[U-Boot-Users] relocate_code

[YanMin Qiao <sepherosa@sohu.com>]

Murray Jensen wrote:

>On Sat, 04 Jan 2003 00:22:21 +0800, YanMin Qiao <sepherosa@sohu.com> writes:
>...
>  
>
>>   cmplw    cr1,r3,r4
>>    
>>
>...
>  
>
>>   bge    cr1,2f
>>    
>>
>...
>  
>
>>1:    lwzu    r0,4(r8)
>>   stwu    r0,4(r7)
>>   bdnz    1b
>>   b    4f
>>
>>2:    slwi    r0,r0,2        <<----------------- i think above code will 
>>be enough for the two        
>>   add    r8,r4,r0                                      situation
>>   add    r7,r3,r0
>>3:    lwzu    r0,-4(r8)
>>   stwu    r0,-4(r7)
>>   bdnz    3b
>>    
>>
>
>One is copying forwards, the other is copying backwards - you can see the
>lwzu/stwu pair of instructions either add +4 or -4 to the registers
>containing the source and destination addresses (the "u" means to update the
>index register with the resulting memory address).
>
    First thanks for your answer, howerever, my question is
    Why we copy _forward_ when  _memory address_ <CFG_MONITOR_BASE, but 
when _memory address_>=CFG_MONITOR_BASE we will have to _copy backword_? 
 Is it because some overlapping problem when copy?

>
>Be careful with the first "bge" above - it isn't testing the result of the
>previous instruction, it is testing the result of the "cmplw" done quite a
>few instructions beforehand and stored in "cr1" i.e. it will branch if "r3"
>(the destination address) is >= "r4" (the source address) i.e. if it is
>possible that the copy will overwrite the source.
>
    thanks, I know

>
>The Powerpc CPU has four(?) condition code sets where results of operations
>can be stored. While I don't understand the details, I presume it has
>something to do with the instruction pipeline and avoiding unnecessary
>flushes - esp. when you consider that the programmer can indicate whether a
>branch is likely or not with a bit in the instruction - I presume (again)
>that this determines whether the processor does pre-fetching or something
>like that. Details of all this stuff is in the Powerpc Programmer's
>Environment Manual (I think).
>
>If you look through the latest linux kernel source you often see things like
>
>	if (unlikely(<condition>)) {
>
>or
>
>	if (likely(<condition>)) {
>
>On powerpc (I'm guessing) these macros will set an attribute for the compiler
>which will result in this bit being set or cleared in the branch instruction
>of the resulting assembly code - I guess it does something similar for later
>Pentium processors (and nothing at all if the processor doesn't have these
>sort of features). Cheers!
>								Murray...
>--
>Murray Jensen, CSIRO Manufacturing & Infra. Tech.      Phone: +61 3 9662 7763
>Locked Bag No. 9, Preston, Vic, 3072, Australia.         Fax: +61 3 9662 7853
>Internet: Murray.Jensen at csiro.au
>
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>
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>  
>