Hardware Stack
The ATtiny15L uses a 3-level-deep Hardware Stack for subroutines and interrupts. The
Hardware Stack is nine bits wide and stores the Program Counter (PC) return address
while subroutines and interrupts are executed.
RCALL instructions and interrupts push the PC return address onto Stack level 0, and
the data in the other Stack levels 1 - 2 are pushed one level deeper in the Stack. When
a RET or RETI instruction is executed the returning PC is fetched from Stack level 0,
and the data in the other Stack levels 1 - 2 are popped one level in the Stack.
If more than three subsequent subroutine calls or interrupts are executed, the first values
written to the Stack are overwritten. Pushing four return addresses A1, A2, A3, and
A4 followed by four subroutine or interrupt returns, will pop A4, A3, A2, and once more
A2 from the Hardware Stack.