Provided by: avr-libc_1.8.0-4.1_all 
NAME
todo - Todo List
Group avr_boot
From email with Marek: On smaller devices (all except ATmega64/128), __SPM_REG is in the I/O space,
accessible with the shorter 'in' and 'out' instructions - since the boot loader has a limited size,
this could be an important optimization.