Lecture 05 – AVR Addressing Modes

Program and Data addressing modes in AVR

Table of Contents

• Lecture 05 – AVR Addressing Modes
  • Table of Contents
  • Introduction
  • Type 1: Register Direct (Single Reg.)
  • Type 2: Register Direct (Two Reg.)
  • Type 3: Immediate Mode
  • Type 4: I/O Direct
  • Type 5: Data Direct
  • Type 6: Data Indirect
  • I/O Ports using Indirect
  • Extended I/O
  • Type 7: Direct program addressing
  • Type 8: Implicit Addressing
  • Type 9: Indirect Program Addressing
  • Type 10: Relative program addressing

Introduction

• The Instruction set of a processor is like the vocabulary to command the processor.

  • Each instruction controls some part of the processor and allows the programmer to manipulate data in the memory as well as input and output devices.

• Program and Data Addressing Modes:

  • The instructions can be categorized based on how they access data and operate upon it. This is called the program and data addressing modes of the processor.

  • The various AVR instructions can be categorized in about 10 different addressing modes.

• Each instruction has an 2 parts:

  • Opcode: indicates to ALU what to do.

  • Operands: on which the opcode operates.

Type 1: Register Direct (Single Reg.)

• Register Direct instructions can operate on any of the 32 registers.
• Instruction Reads the contents of a register, operates on the contents, and then stores the result back into the same register.

Example	Decription
INC R0	++i;
DEC R5	--j;
LSL R9	Shift R9, X<<1;

Type 2: Register Direct (Two Reg.)

• Two registers are involved.

  • Rs: Source register

  • Rd: Destination register

  • Instruction reads the two registers and operates on their contents and stores the result back into the destination register.

Example	Description
ADD R1, R3	i = i+j; //i+=j;
SUB R5, R7	i = i-j;

Type 3: Immediate Mode

• A constant value is provided in the instruction
• the constant is therefore stored within the program code in the program memory space (Flash memory on AVR)

SUBI R4, 8    ;Subtract Constant from Register   
ADIW R26, 5   ;Add Immediate to Word  
              ; R27:R26 ← R27:R26 + 5  --  

like

i-=8;  
a+=29;  

• The second instruction, with the notable (W), is an example of an AVR double-register operation. These use uses special register pairs. In this case R26 indicates the pair R27:R26. (R27 is the MSW)

Type 4: I/O Direct

• These instructions are used to access the I/O space, I/O Registers, but not Extended I/O Registers.

• The I/O registers can be accessed using

  • in Rd, PORTADDRESS
  • out PORTADDRESS, Rs
  • Rd, Rs: any register from register file.
  • PORTADDRESS : any register from
    the entire range of 0x00 to 0x3F (not 0x20-0x5F) (a total of 64 I/O registers).

C

unsigned char i = PINB;  
unsigned char k = 45;  
PORTC = k;

accomplished using

IN R10, PINB if the input to PORTB is 0x03 and all the pins are configured as an inputs, R10 becomes 0x03

OUT PORTC, R1 the contents of R1 are used to set the output register of PORTC

Table with Register Addresses, and Corresponding Memory Address:

Type 5: Data Direct

• These are two-word instructions
• One of the words is the address of the data memory space.
  • What is the maximum data memory that can be accessed in this way?

STS K, Rs

• STS: Store Direct to SRAM

LDS RD, K

• LDS: Load Direct from SRAM
  K is a 16-bit address

Note writing a constant to memory requires writing it to a register first

Type 6: Data Indirect

• Similar to the data direct type.

• But they are one word each, and a pointer register (X, Y, or Z) is used that has the base address of the data memory.

• One variant is depicted in this this figure:

Look at Page 378 of data sheet

• Examples:

  Indirect
  LD Rd, X

  • X == R27:R26

  Indirect with post-increment
  LD Rd, X+

  • Rd ← (X), X ← X + 1

  Indirect with displacement (indirect with index)
  LDD Rd, Y+q

  • Rd ← (Y + q)

  Indirect with pre-decrement
  ST -Y, Rs

  • Y ← Y -- 1, (Y) ← Rs

I/O Ports using Indirect

The I/O ports can also be accessed using SRAM access commands, e.g. ST and LD.
Add 0x20 to the port number (the first 32 addresses are the registers!)

Example:

.DEF SomeRegister = R16 

  LDI ZH,HIGH(PORTB+32) 
  LDI ZL,LOW(PORTB+32) 
  LD SomeRegister,Z 

Extended I/O

For I/O registers located in extended I/O map,
I/O register direct commands like "IN", "OUT", "SBIS", "SBIC", "CBI", and "SBI" cannot be used.
Use direct (memory) and indirect (memory) instructions that allow access to extended I/O, typically "LDS" and "STS" combined with "SBRS", "SBRC", "SBR", and "CBR" if modifying a single pin

Type 7: Direct program addressing

• Direct program addressing

CALL k ;

• Direct Subroutine Call
• PC ← k and STACK = PC+1

Type 8: Implicit Addressing

CLC

• Clear Carry C ← 0
• Data Addressing

RET

• Subroutine Return
• PC ← STACK // Program Addressing

Type 9: Indirect Program Addressing

In these types of instructions, the Z register is used to point to

the program memory. (Up to 64 Kbytes of program memory)

IJMP ;Indirect Jump to (Z) PC ← Z

ICALL ;Indirect Call to (Z) PC ← Z,STACK = PC + 1

Type 10: Relative program addressing

• Type RJMP and RCALL,
  • offset of +/- 2K to the PC

RCALL k ;Relative Subroutine Call PC ← PC + k + 1 , STACK=PC+1
RJMP k ;Relative Jump PC ← PC + k + 1