Datasheet

13
8018P–AVR–08/10
ATmega169P
6.4 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. Note that the Stack is implemented as
growing from higher to lower memory locations. The Stack Pointer Register always points to the
top of the Stack. The Stack Pointer points to the data SRAM Stack area where the Subroutine
and Interrupt Stacks are located. A Stack PUSH command will decrease the Stack Pointer.
The Stack in the data SRAM must be defined by the program before any subroutine calls are
executed or interrupts are enabled. Initial Stack Pointer value equals the last address of the
internal SRAM and the Stack Pointer must be set to point above start of the SRAM, see Figure
7-2 on page 21.
See Table 6-1 for Stack Pointer details.
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 implementa-
tions of the AVR architecture is so small that only SPL is needed. In this case, the SPH Register
will not be present.
Table 6-1. Stack Pointer instructions
Instruction Stack pointer Description
PUSH Decremented by 1 Data is pushed onto the stack
CALL
ICALL
RCALL
Decremented by 2
Return address is pushed onto the stack with a subroutine call or
interrupt
POP Incremented by 1 Data is popped from the stack
RET
RETI
Incremented by 2 Return address is popped from the stack with return from
subroutine or return from interrupt