ATtiny13A datasheet(11/174 Pages) ATMEL

11 / 174 page

8126D–AVR–11/09

ATtiny13A

Figure 4-3.

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 the instruction set reference for details).

4.5

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. The Stack Pointer Register always points

to the top of the Stack. Note that the Stack is implemented as growing from higher memory loca-

tions 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 Interrupt

Stacks are located. This Stack space in the data SRAM is automaticall defined to the last

address in SRAM during power on reset. 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 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.

4.5.1

SPL – Stack Pointer Low

X-register

R27 (0x1B)

R26 (0x1A)

Y-register

R29 (0x1D)

R28 (0x1C)

Z-register

070

R31 (0x1F)

R30 (0x1E)

Bit

76543210

0x3D

SP7

SP6

SP5

SP4

SP3

SP2

SP1

SP0

SPL

Read/Write

R/W

Initial Value

10011111

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