What registers are used by the C compiler?
• Data types: char is 8 bits, int is 16 bits, long is 32 bits, long long is 64 bits, float and double are 32 bits (this is the only supported floating point format), pointers are 16 bits (function pointers are word addresses, to allow addressing up to 128K program memory space). There is a -mint8 option (see Options for the C compiler avr-gcc) to make int 8 bits, but that is not supported by avr-libc and violates C standards (int must be at least 16 bits). It may be removed in a future release. • Call-used registers (r18-r27, r30-r31): May be allocated by gcc for local data. You may use them freely in assembler subroutines. Calling C subroutines can clobber any of them – the caller is responsible for saving and restoring. • Call-saved registers (r2-r17, r28-r29): May be allocated by gcc for local data. Calling C subroutines leaves them unchanged. Assembler subroutines are responsible for saving and restoring these registers, if changed. r29:r28 (Y pointer) is used as a frame pointer (p
