: (+49)-8142-4596-88  Email: wd at denx.de
Real computer scientists despise the idea of actual  hardware.  Hard-
ware has limitations, software doesn't. It's a real shame that Turing
machines are so poor at I/O.

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