8
8331B–AVR–03/12
Atmel AVR XMEGA AU
The arithmetic logic unit (ALU) supports arithmetic and logic operations between registers or
between a constant and a register. Single-register operations can also be executed in the ALU.
After an arithmetic operation, the status register is updated to reflect information about the result
of the operation.
The ALU is directly connected to the fast-access register file. The 32 x 8-bit general purpose
working registers all have single clock cycle access time allowing single-cycle arithmetic logic
unit operation between registers or between a register and an immediate. Six of the 32 registers
can be used as three 16-bit address pointers for program and data space addressing, enabling
efficient address calculations.
The memory spaces are linear. The data memory space and the program memory space are
two different memory spaces.
The data memory space is divided into I/O registers, SRAM, and external RAM. In addition, the
EEPROM can be memory mapped in the data memory.
All I/O status and control registers reside in the lowest 4KB addresses of the data memory. This
is referred to as the I/O memory space. The lowest 64 addresses can be accessed directly, or as
the data space locations from 0x00 to 0x3F. The rest is the extended I/O memory space, ranging
from 0x0040 to 0x0FFF. I/O registers here must be accessed as data space locations using load
(LD/LDS/LDD) and store (ST/STS/STD) instructions.
The SRAM holds data. Code execution from SRAM is not supported. It can easily be accessed
through the five different addressing modes supported in the AVR architecture. The first SRAM
address is 0x2000.
Data addresses 0x1000 to 0x1FFF are reserved for memory mapping of EEPROM.
The program memory is divided in two sections, the application program section and the boot
program section. Both sections have dedicated lock bits for write and read/write protection. The
SPM instruction that is used for self-programming of the application flash memory must reside in
the boot program section. The application section contains an application table section with sep-
arate lock bits for write and read/write protection. The application table section can be used
forsave storing of nonvolatile data in the program memory.
3.4
ALU - Arithmetic Logic Unit
The arithmetic logic unit supports arithmetic and logic operations between registers or between
a constant and a register. Single-register operations can also be executed. The ALU operates in
direct connection with all 32 general purpose registers. In a single clock cycle, arithmetic opera-
tions between general purpose registers or between a register and an immediate are executed
and the result is stored in the register file. After an arithmetic or logic operation, the status regis-
ter is updated to reflect information about the result of the operation.
ALU operations are divided into three main categories – arithmetic, logical, and bit functions.
Both 8- and 16-bit arithmetic is supported, and the instruction set allows for efficient implementa-
tion of 32-bit aritmetic. The hardware multiplier supports signed and unsigned multiplication and
fractional format.
