ATMEGA649 datasheet(13/375 Pages) ATMEL

13 / 375 page

ATmega329/3290/649/6490

2552H–AVR–11/06

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 6.

Figure 6. The X-, Y-, and Z-registers

In the different addressing modes these address registers have functions as fixed dis-

placement, automatic increment, and automatic decrement (see the instruction set

reference for details).

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. The Stack Pointer Regis-

ter always points to the top of the Stack. Note that the Stack is implemented as growing

from higher memory locations to lower memory locations. This implies that a Stack

PUSH command decreases the Stack Pointer.

The Stack Pointer points to the data SRAM Stack area where the Subroutine and Inter-

rupt Stacks are located. This Stack space 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 0x60. The Stack Pointer is decremented by one

when data is pushed onto the Stack with the PUSH instruction, and it is decremented by

two when the return address is pushed onto the Stack with subroutine call or interrupt.

The Stack Pointer is incremented by one when data is popped from the Stack with the

POP instruction, and it is incremented by two when data is popped from the Stack with

return from subroutine RET or return from interrupt RETI.

The AVR Stack Pointer is implemented as two 8-bit registers in the I/O space. The num-

ber 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)

Bit

151413

1211

0x3E (0x5E)

SP15

SP14

SP13

SP12

SP11

SP10

SP9

SP8

SPH

0x3D (0x5D)

SP7

SP6

SP5

SP4

SP3

SP2

SP1

SP0

SPL

765

432

Read/Write

R/W

Initial Value

000

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