Rainbow Electronics ATmega128L - Manual (Page 11)

11
ATmega128(L)
2467B–09/01
Figure 4. 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 single cycle instructions.
As shown in Figure 4, 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 phys-
ically 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.
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 described in
Figure 5.
Figure 5. 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).
7 0 Addr.
R0 $00
R1 $01
R2 $02
…
R13 $0D
General R14 $0E
Purpose R15 $0F
Working R16 $10
Registers R17 $11
…
R26 $1A X-register low byte
R27 $1B X-register high byte
R28 $1C Y-register low byte
R29 $1D Y-register high byte
R30 $1E Z-register low byte
R31 $1F Z-register high byte
15 XH XL 0
X - register 7 0 7 0
R27 ($1B) R26 ($1A)
15 YH YL 0
Y - register 7 0 7 0
R29 ($1D) R28 ($1C)
15 ZH ZL 0
Z - register 7 0 7 0
R31 ($1F) R30 ($1E)