Six of the 32 registers can be used as three 16-bit indirect address register pointers for data space addressing – enabling
efficient address calculations. One of the these address pointers can also be used as an address pointer for look up tables in
Flash program memory. These added function registers are the 16-bit X-, Y-, and Z-register, described later in this section.
The 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.
Program flow is provided by conditional and unconditional jump and call instructions, able to directly address the whole
address space. Most AVR instructions have a single 16-bit word format. Every program memory address contains a 16- or
32-bit instruction.
Program flash memory space is divided in two sections, the boot program section and the application program section. Both
sections have dedicated Lock bits for write and read/write protection. The SPM (store program memory) instruction that
writes into the application flash memory section must reside in the boot program section.
during interrupts and subroutine calls, the return address program counter (PC) is stored on the stack. The stack is
effectively allocated in the general data SRAM, and consequently the stack size is only limited by the total SRAM size and
the usage of the SRAM. All user programs must initialize the SP in the reset routine (before subroutines or interrupts are
executed). The stack pointer (SP) is read/write accessible in the I/O space. The data SRAM can easily be accessed through
the five different addressing modes supported in the AVR architecture.
The memory spaces in the AVR® architecture are all linear and regular memory maps.
A flexible interrupt module has its control registers in the I/O space with an additional global interrupt enable bit in the status
register. All interrupts have a separate interrupt vector in the interrupt vector table. The interrupts have priority in accordance
with their interrupt vector position. The lower the interrupt vector address, the higher is the priority.
The I/O memory space contains 64 addresses for CPU peripheral functions as control registers, SPI, and other I/O functions.
The I/O memory can be accessed directly, or as the data space locations following those of the register file, 0x20 - 0x5F. In
addition, the Atmel ATmega16/32/64/M1/C1 has extended I/O space from 0x60 - 0xFF in SRAM where only the
ST/STS/STD and LD/LDS/LDD instructions can be used.
3.3
3.4
ALU – Arithmetic Logic Unit
The high-performance AVR ALU operates in direct connection with all the 32 general purpose working registers. Within a
single clock cycle, arithmetic operations between general purpose registers or between a register and an immediate are
executed. The ALU operations are divided into three main categories – arithmetic, logical, and bit-functions. Some
implementations of the architecture also provide a powerful multiplier supporting both signed/unsigned multiplication and
fractional format. See the “Instruction Set” section for a detailed description.
Status Register
The status register contains information about the result of the most recently executed arithmetic instruction. This
information can be used for altering program flow in order to perform conditional operations. Note that the status register is
updated after all ALU operations, as specified in the Instruction Set Reference. This will in many cases remove the need for
using the dedicated compare instructions, resulting in faster and more compact code.
The status register is not automatically stored when entering an interrupt routine and restored when returning from an
interrupt. This must be handled by software.
The AVR Status Register – SREG – is defined as:
Bit
7
6
5
4
3
V
2
N
1
Z
0
C
I
T
H
S
SREG
Read/Write
Initial Value
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
R/W
0
• Bit 7 – I: Global Interrupt Enable
The global interrupt enable bit must be set to enabled the interrupts. The individual interrupt enable control is then performed
in separate control registers. If the global interrupt enable register is cleared, none of the interrupts are enabled independent
of the individual interrupt enable settings. The I-bit is cleared by hardware after an interrupt has occurred, and is set by the
RETI instruction to enable subsequent interrupts. The I-bit can also be set and cleared by the application with the SEI and
CLI instructions, as described in the instruction set reference.
12
ATmega16/32/64/M1/C1 [DATASHEET]
7647O–AVR–01/15
MICROCHIP [ MICROCHIP ]
