In message 2214.1041574514@msa.cmst.csiro.au 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:
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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 blrWhy do we need to calculate the absolutely address? Why should we do it here, instead of everywhere?
U-Boot (aka ppcboot - since you are looking at cpu/mpc8xx) is compiled using the "-mrelocatable" flag i.e. my understanding of this is that all text references will be relative so that the code may be relocated at run time.
The "-mrelocatable" thingy has nothing to do with this part of the code. Also, "-mrelocatable" does _not_ mean that you can just copy the code to a new address and jump into it - it just means that the compiler will generate enough information (especially the GOT) so that you can relocate the code _manually_.
The bit of code above is special - at this point the CPU will be executing at whatever address the reset configuration dictates (for 8xx I think the CPU
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This code jumps to the address that the boot device will appear at after you program the memory controller registers, so that the boot device will not disappear out from underneath you.
_This_ is the correct explanation.
Ummm...
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Can you please shut off this crap when posting to public mailing lists?
Best regards, and a Happy New Year!
Wolfgang Denk