This chapter is only an overview of the AVR architecture, with special emphasis on the ATmega328 and ATmega2560 devices. The final word on all things AVR lies within the product datasheets, which are freely available from the Atmel web site:
Be sure to also look for any applicable errata sheets on your particular device. Any machine as complex as the AVR microcontroller is going to have a few adjustments along the way. Each revision of the silicon has incremental improvements, bug fixes, and documented workarounds that are collected and published by the manufacturer.
You’re encouraged ...