AVR Libc Home Page
AVR Libc Development Pages
Main Page
User Manual
Reference
FAQ
Example Projects
Todo List
Module
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.