User Manual

ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]

8271E–AVR–07/2012

7.5 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 Table 8-3 on page 18.

See Table 7-1 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 implementations of the AVR architecture is so small

that only SPL is needed. In this case, the SPH Register will not be present.

7.5.1 SPH and SPL – Stack Pointer High and Stack Pointer Low Register

Table 7-1. 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

Bit 151413121110 9 8

0x3E (0x5E) SP15 SP14 SP13 SP12 SP11 SP10 SP9 SP8 SPH

0x3D (0x5D) SP7 SP6 SP5 SP4 SP3 SP2 SP1 SP0 SPL

76543210

Read/Write R/W R/W R/W R/W R/W R/W R/W R/W

R/W R/W R/W R/W R/W R/W R/W R/W

Initial Value

RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND