[U-Boot-Users] U-Boot Boot Sequence

Martin Winistörfer martinwinistoerfer at gmx.ch
Sun Jan 12 20:06:16 CET 2003

Hello all,

Im currently looking closely to the boot sequence for the MPC8xx but cant
figure it out how it works. Every hint is appreciated. 

Since most MPC8xx and MPCxx both start executing at adress 0x100 there
must reside some code at this location at boot time. I assume the U-Boot
image will
be burned to the external flash (CS0) and we start from there (0x100). 

What I dont understand is, that the label _start (In cpu/mpc8xx/start.S)
will be placed to the runtime
adress 0x100, but resides in the .text section which according to the linker
(take as example board/cogent/u-boot.lds)  wont probably start at adresse
in the image. The .text section is located after the .interp section.

Are this assumptions true?

I have another question concerns the adress calculation 
to in_flash label (Some guys already mentioned something about this on this
The U-Boot image is located in flash (CS0) we start executing at adress
in flash somehow. MPC is now executing instruction by instruction
until the following lines:

 >         /*
 >           * Calculate absolute address in FLASH and jump there
 >           */
 >          lis     r3, CFG_MONITOR_BASE at h
 >          ori     r3, r3, CFG_MONITOR_BASE at l
 >          addi    r3, r3, in_flash - _start + EXC_OFF_SYS_RESET
 >          mtlr    r3
 >          blr

My question is why to calculate the branch to in_flash and why
adding the CFG_MONITOR_BASE? (dont be angry I know the 
same question has alredy been asked but....)

Since _start label is placed to location 0x100

>	.globl	_start
>                _start:

and location counter will increase until to the in_flash label (and further
of course),
the in_flash label will be always in a specific offset from the label

Why we cant simply leave it (the branch calculation) and simply executing
the next 
instruction? (first instrution in in_flash)

I really dont know whether I simply make a huge mess but please help me
to clarify it. Best regards,


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