The AVR Context
[RTOS Implementation Building Blocks]

A context switch requires the entire execution context to be saved. On the AVR microcontroller the context consists of:

32 general purpose processor registers. The gcc development tools assume register R1 is set to zero.
Status register. The value of the status register affects instruction execution, and must be be preserved across context switches.
Program counter. Upon resumption, a task must continue execution from the instruction that was about to be executed immediately prior to its suspension.
The two stack pointer registers.