[avr-gcc-list] Writing ASM libraries for avr-gcc

[Larry Barello]

Sometime ago there was a file "ctoasm.inc" which detailed the parameter
passing sequence and the registers one could clobber in their assembly
routine.  I cannot find it anywhere (well I found an old pre 2.97 one) and
wanted to verify that R30:31 are clobberable registers in the subroutine.

Currently this is my understanding:

R24:25 - P1
R22:23 - P2 Byte, Word, Long
R20:21 - P3 etc
R18:19 - P4 etc

All the above registers along with  R0 and R1 (with restoring to 0) R26, 27,
30 and 31 are freely clobberable.  R1-R17 and (maybe) R27:28 need to be
preserved.

Thanks!

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