BTEC101/COMP200 OS Introduction

Introduction to Operating Systems

• Operating system concepts include what OS does, computer system organization, computer system architecture, OS operations, resource management, security, virtualization, distributed systems, kernel data structures, computing environments, and free/libre and open-source OS.

What Does the Term Operating System Mean?

• An operating system is not just for computers, but can also be found in cars, airplanes, printers, washing machines, toasters, compilers, etc.

Bits and Bytes

• A bit can contain one of two values, 0 and 1.
• All other storage in a computer is based on collections of bits.
• A byte is 8 bits, and on most computers, it is the smallest convenient chunk of storage.
• A word is a given computer architecture's native unit of data, made up of one or more bytes.
• Computer storage and throughput are generally measured and manipulated in bytes and collections of bytes.
• Kilobyte (KB), megabyte (MB), gigabyte (GB), terabyte (TB), and petabyte (PB) are units of measurement for computer storage.

Transition from User to Kernel Mode

• When a user is running, the mode bit is "user".
• When kernel code is executing, the mode bit is "kernel".
• System call changes mode to kernel, and return from call resets it to user.
• Some instructions are designated as privileged and can only be executed in kernel mode.

Timer

• A timer is used to prevent infinite loops or process hogging resources.
• The timer is set to interrupt the computer after some time period.
• The operating system sets the counter (privileged instruction).
• When the counter is zero, an interrupt is generated.

Process Management

• A process is a program in execution.
• A process needs resources to accomplish its task.
• Process termination requires reclaiming any reusable resources.
• A single-threaded process has one program counter specifying the location of the next instruction to execute.
• A multi-threaded process has one program counter per thread.
• The operating system is responsible for creating and deleting processes, suspending and resuming processes, providing mechanisms for process synchronization, and providing mechanisms for process communication.

Memory Management

• Memory management determines what is in memory and when.
• Optimizing CPU utilization and computer response to users.
• Memory management activities include keeping track of which parts of memory are currently being used, deciding which processes and data to move into and out of memory, and allocating and deallocating memory space as needed.

File-System Management

• The OS provides a uniform, logical view of information storage.
• Abstracts physical properties to logical storage unit - file.
• Each medium is controlled by a device (e.g., disk drive, tape drive).
• File-system management includes creating and deleting files and directories, primitives to manipulate files and directories, mapping files onto secondary storage, and backup files onto stable storage media.

Mass-Storage Management

• Mass storage is usually used to store data that does not fit in main memory or data that must be kept for a long period of time.
• Proper management is of central importance.
• Operating system activities include mounting and unmounting, free-space management, storage allocation, disk scheduling, partitioning, and protection.

Caching

• Caching is an important principle performed at many levels in a computer (in hardware, operating system, software).
• Information in use is copied from slower to faster storage temporarily.
• Faster storage (cache) is checked first to determine if the information is there.
• Cache management is an important design problem.

Characteristics of Various Types of Storage

• Movement between levels of storage hierarchy can be explicit or implicit.

Migration of Data

• Multitasking environments must be careful to use the most recent value, no matter where it is stored in the storage hierarchy.
• Multiprocessor environment must provide cache coherency in hardware.
• Distributed environment situation is even more complex.

I/O Subsystem

• The I/O subsystem is responsible for memory management of I/O, including buffering, caching, and spooling.
• General device-driver interface and drivers for specific hardware devices.

Protection and Security

• Protection is any mechanism for controlling access of processes or users to resources defined by the OS.
• Security is the defense of the system against internal and external attacks.
• Systems generally first distinguish among users, to determine who can do what.
• User identities, group identifiers, and privilege escalation are used for access control.

Virtualization

• Allows operating systems to run applications within other OSes.
• Emulation is used when the source CPU type is different from the target type.

Cloud Computing

• Allows operating systems to run applications within other OSes over the Internet.
• Types of cloud computing include public, private, hybrid, Software as a Service (SaaS), Platform as a Service (PaaS), and Infrastructure as a Service (IaaS).

Real-Time Embedded Systems

• Real-time embedded systems are the most prevalent form of computers.
• They vary considerably, are special purpose, and have limited purpose OS.
• Real-time OS has well-defined fixed time constraints.

Free and Open-Source Operating Systems

• Operating systems made available in source-code format rather than just binary closed-source and proprietary.
• Examples include GNU/Linux and BSD UNIX.
• Can use VMM like VMware Player or Virtualbox to run guest operating systems for exploration.