Fundamentals of Embedded Operating Systems

Refers to the use of electronics and software within a product that is designed to perform a dedicated function

• In many cases, embedded systems are part of a larger system or product

• Antilock braking system in a car would be an example

Discuss the architecture of embedded systems.

Characteristics of Embedded OS

• Real-time operation

• Reactive operation

• Configurability

• I/O device flexibility

• Streamlined protection mechanisms

• Direct use of interrupts

Two general approaches:

• Take an existing OS and adapt it for the embedded application

• Design and implement an OS intended solely for embedded use

An existing commercial OS can be used for an embedded system by adding:

• Real-time capability

• Streamlining operation

• Adding necessary functionality

Advantage:

• Familiar interface

Disadvantage:

• Not optimized for real-time and embedded applications

Typical characteristics include:

• Fast and lightweight process or thread switch

• The scheduling policy is real-time and dispatcher module is part of scheduler

• Small size

• Responds to external interrupts quickly

• Minimizes intervals during which interrupts are disabled

• Provides fixed or variable-sized partitions for memory management

• Provides special sequential files that can accumulate data at a fast rate

Timing Constraints in Embedded OS

• Provides bounded execution time for primitives

• Maintains a real-time clock

• Provides for special alarms and timeouts

• Supports real-time queuing disciplines

• Provides primitives to delay processing by a fixed amount of time and to suspend or resume execution

A version of Linux running in an embedded system

• Embedded devices typically require support for a specific set of devices, peripherals, and protocols, depending on the hardware that is present in a given device and the intended purpose of that device

• An embedded Linux distribution is a version of Linux customized for the size and hardware constraints of embedded devices

• Includes software packages that support a variety of services and applications on those devices

• An embedded Linux kernel will be far smaller than an ordinary Linux kernel

• A key differentiator between desktop/server and embedded Linux distributions is that desktop and server software is typically compiled on the platform where it will execute

• Embedded Linux distributions are usually compiled on one platform but are intended to be executed on another

  • The software used for this purpose is referred to as a cross-compiler

The file system must be as small as possible

Commonly used examples:

• cramfs

  a simple read-only file system that is designed to minimize size by maximizing the efficient use of underlying storage files is compressed in units that match the Linux page size

• squashfs

  a compressed, read-only file system that was designed for use on low-memory or limited-storage size environments

• jffs2

  a log-based file system that is designed for use on NOR and NAND flash devices with special attention to flash-oriented issues such as wear-leveling

• ubifs

  provides better performance on larger flash devices and also supports write caching to provide additional performance improvements

• yaffs2

  provides a fast and robust file system for large flash devices

Advantages of using Linux as the basis for an embedded OS include the following:

• vendor independence

  The platform provider is not dependent on a particular vendor to provide needed features and meet deadlines for deployment

• varied hardware support

  Linux's support for a wide range of processor architectures and peripheral devices makes it suitable for virtually any embedded system

• low cost

  The use of Linux minimizes cost for development and training

• open source

  The use of Linux provides all of the advantages of open source

The focus of Android lies in the vertical integration of the Linux kernel and the Android user-space components

• Many embedded Linux developers do not consider Android to be an instance of embedded Linux

• From the point of view of these developers, a classic embedded device has a fixed function, frozen at the factory

• Android is an operating system for mobile devices. It is mostly used for smartphones, like Google's own Google Pixel, as well as by other phone manufacturers like HTC and Samsung. It has also been used for tablets such as the Motorola Xoom and Amazon Kindle. A modified Linux kernel is used as Android's kernel.

• Google says that over 1.3 million Android smartphones are sold every day. Most mobile phones run Android, making it the most popular mobile operating system. It is also the most popular operating system in general.

• It supports multitasking and two-dimensional and three-dimensional graphics.

• Security updates are not provided on versions older than Android 8.0. Newer versions receive security updates if the phone manufacturer supports them. Phones that have Android 10 and above receive security updates (like app updates) directly from Google Play.

• The current stable version is Android 11, released on September 8, 2020, and Android 10 is currently the most popular Android version.

Discuss the Different Version of Android Operating System.

• Streamlines to a very minimal OS for embedded systems

• Core OS requires 400 bytes of code and data memory combined

• Not a real-time OS

• There is no kernel

• There are no processes

• OS doesn’t have a memory allocation system

• Interrupt and exception handling are dependent on the peripheral

• It is completely nonblocking, so there are few explicit synchronization primitives

• Has become a popular approach to implementing wireless sensor network software

Discuss the Structure of TinyOS

Discuss the WSN Topology.

• With the tiny distributed sensor application in mind, the following goals were set for TinyOS:

• Allow high concurrency

• Operate with limited resources

• Adapt to hardware evolution

• Support a wide range of applications

• Support a diverse set of platforms

• Be robust

• Embedded software systems built with TinyOS consist of a set of modules (called components), each of which performs a simple task and which interface with each other and with hardware in limited and well-defined ways

• The only other software module is the scheduler

• Because there is no kernel, there is no actual OS

• The application area of interest is the wireless sensor network (WSN)

A software component implements one or more tasks

• Each task in a component is similar to a thread in an ordinary OS

• Within a component, tasks are atomic

• Once a task has started it runs to completion

• A command is a nonblocking request

• A task that issues a command does not block or spin wait for a reply from the lower-level component

• Is typically a request for the lower-level component to perform some service

• The effect on the component that receives the command is specific to the command given and the task required to satisfy the command

• A command cannot preempt the currently running task

• A command does not cause a preemption in the called component and does not cause blocking in the calling component

• Events in TinyOS may be tied either directly or indirectly to hardware events

• Lowest-level software components interface directly to hardware interrupts

• There may be external interrupts, timer events, or counter events

• An event handler in a lowest-level component may handle the interrupt itself or may propagate event messages up through the component hierarchy

• A command can post a task that will signal an event in the future

• In this case, there is no tie of any kind to a hardware event

• Operates across all components

• Only one task is executed at a time

• The scheduler is a separate component

  • it is the one portion of TinyOS that must be present in any system

• The default scheduler is a simple FIFO queue

• The scheduler is power-aware

  • puts processor to sleep when there is no task in the queue

TinyOS provides a simple but powerful set of conventions for dealing with resources

Dedicated

• A resource that a subsystem needs exclusive access to at all times

• No sharing policy is needed

• Examples include interrupts and counters

Virtualized

• Every client of a virtualized resource interacts with it as if it were a dedicated resource

• An example is a clock or timer

Shared

• Abstraction that provides access to a dedicated resource through an arbiter component

• The arbitrator determines which client has access to the resource at what time

Discuss the Conclusion of the Course