7.6 Stack Pointer
The Stack is mainly used for storing temporary data, for storing local variables and for storing return addresses
after interrupts and subroutine calls. The Stack Pointer Register always points to the top of the Stack. Note that the
Stack is implemented as growing from higher memory locations to lower memory locations. This implies that a
Stack PUSH command decreases the Stack Pointer.
The Stack Pointer points to the data SRAM Stack area where the Subroutine and Interrupt Stacks are located. This
Stack space in the data SRAM must be defined by the program before any subroutine calls are executed or inter-
rupts are enabled. The Stack Pointer must be set to point above 0x0200. The initial value of the stack pointer is the
last address of the internal SRAM. The Stack Pointer is decremented by one when data is pushed onto the Stack
with the PUSH instruction, and it is decremented by two for ATmega640/1280/1281 and three for
ATmega2560/2561 when the return address is pushed onto the Stack with subroutine call or interrupt. The Stack
Pointer is incremented by one when data is popped from the Stack with the POP instruction, and it is incremented
by two for ATmega640/1280/1281 and three for ATmega2560/2561 when data is popped from the Stack with
return from subroutine RET or return from interrupt RETI.
The AVR Stack Pointer is implemented as two 8-bit registers in the I/O space. The number of bits actually used is
implementation dependent. Note that the data space in some implementations of the AVR architecture is so small
that only SPL is needed. In this case, the SPH Register will not be present.
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