ATmega325P datasheet(15/364 Pages) ATMEL

15 / 364 page

8023F–AVR–07/09

ATmega325P/3250P

6.5.1

The X-register, Y-register, and Z-register

The registers R26..R31 have some added functions to their general purpose usage. These reg-

isters are 16-bit address pointers for indirect addressing of the data space. The three indirect

address registers X, Y, and Z are defined as described in Figure .

The X-, Y-, and Z-registers

In the different addressing modes these address registers have functions as fixed displacement,

automatic increment, and automatic decrement (see the instruction set reference for details).

6.6

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. 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.

X-register

R27 (0x1B)

R26 (0x1A)

Y-register

R29 (0x1D)

R28 (0x1C)

Z-register

070

R31 (0x1F)

R30 (0x1E)

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

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