Datasheet
11
8197C–AVR–05/11
ATtiny261A/461A/861A
In different addressing modes these address registers function as automatic increment and
automatic decrement (see the instruction set reference for details).
4.5 Stack Pointer
The Stack is mainly used for storing temporary data, local variables and return addresses for
interrupts and subroutine calls. The Stack Pointer Register always points to the top of the Stack,
in the data SRAM Stack area where the subroutine and interrupt stacks are located.
The Stack in the data SRAM must be defined by the program before any subroutine calls are
executed or interrupts are enabled. The Stack Pointer must be set to point above start of the
SRAM (see Figure 5-2 on page 16). The initial Stack Pointer value equals the last address of the
internal SRAM.
Note that the Stack is implemented as growing from higher to lower memory locations. This
means a Stack PUSH command decreases the Stack Pointer. See Table 4-1.
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.
4.5.1 SPH and SPL – Stack Pointer Register
15 YH YL 0
Y-register 707 0
R29 (0x1D) R28 (0x1C)
15 ZH ZL 0
Z-register 7070
R31 (0x1F) R30 (0x1E)
Table 4-1. Stack Pointer instructions
Instruction Stack pointer Description
PUSH Decremented by 1 Data is pushed onto the stack
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
RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND RAMEND