ATMEGA161 View Datasheet(PDF) - Atmel Corporation

Part Name

Description

Manufacturer

ATMEGA161

8-bit Microcontroller with 16K Bytes In-System Programmable Flash

Atmel Corporation 

ATmega161(L)

General Purpose Register File

Figure 6 shows the structure of the 32 general purpose working registers in the CPU.

Figure 6. AVR CPU General Purpose Working Registers

General

Purpose

Working

Registers

7

0 Addr.

R0

$00

R1

$01

R2

$02

…

R13

$0D

R14

$0E

R15

$0F

R16

$10

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

All the register operating instructions in the instruction set have direct and single cycle access to all registers. The only

exceptions are the five constant arithmetic and logic instructions SBCI, SUBI, CPI, ANDI, and ORI between a constant and

a register, and the LDI instruction for load immediate constant data. These instructions apply to the second half of the reg-

isters in the register file – R16..R31. The general SBC, SUB, CP, AND, and OR, and all other operations between two

registers or on a single register apply to the entire register file.

As shown in Figure 6, each register is also assigned a data memory address, mapping them directly into the first 32 loca-

tions 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-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 address pointers

for indirect addressing of the Data Space. The three indirect address registers X, Y and Z are defined as:

Figure 7. X, Y and Z-registers

X-register

15

7

R27 ($1B)

07

0

0

R26 ($1A)

Y-register

15

7

R29 ($1D)

07

0

0

R28 ($1C)

Z-register

15

7

R31 ($1F)

07

0

0

R30 ($1E)

In the different addressing modes, these address registers have functions as fixed displacement, automatic increment and

decrement (see the descriptions for the different instructions).

9

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