Re: [avr-gcc-list] Modifying compiler output to suit Tiny13 bootloader

[David Carr]

Hope everyone is enjoying this thread as much as I am ;)

For posterity and confirmation of my understanding:

It looks like the rjmp at address 0 is generated by the file 
/usr/avr/lib/crttnXX.o.  This file seems to be generated during the 
avr-libc build process from a file called gcrt1.S in the avr-libc 
distribution.  To override this behavior it seems that I'd have to make 
a custom start up assembly file and use it for my bootloader-C 
projects.  That sounds messy.

After perusing this page 
[http://www.nongnu.org/avr-libc/user-manual/mem_sections.html], it 
appears that a better solution might be to override the weak 
.init0/__init() section.  The new .init0/__init() would rjmp to my 
bootloader.  My bootloader could then rjmp back to .init1 or .init2 so 
long as those were placed at a fixed location.  Linker script to do that?

Lastly, the avr-libc startup code seems to zero r1, zero the status 
register, and load the stack pointer.  I'm not sure if there are any 
other pieces of state I should be concerned about.

-DC

David Carr wrote:
> Hi all,
>
> I've been writing a bootloader for the Tiny13 for the past few days.  
> At this point, the bootloader can download, program, and execute 
> simple programs written in assembly.  Now I'd like to add support for 
> programs compiled with avr-gcc.
>
> I wrote a very simple C program and looked at the assembly output 
> produced by avr-gcc.  The first line is:
>
> 00000000 <__vectors>:
>   0:   10 c0           rjmp    .+32            ; 0x22 <__ctors_end>
>
> My bootloader lives in the last 192 bytes of flash, so I'd like to 
> replace this instruction (at address 0) with an rjmp to that 
> location.  Where/how does this line get generated?  I assume its deep 
> in the architecture specific start up code somewhere.
>
> Secondly, what assumptions does the compiler make about the state of 
> the machine/registers on boot?
> It seems there is some initialization going on in the __ctors_end 
> section, such as zeroing r1.
>
> Lastly, where do the linker scripts live now?
> The avr-libc webpage mentions /usr/avr/lib, but I only have a bunch 
> files ending in .o there.  No files ending in .x.  Am I doing 
> something silly?
>
> I really appreciate any help you can give me.  Understanding the 
> internals of gcc is a formidable challenge for me.
> -David Carr
>
>
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