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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