Datasheet

ATmega48PA/88PA/168PA [DATASHEET]

9223F–AVR–04/14

7.4.1 The X-Register, Y-Register, and Z-Register

The registers R26...R31 have some added functions to their general purpose usage. These registers 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 7-3.

Figure 7-3. 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 Section 32. “Instruction Set Summary” on page 317).

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 17.

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.

15 XH XL 0

X-register 7 0 7 0

R27 (0x1B) R26 (0x1A)

15 YH YL 0

Y-register 7 0 7 0

R29 (0x1D) R28 (0x1C)

15 ZH ZL 0

Z-register 7 0 7 0

R31 (0x1F) R30 (0x1E)

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