Datasheet

11
7701F–AVR–10/12
Atmel ATtiny24/44/84 [Preliminary]
Figure 5-3. The X-, Y-, and Z-registers
In the different addressing modes, these address registers have functions as fixed displace-
ment, automatic increment, and automatic decrement (see the instruction set summary for
details).
5.6 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. The stack pointer register 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 instruction 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 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 are pushed onto the
stack with the PUSH instruction, and it is decremented by two when the return address is
pushed onto the stack with a subroutine call or interrupt. The stack pointer is incremented by
one when data are popped from the stack with the POP instruction, and it is incremented by
two when data are popped from the stack with a return from subroutine RET instruction or
return from interrupt RETI instruction.
The Atmel 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 the SPL register is needed. In
this case, the SPH register will not be present.
5.6.1 SPH and SPL – Stack Pointer High and Low
15 XH XL 0
X-register 707 0
R27 (0x1B) R26 (0x1A)
15 YH YL 0
Y-register 707 0
R29 (0x1D) R28 (0x1C)
15 ZH ZL 0
Z-register 70 7 0
R31 (0x1F) R30 (0x1E)
Bit 151413121110 9 8
0x3E (0x5E) SP15 SP14 SP13 SP12 SP11 SP10 SP9 SP8 SPH
0x3D (0x5D) SP7 SP6 SP5 SP4 SP3 SP2 SP1 SP0 SPL
76543210
Read/Write R/W R/W R/W R/W R/W R/W R/W R/W
R/W R/W R/W R/W R/W R/W R/W R/W
Initial Value00000000
00000000