At 09:19 PM 1/3/2003 -0500, bren@sjtu.edu.cn wrote:

Murray Jensen wrote:

...

On Fri, 03 Jan 2003 11:47:22 +0800, Laudney Ren bren@sjtu.edu.cn

writes:

...

...

I'm reading cpu/mpc8xx/start.S and found the following:

  /*
    * Calculate absolute address in FLASH and jump there
    */

   lis     r3, CFG_MONITOR_BASE@h
   ori     r3, r3, CFG_MONITOR_BASE@l
   addi    r3, r3, in_flash - _start + EXC_OFF_SYS_RESET
   mtlr    r3
   blr

Why do we need to calculate the absolutely address? Why should we do it here, instead of everywhere?

Another part of the puzzle for doing an absolute address jump is because the PowerPC relative jump is limited 24 bits (+/- 8Mbytes). If you are jumping to a location beyond the 24 bit jump range, for instance if your vector is at 0x100 but your ultimate u-boot code will be based at 0x80000000 (to pick an arbitrary example to illustrate the problem), a relative jump won't get you there. Since u-boot doesn't know a priori where the addresses will be on a given piece of hardware, this is the safe and effective method of getting to the right place on every system.

Once you get to your final destination, relative addresses are fine since Wolfgang will never let u-boot code exceed 8Mbytes :-).

gvb

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