Lecture notes operating systems for embedded systems

OSES notes

2019-2020 Prof. Massimo Violante

Francesco Malacarne 1

Sommario

Introduction .................................................................................................................................................. 5

Cross compilation flow .............................................................................................................................. 6

Variables ................................................................................................................................................... 8

Modifiers ............................................................................................................................................... 9

Hardware basics .......................................................................................................................................... 13

Computer architecture and basic knowledge ......................................................................................... 13

Embedded system architecture .............................................................................................................. 15

CPU peripheral interface ......................................................................................................................... 18

SPI bus ................................................................................................................................................. 19

I2C communication ............................................................................................................................. 20

Polling and interrupt ........................................................................................................................... 22

DMA .................................................................................................................................................... 25

OS overview ................................................................................................................................................ 27

Operating system architecture ............................................................................................................... 28

Flat architecture .................................................................................................................................. 28

Layered architecture with monolithic kernel...................................................................................... 30

Microkernel ......................................................................................................................................... 33

Process management A .............................................................................................................................. 36

Process in memory .................................................................................................................................. 36

Process state diagram ............................................................................................................................. 40

Process control block PCB ....................................................................................................................... 42

CPU scheduler ......................................................................................................................................... 43

Basic task ............................................................................................................................................. 46

Extended task ...................................................................................................................................... 47

Task in Micrium ................................................................................................................................... 47

Critical section definition ........................................................................................................................ 49

Busy waiting ........................................................................................................................................ 52

Semaphore .......................................................................................................................................... 52

Scheduling algorithm .............................................................................................................................. 58

Round Robin scheduling algorithm ..................................................................................................... 58

Priority-based scheduler ..................................................................................................................... 59 2

Real-time and non real-time systems ..................................................................................................... 61

Device management A ................................................................................................................................ 63

Introduction ............................................................................................................................................ 63

Programming .......................................................................................................................................... 67

Pin multiplexing ...................................................................................................................................... 68

Digital I/O ................................................................................................................................................ 69

Digital level-triggered output .............................................................................................................. 69

Digital level-triggered input, polling and interrupt code .................................................................... 76

Process management B ............................................................................................................................... 80

Task definition ......................................................................................................................................... 80

Scheduling algorithm assumptions and definitions ................................................................................ 82

Timeline scheduling ................................................................................................................................ 86

Exam question + solution .................................................................................................................... 88

Rate monotonic scheduling .................................................................................................................... 91

Previous exercise solved with the RMS .............................................................................................. 95

Exercise about RMS (exam) ................................................................................................................ 97

Earliest Deadline First scheduling ........................................................................................................... 98

Scheduling algorithm for aperiodic tasks .............................................................................................. 101

Jackson’s algorithm ........................................................................................................................... 102

Horn’s algorithm ............................................................................................................................... 103

Exercises taken from past exams ...................................................................................................... 104

Scheduling algorithms for periodic and aperiodic tasks ....................................................................... 106

Background scheduling ..................................................................................................................... 107

Polling server ..................................................................................................................................... 108

Exercise about background scheduler and polling server ................................................................ 114

Deferrable server .............................................................................................................................. 117

Priority inversion ................................................................................................................................... 120

NPP .................................................................................................................................................... 124

IPC ..................................................................................................................................................... 125

PIP ..................................................................................................................................................... 126

Factors influencing the determinism of the scheduler ......................................................................... 127

Focus about the LAB 1 – possible exam exercise ...................................................................................... 131 3

Device management part 2 ....................................................................................................................... 136

PWM ..................................................................................................................................................... 136

ADC operations ......................................................................................................................................... 146

Polling with NXP board ......................................................................................................................... 150

Interrupt with the NXP board ............................................................................................................... 153

Memory management .............................................................................................................................. 159

Static partitioning

.................................................................................................................................. 161

Dynamic partitioning............................................................................................................................. 164

Exercise ................................................................................................................................................. 168

Memory management and real time .................................................................................................... 170

Stack sharing ......................................................................................................................................... 171

Exercise ................................................................................................................................................. 176

Mass memory management ................................................................................................................. 176

Virtual file system ............................................................................................................................. 181

Memory technology .......................................................................................................................... 182

File system implementation .............................................................................................................. 183

FAT .................................................................................................................................................... 189

Journaling .......................................................................................................................................... 189

Exam questions about this part ........................................................................................................ 194

AUTOSAR ................................................................................................................................................... 195

AUTOSAR classic .................................................................................................................................... 198

Application layer ............................................................................................................................... 200

RTE .................................................................................................................................................... 201

Services layer .................................................................................................................................... 202

ECU abstraction layer ........................................................................................................................ 206

Microcontroller abstraction layer ..................................................................................................... 207

Complex driver .................................................................................................................................. 208

Example ................................................................................................................................................. 209

Embedded Linux ........................................................................................................................................ 215

Components of the embedded Linux system ....................................................................................... 216

Bootloader ............................................................................................................................................ 218

First scenario ..................................................................................................................................... 219 4

Second scenario ................................................................................................................................ 220

Kernel .................................................................................................................................................... 223

Device tree ............................................................................................................................................ 224

System program .................................................................................................................................... 225

Application ............................................................................................................................................ 225

Root file system .................................................................................................................................... 226

Build system .......................................................................................................................................... 227

Workflow ........................................................................................................................................... 228

Build systems .................................................................................................................................... 231

Yocto ..................................................................................................................................................... 233

C language review ..................................................................................................................................... 241

Functions ............................................................................................................................................... 241

Data types ............................................................................................................................................. 241

Numbers ................................................................................................................................................ 242

How to handle global variables ............................................................................................................. 243 5

Lectures

Introduction

What is an embedded system? An embedded system is a special-purpose computer designed for specific

applications. It is very different from a general-purpose computer since it can only run specific tasks and

it often obey to real-time computing constraints. An embedded system interacts with a physical system

by means of actuators/sensors.

In this course we will not deal with hardware, we will only program it at the basic software level.

The top part of the chart is the application. The application is the software where we implement the

features that our customer is asking for. The application needs to interact with hardware to provide

results. In order to do this a basic software is necessary. The basic software is the collection of all the

modules that are needed to abstract the operation of the hardware. Hence, the basic software provides

services to use and manage hardware. The platform is the combination between the hardware and the

basic software. 6

Cross compilation flow

Which is the compilation flow we will use in our practical experiences? Let’s start by analysing the

standard process that happens when programming on a development PC: when coding in C with

Codeblocks (for example) we are programming our machine, hence the created file will be executed by

the same device where the coding is performed. What is the process behind such a condition? The source

files that compose our program are analysed by a special program called compiler, that will first analyse

the syntax of our code and then, if correct, it will translate the source files into object files. Such object

files will be provided to another program, the linker, which will take care of gluing together the source

files with the libraries to produce the executable file. In general, such a flow can be extended for multiple

files so that the program appears in a cleaner fashion. Moreover, it is possible to divide the source file

into a .c file and a .h file. The .c file contains the declaration of variables and the operations to be

performed, the .h file contains the datatypes and the libraries. The compiler will create object files based

on the source files and then the linker will glue them together to obtain the executable file implemented

on the application.

This was the procedure that generally happens on our development PC; when dealing with embedded

systems the situation is slightly different: the processor that is coming with our embedded system is

simpler than the one on the PC and, in general, w