[U-Boot-Users] RE: [u-boot-users]: Relocation code on Nios
ScottM at orbacom.com
Mon Jul 12 19:01:26 CEST 2004
>> I don"t understand this particular line of code:
>> ld %g7, [%g7]
This moves the value pointed to by %g7 into register
%g7. %g7 points to the location that stores the address
of _start. This is the address where u-boot is to be
copied (the destination address).
>> Per my understanding, %g7 keeps the return address.
>> This address should point to the next instruction
>> of code just after the delay slot of CALL or BSR
>> used to call u-boot.
Yes, you are correct. %g7 points to where the (link time)
address of _start is stored (not the current execution
address of _start).
>> later this value (instruction bytes) is compared with
>> %g5 that keeps the end of data segment according to
>> u-boot.lds. I can"t understand why code address is
>> being compared with instruction bytes. Please
>> explain if possible - what does [%g7] keep and where
>> am I wrong?
This just lets you store u-boot in memory at any address.
When you being execution, the code is simply copied to
TEXT_BASE, that's all. This initial bsr just lets us
know where we begin execution, the value in %g7 after
the bsr is the target address plus 4.
Currently, for convenience and simplicity, the u-boot
code, data, and command table are currently located
There's no magic here ... just load the binary to
some arbitrary address, then single step through the
code -- it'll help you understand :-)
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