Intro

What Operating Systems Do?

A program that acts as an intermediary between a user of a computer and the computer hardware.

In user view, depending the number using this computer, operating system does differently:

For single user: the operating system is designed mostly for ease of use and non paid to resource utilization
For many user: maximize resource utilization

In system view, operating system is

Resource allocator
Control program

Defining Operating System

In general, we have no completely adequate definition of an operating system.

A common definition is that operating system is the one program running at all times on the computer. This program is usually called as kernel. Along with kernel, we have two types of programs:

System programs: associated with the operating system but are not necessarily part of the kernel
Application programs

Operating System Structure

I/O Structure

There are two I/O methods:

Synchronous: after I/O starts, control returns to user program only upon I/O completion.
Asynchronous: after I/O starts, control returns to user program without waiting for I/O completion.

Direct Memory Access Structure

Device controller transfers blocks of data from buffer storage directly to main memory without CPU intervention, used for hight-speed I/O devices able to transmit information at close to memory speeds.

Storage Structure

Storage systems organized in hierarchy, as speed, cost, volatility.

Caching: copying information into faster storage system, main memory can be viewed as a last cache for secondary storage.

Computer System Architecture

Single-processor Systems
Multi-processor Systems
Clustered Systems

Operation System Structure

Multiprogramming need for efficiency. Single user cannot keep CPU and I/O devices busy at all times. Multiprogramming organizes jobs so CPU always has one to execute. Several jobs are kept in main memory at the same time, and the CPU is multiplexed among them.

Timesharing(multitasking) is logical extension in which CPU switches jobs so frequently that users can interact with each jobs while it is running, creating interactive computing.

OS need these features for multiprogramming:

I/O routine supplied by the system
Memory management, the system must allocate the memory to several jobs
CPU scheduling, the system must choose among several jobs ready to run
Allocation of devices

Operating System Operations

Interupt

Interrupt driven by hardware
Software error or request create exception or trap
Other process problems: infinite loop, processes modifying each other or operating system.

Dual Mode

Dual-mode operation allows OS to protect itself and other system components.

User Mode
Kernel Mode.

And there is a mode bit provided by hardware, providing ability to distinguish when system is running user code or kernel code. Some instructions designated as privileged, only executable in kernel mode. System call change mode to kernel mode, return from call resets it to user mode.

Transition from user to kernel mode:

Timer to prevent infinite loop/ process hogging resources.

Set interrupt after specific period
Operating system decrements counter
When counter zero generate an interrupt
Set up before scheduling process to regain control or terminate program that exceeds alloted time.

Process Management

A process is a program in execution. It is a unit of work within the system. Program is a passive entity, process is an active entity. Process needs resources to accomplish its task:

CPU, memory, I/O and files
data

And process termination requires reclaim of any reusable resources.

Single-threaded process has one program counter specifying location of next instruction to execute. Multi-threaded process has one program counter per thread.

Operating system is responsible for the following activities in connection with process management:

Creating and deleting both user and system process
Suspending and resuming processes
Providing mechanisms for process synchronization
Providing mechanism for process communication
Providing mechanisms for deadlock handling

Memory Management

Memory management determines what is in memory when optimizing CPU utilization and computer response to users.

Keeping track of which parts of memory are currently being used and by whom
Deciding which processes and data to move into and out of memory
Allocating and deallocating memory space as needed

Storage Management

OS provides uniform, logical view of information storage.

Abstracts physical properties to logical storage unit: file
Each medium is controlled by device
File System management

Mass-Storage Management

Proper management is of central importance.

I/O Subsystem

One purpose of OS is to hide peculiarities of hardware devices from the user.

I/O subsystem responsible for

Memory management of I/O including buffering, caching, spooling
General device-driver interface
Drivers fro specific hardware devices

Protection and Security

Protection: any mechanism for controlling access of processes or users to resources defined by the OS.

Security: defense of the system against internal and external attacks.

Systems generally first distinguish among users, to determine who can do what.

User identities include name and associated number, one per user
User ID then associated with all files, processes of tha user to determine access control
Group ID allows set of users to be defined and controls managed, then be associated with processes and files

And we have privilege escalation allows user to change to effective ID with more rights.

System and Computing Environment

The CPU is multiplexed among serveral jobs that are kept in memory and on disk, and the concept time slot is used to describe one single job use the CPU time.

Parallel Systems

Multiprocessor systems with more than one CPU in close communication. The advantages of parallel system:

Increased throughput
Economical
Increased reliability

There are two kinds of paralled systems:

Symmetric multiprocessing SMP

Each processor runs identical copy of the operating system. Many processes can run at once without performace deterioration. Most modern operating systems support SMP
Asymmertic multiprocessing

Each processor is assigned a specific task, master processor schedules and allocated work to slave processors. Most common in extremely large systems.

Distributed Systems

Distribute the computation among serveral physical processors. Loosely coulped system: each processor has its own local memory, processors communicate with one another through various communications lines,such as high-speed buses or telephone lins. Distributed systems has advantages:

Resources sharing
Computation speed up - load sharing
Reliability
Communications

Distributed systems require networking infratructure, local area networks (LAN) or wide area networks (WAN) both are ok. A distribued system can be either client-server or peer-to-peer systems.

Clustered Systems

Clustering allows two or more systems to share storage, provides high reliability.

Asymmetric clustering: one server runs the application while other servers standby.
Symmetic clustering: all N hosts are running the application

Real-Time Systems

Ofter used as a control device in a dedicated application such as controlling scientific experiments, medical imaging systems, industrial control systems and some display systems. Real time systems has well-defined fixed-time constraints.

Real time systems may be either hard or soft real-time.

Hard real-time: secondary storage limited or absent, data stored in short term memory or read-only memory. This mode will conflicts with time-sharing systems, not supported by general-purpose operating systems
Soft real-time: limited utility in industrial control of robotics, useful in applications but requiring advanced operating-system features

Handheld Systems

The handheld systems is the operating systems used by personal digital assistants (PDAs) or cellular telephones. The issues with handheld systems are:

Limited memory
Slow processors
Small display screens

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