Re: [avr-gcc-list] How to get low byte off a function address?

[Erik Christiansen]

On Sat, Jan 19, 2008 at 04:15:35PM +0100, Wouter van Gulik wrote:
> 
> How do I do such a thing? Using the lower 8 bits is possible when loading a
> register so why not in a table?

In the past, we've encountered other relocations that aren't handled by
the avr port of binutils. It does look like this is another case that
avr-ld hasn't been tweaked to handle.

It sounds like you've tried one work-around, i.e. loading a register,
then writing to the table, now necessarily in RAM. That's workable, code
space allowing.

The learning curve for binutils internals being a bit too steep for a
quick toolchain tweak, I'd alternatively be tempted to invoke a few
lines of awk (from the makefile) to snaffle the absolute addresses from
the map file, insert them in the table, reassemble that file, and link
again. (Pre-existing dummy .byte lines would ensure addresses don't move
in flash.) That's perhaps worthwhile if you're chasing this either
because a RAM-resident table, or the copy loop, is intolerable in the
tiny bootloader. Granted, this comes close to winning an ugly contest,
but it pretty much "has to work"(tm).

If the file with the function pointer table is linked last, then the
others can be incrementally linked, and the table file linked after
being "awked".

An afterthought: You could alternatively put the foo() functions into a
separate output section, allowing the linker script to place the block
of them at a fixed address. (Each could in fact be placed in an
individual section.) The table could in the latter case be filled with
constants. (It's not real pretty either, is it?)

> 
> Hope you can help

Hope some of the above does, at least a little. :-)

Erik