10
ATmega8A [DATASHEET]
8159E–AVR–02/2013
Figure 7-2.
AVR CPU General Purpose Working Registers
Most of the instructions operating on the Register File have direct access to all registers, and most of them are sin-
gle cycle instructions.
As shown in
Figure 7-2, each register is also assigned a Data memory address, mapping them directly into the first
32 locations of the user Data Space. Although not being physically implemented as SRAM locations, this memory
organization provides great flexibility in access of the registers, as the X-, Y-, and Z-pointer Registers can be set to
index any register in the file.
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
Figure 7-3.
The X-, Y- and Z-Registers
In the different addressing modes these address registers have functions as fixed displacement, automatic incre-
ment, and automatic decrement (see the Instruction Set Reference for details).
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
70
Addr.
R0
0x00
R1
0x01
R2
0x02
…
R13
0x0D
General
R14
0x0E
Purpose
R15
0x0F
Working
R16
0x10
Registers
R17
0x11
…
R26
0x1A
X-register Low Byte
R27
0x1B
X-register High Byte
R28
0x1C
Y-register Low Byte
R29
0x1D
Y-register High Byte
R30
0x1E
Z-register Low Byte
R31
0x1F
Z-register High Byte
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)