ATtiny261A datasheet(11/292 Pages) ATMEL

11 / 292 page

8197A–AVR–10/09

ATtiny261A/461A/861A

Figure 4-3.

The X-, Y-, and Z-registers

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.

X-register

R27 (0x1B)

R26 (0x1A)

Y-register

R29 (0x1D)

R28 (0x1C)

Z-register

R31 (0x1F)

R30 (0x1E)

Table 4-1.

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

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