https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115817
--- Comment #6 from Dmytro Bagrii <dimich.dmb at gmail dot com> --- (In reply to Georg-Johann Lay from comment #5) Thank you for explanation. > LDI cannot be used since it only allows R16...R31. I meant do not treat 0 as a special value and use LDI R24,0x00 The same as for other values. Now I see it requires GCC to treat ISR code generation as a special case on another level, which is not implemented.