ARM newbie

ARM newbie

[Renaud Barbier]

I am doing my first port to an ARM CPU (Broadcom chip). For testing I
built the friendlyarm-tiny210 and created an objdump to see what the
code look like:

barebox:     file format elf32-littlearm


Disassembly of section .text:

23e00000 <start>:
23e00000:   ea000012        b       23e00050 <start+0x50>
...
23e00050:   ea000013        b       23e000a4 <barebox_arm_reset_vector>
...

23e000a4 <barebox_arm_reset_vector>:
23e000a4:   e52de004        push    {lr}       ; (str lr, [sp, #-4]!)

I am a bit surprise to see a push to stack as the first instruction of
barebox_arm_reset_vector as the stack pointer has not been defined yet.
Or is there a pre boot loader that is not built as part of the
friendlyarm-tiny210  build.

Other BSP I have built do not produce this push instruction.
Where is the difference coming from in the build?

Are all BSP having a lowlevel.c file assumed to have a pre-bootloader?

Cheers,
R.










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Re: ARM newbie

[Sascha Hauer]

Hi Renaud,

On Thu, Jan 22, 2015 at 11:53:56AM +0000, Renaud Barbier wrote:
> 
> I am doing my first port to an ARM CPU (Broadcom chip). For testing I
> built the friendlyarm-tiny210 and created an objdump to see what the
> code look like:
> 
> barebox:     file format elf32-littlearm
> 
> 
> Disassembly of section .text:
> 
> 23e00000 <start>:
> 23e00000:   ea000012        b       23e00050 <start+0x50>
> ...
> 23e00050:   ea000013        b       23e000a4 <barebox_arm_reset_vector>
> ...
> 
> 23e000a4 <barebox_arm_reset_vector>:
> 23e000a4:   e52de004        push    {lr}       ; (str lr, [sp, #-4]!)
> 
> I am a bit surprise to see a push to stack as the first instruction of
> barebox_arm_reset_vector as the stack pointer has not been defined yet.
> Or is there a pre boot loader that is not built as part of the
> friendlyarm-tiny210  build.
> 
> Other BSP I have built do not produce this push instruction.
> Where is the difference coming from in the build?
> 
> Are all BSP having a lowlevel.c file assumed to have a pre-bootloader?

barebox_arm_reset_vector should have a __naked like the other boards
have, then the stack won't be used. Does the board work? If yes then the
stack pointer seems to be initialized with some value where it doesn't
hurt to push to, but it doesn't look intentional.

Sascha

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