Datasheet

13
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
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 “Instruction Set Summary” on page 418 for detailed information).
4.6 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 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 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 0x0100. The initial value of the stack
pointer is the last address of the internal SRAM. The Stack Pointer is decremented by one when data is pushed
onto the Stack with the PUSH instruction, and it is decremented by three 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 three 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 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)
Bit 1514131211109 8
SP15 SP14 SP13 SP12 SP11 SP10 SP9 SP8 SPH
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 Value 0 0 1 0 0 0 0 0
11111111