introduction to assembly language

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USING THE AVR MICROPROCESSOR

M. Neil - Microprocessor Course

Introduction to Assembly language

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Outline


Introduction to Assembly CodeThe AVR MicroprocessorBinary/Hex NumbersBreaking down an example microprocessor

programAVR instructions overviewCompiling and downloading assembly codeRunning and debugging assembly code

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A really simple program

Some variables (i,j)Set variables to

initial valuesA loop

Check a condition to see if we are finished

Do some arithmetic Output some

information Increment the loop

counter

int main(void){

char i;char j;i=0;j=0;while (i<10) {

j = j + i;

PORTB = j;

PORTB = i;

i++;}return 0;

}M. Neil - Microprocessor Course

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Running a program on a microprocessor

When you want to turn your program into something which runs on a microprocessor you typically compile the program

This creates a program in the “machine language” of the microprocessor A limited set of low level instructions which can be

executed directly by the microprocessorWe can also program in this language using an

assemblerWe can have a look at the assembly language the

c compiler generates for our simple program to understand how this works


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Translating our program into assembly language

000000be <main>: be: 80 e0 ldi r24, 0x00 ; 0 c0: 90 e0 ldi r25, 0x00 ; 0 c2: 98 0f add r25, r24 c4: 92 bb out 0x18, r25 ; 18 c6: 82 bb out 0x18, r24 ; 18 c8: 8f 5f subi r24, 0xFF ; 255 ca: 8a 30 cpi r24, 0x0A ; 10 cc: d1 f7 brne .-12 ; 0xc2 <main+0x4> ce: 80 e0 ldi r24, 0x00 ; 0 d0: 90 e0 ldi r25, 0x00 ; 0 d2: 08 95 ret

int main(void){


j = j + i;PORTB

= j; PORTB = i;

i++;}return 0;

}

Location in

program memory

(Address)

Opcodes – the

program code

The Assembly language

equivalent of the opcodes


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AVR Microprocessor architecture

Program MemoryYour code goes here

RegistersStorage for numbers

the processer will perform arithmetic

on

Arithmetic Logic Unit (ALU)

The heart of the processor which handles logic/math

Input/OutputInterface to the outside world


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Numbers on a microprocessor

We’ve seen that our program is converted into a series of numbers for execution on the microprocessor

Numbers are stored in a microprocessor in memory They can be moved in an out of registers and memory Calculations can be done Numbers can be sent to Output devices and read from Input

devicesThe numbers are stored internally in binary

representations. We will study precisely how this is done shortly, and even

build some simple memory devices.


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Binary/Hexadecimal Numbers

A “Bit” can be a 0 or 1 The value is set using transistors

in the hardware Bits are organized into groups

to represent numbers 4 Bits is a “Nybble”

Can store numbers 0-15 8 Bits is a “Byte”

Can store numbers 0-255 16 Bits is a word

Can store numbers 0-65535 Hexadecimal is a very handy

representation for binary numbers Each 4 bits maps onto a HEX

number Can quickly convert from HEX to

binary

Binary Hex Decimal

0000 0 0

0001 1 1

0010 2 2

0011 3 3

0100 4 4

0101 5 5

0110 6 6

0111 7 7

1000 8 8

1001 9 9

1010 A 10

1011 B 11

1100 C 12

1101 D 13

1110 E 14

1111 F 15M. Neil - Microprocessor Course

Binary Representation

• This representation is based on powers of 2. Any number can be expressed as a string of 0s and 1s


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Example: 9 = 10012 = 1* 23 + 0* 22 + 0*21 + 1*20

Example: 5 = 1012 = 1* 22 + 0*21 + 1*20

Exercise: Convert the numbers 19, 38, 58 from decimal to binary. (use an envelope, a calculator or C program)

Hexadecimal Representation

• This representation is based on powers of 16. Any number can be expressed in terms of:

0,1,2,…,9,A,B,C,D,E,F (0,1,2,…,9,10,11,12,13,14,15)


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Example: 1002 = 3EA16 = 3* 162 + 14*161 + 10*160

Example: 256 = 10016 = 1* 162 + 0*161 + 0*160

Exercise: Convert the numbers 1492, 3481, 558 from decimal to hex. (use calculator or C program)

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HEX/Binary Conversion

Converting HEX/Binary to Decimal is a bit painful, but converting HEX/Binary is trivial

We often use the notations 0x or $ to represent a HEX number Example 0x15AB for the HEX number 15AB In assember language we see the notation $A9 for the

HEX number A9

Exercise: Convert the numbers 0xDEAD 0xBEEF from Hex to binary.

Now convert them to Decimal


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8 Bit Microprocessors

The AVR microprocessor we will be using is an “8 bit” processor

The operations the processor performs work on 8 bit numbers Data is copied from memory into the processors

internal “registers” 8 bits at a time Operations (addition/subtraction/etc..) can be

performed on the 8 bit registers We can of course do calculation with bigger numbers,

but we will have to do this as a sequence of operations on 8 bit numbers We will see how to do this later – using a “carry” bit


Operations


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Exercise: (1) Find NOT(AAA) (2) Find OR(AAA; 555) (3) Find AND (AEB123; FFF000)

Why is shift important ?Try SHIFT R(011) SHIFT L(011)

Operation Register A Register B

Result

Add A,B 00001111 00000001 00010000

Not A 01010101 10101010

OR A,B 00010101 00101010 00111111

ShiftL A 00001111 00011110

ShiftR A 00001111 00000111

AND A,B 01010101 10101010 00000000

The processor can perform several operations Including “Boolean” algebra

What About Subtraction: Negative Numbers

With 4 bits, you can represent 0 to +15 -8 to + 7

There are three ways to represent these negative numbers


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Integer Sign Magnitude

1’s complement

2’s complement

+7 0111 0111 0111

+6 0110 0110 0110

+5 0101 0101 0101

+4 0100 0100 0100

+3 0011 0011 0011

+2 0010 0010 0010

+1 0001 0001 0001

0 0000 0000 0000

-1 1001 1110 1111

-2 1010 1101 1110

-3 1011 1100 1101

-4 1100 1011 1100

-5 1101 1010 1011

-6 1110 1001 1010

-7 1111 1000 1001

-8 1000 (-0) 11111 1000

Sign/Magnitude: Set the top bit to 1 1’s complement : Take the complement

of the number 2’s complement : Take the complement

of the number and then add 1

What About Subtraction: Negative Numbers


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Exercise: Fill out the same table using sign magnitude numbers. Do you understand now why 2’s complement is a useful representation!

There is a good reason to use a 2’s complement representation Binary addition of a two’s complement numbers “just

works” whether the numbers are positive or negative

3 + 4 3 + -4 -3 + 4 -3 + -4

0011 0011 1101 1101

+ 0100 + 1100 + 0100 + 1100

= 0111 = 1111 = 0001 = 1001

(7) (-1) (+1) (-7)

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8 Bit representations

8 bits can be used for The numbers 0-255 The numbers -128 to + 127 Part of a longer number A “Character”

Hence “char” in c This is a bit out of date Unicode uses 16 bits


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Back to our program


Address Value

00BE 80

00BF E0

00C0 90

00C1 E0

00C2 98

00C3 0F

00C4 92

• The program is stored in program memory

• There are 64Kilobytes of program memory (2^16 bytes)

• Each location stores an 8 bit number

• The location is specified with a 16 bit Address• (0x0000-0xFFFF)


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Registers on the AVR

There are 32 General purpose registers on the AVR microprocessor (R0-R31)

Each of these can hold an 8 bit number

R26:R27 is also a 16 bit register called X

R28:R29 is Y

R30:R31 is Z

You can perform calculations on these registers very quickly

AVR Registers – where the numerical work is done in a program


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Back to our program: Loading a register


Address Value

00BE 80

00BF E0

00C0 90

00C1 E0

00C2 98

00C3 0F

00C4 92

• The first instruction is loading a value of 0x00 into register r24

• This instruction is encoded in the 16 bit opcode stored at address 00BE

• The Assember code is • ldi r24,0x00• LoaD Immediate r24 with

0x00


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Decoding an Opcode

be: 80 e0 ldir24, 0x00

Words are stored “little endian”Read into the processor as E080E080=1110 0000 1000 0000dddd: 1000=8 -> 16+8 = r24KKKKKKKK=0x00Exercise: what is the opcode for

ldi r19, 0x3F


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Back to our program: Loading a register


The second instruction is loading a value of 0x00 into register r25This adds r24 to r25 and stores the result into register r24

The next instructions output r24 and r25 (we’ll learn where later)This is subtracting a value of 0xFF from register r24This is the compiler cleverly adding 1 This instruction is comparing the value 0x0A to register r24

If they are not equal, the next instruction will branch back to location 0xC2 that is loop back if they are equal it continues on (and returns from main)M. Neil - Microprocessor Course

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Compare assembly language to c


int main(void){


j = j + i;PORTB

= j;PORTB

= i; i++;

}return 0;

}

• Here the variables are stored in registers r24, r25

• Pretty easy to see how this program is translated

• More complex programs quickly become very complicated to understand in assembly languageM. Neil - Microprocessor Course

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Programming in this course

We will be programming exclusively in assembler Allows us to understand precisely what the

microprocessor is going to do and how long it will take to do so important for time critical applications

Full access to all functions of the microprocessor With care can make very efficient use of resources

Sometimes very important for small microprocessors

Programming in assembler requires some discipline Code can be very difficult to understand

The code is very low level Line by line comments very important


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The AVR instruction set

We’ve seen a few sample instructions which cover most of the basic type of operations

Arithmetic and Logic instructions (ADD, SUB, AND, OR, EOR, COM, INC, DEC, …)

Branch Instructions Jump to a different location depending on a test

Data transfer instructions Move data to/from Registers and memory

Bit setting and testing operations Manipulate and test bits in registers


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Arithmetic and Logic Instructions

Additionadd r20,r21R20 r20+r21

Incrementinc r20R20 r20+1

Subtractionsubi r20,$22R20 r20-$22sub r20,r21R20 r20-r21

Logicand r20,r24R20 AND(r20,r24)

Many instructions work either with two registers, or with “immediate” data values (stored in the opcode)


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Arithmetic and Logic Instructions – The full set


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The Status Register

Every time the processor performs an operation it sets bits in the Status Register (SREG) Example cpi r01,$77 This register is in the I/O region

SREG can be examined/set with the in and out instructions in r17,SREG out SREG,r22

The status bits are also tested by branch instructions to decide whether or not to jump to a different location


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Branch Instructions

Branchbreq label1Branch if equal to location label1brlo label2Branch if lower to location label2If the Branch test fails – the next line of code is executedIf the Branch test is successful, the program jumps to the location specified

Call, Returnrcall mysubretCall subroutine mysub. The program saves information about where it currently is executing and then jumps to the code at mysub. When the subroutine is finished it calls ret, which then returns to where the rcall was made.

Jumpjmp label3Jump to label3 – no information about where we jumped from is saved. This is a one way trip


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All Branch Instructions


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Data Transfer Instructions

Loadldi r20,$73R20 $73ld r20,XR20 (X)

Copy Registermov r20,r21R20 r21

Inputin r20,PINDR20 PIND

Outputout PORTD,r24PORTD r24

There are a few quirks about loading and storing to memory. We will cover this in detail soon.


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Data transfer reference


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The Atmega128

The microprocessor you will be using has several input and output ports

Some of these are already connected to switches or LEDs on the boards we are using

Others will be available to you to connect to various devices.

It is time to make some lights blink!


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Setting up the Input/Output ports:

; ******* Port B Setup Code **** ldi r16, $FF ; all bits outout DDRB , r16 ; Port B Direction Registerldi r16, $FF ; Init value out PORTB, r16; Port B value

; ******* Port D Setup Code **** ldi r16, $00 ; all bits in out DDRD, r16 ; Port D Direction Registerldi r16, $FF ; Init value out PORTD, r16 ; Port D value

• For the ports we are using we set the Data Direction Register (DDR) which has a bit for each bit of I/O (1 for input, 0 for output)

• We can then set the initial value of the data bits at the I/O port

• PortB is connected to LEDs on our board

• PortD is connected to the blue switches

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The ATmega128 Microprocessor

In this course you will be using the ATmega128 processor mounted on an ATMEL programming board (STK300)

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The ATMEL BOARD

Connecting the board with your PC

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Where the I/O ports are connected

PORTD Switches

PORTB LEDs


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Setting up a code directory


Create a directory where you will store your code

Download the file Simple.ASM into your code directory from the course web page (Lecture 2b):

DO NOT DOWNLOAD m128def.inc

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Getting Started with STUDIO 4:

Go to Start Programs ATMEL AVR Tools AVR Studio 4

Select New

Project

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You should now see the window:

Pick Atmel AVR Assembler

At theend

Click this and navigate to your code directory

Pick a name for your project

Do not create new file

Create new

folder

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You should now see the window:

Select AVR Simulator

Select ATmega 128

At theend

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Entering files in STUDIO 4 (I):

(1) Right click here and select to ‘Add Files to

Project’

(2) Navigate and find

SIMPLE.ASM

Chose Open to load the file

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Entering files in STUDIO 4 (II):Here is list of files attached

to project

This window is for editing one

of the files

Change the ‘editing window’ by double clicking on a file

icon

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Select the output file format :

Click on Project/ Assembler

Options

Change the format of the output file to

Intel Hex format

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Running your Program in Studio 4 :

Click on Build/ Build and

run

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Running your Program in Studio 4:

Green means ok;

Otherwise click on red and

debug

Program is halted at this instruction

The Build

process generates some

new files

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Open monitoring Windows:

View/ Toolbars/ I/O

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Open monitoring Windows:

InputOutput

Expand views as required

Adjust screen display using Window/…

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Watch your Program Running:

Start here:Everything

zeroed

Click this once

Step through the whole

program and make sure you

understand what is

happening

and again

Use this to run continuously with active

display

Use this to stopUse this to run continuously

without display

Use this to reset

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Exercising the ATmega128 commands:


Download the program simple.asm, assemble it and run it in the simulator

Step through the program and make sure you understand what is happening after each instruction

Try changing the number of times the loop is executed and make sure you understand the outputs on PORTB

Now we will download the program to the development board

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Downloading with AVRISP

Select from Start : AVRISPMake sure Device Atmega128 is selected

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Erase the device


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Select the file to load

Select Load/Flash…

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Chose your .hex file to download


Tell AVRISP which .hex file to download to the Atmega128

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Now download the program into the Flash

Select Program/Flash -- This writes the machine code to be downloaded into the ATmega128 chip

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Running your program

Select Run – If all goes well your program should now be executing

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Programs to write I:


Download the program simple.asm, assemble it download it to the board and run it. What do you see on the LEDs? Do you know why (note that a dark LED 1)?

change the code so that output on the LEDs has a lit LED for a 1 (think about using the neg instruction)

Modify the program so that it changes the number of times the loop is run depending on which switch button is pushed Use the instruction in Rxx,PIND to get the state of the

buttons Make sure that the LED output makes sense when you push

the different buttons

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Programs to write :


Make a counter from 0 – FF, output the values to PORTB and look with your scope probe at the LSB (PB0). How long does it take to make an addition? Why does the B0 bit toggle with a frequency that is twice that of B1? (check this using two scope probes; one on B0 and another on B1)

In the documentation you will find how many clock counts are required to perform an instruction in your program. The ATmega128 has an 8 MHz clock. Predict the time it takes to do an addition and compare with your measurement using the scope.

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