Operating System Structures

Operating System Structures

Ø In this part of the course, we will briefly look at the operating systems from its functional point of view; that is the services which are provided by the operating system. A detailed discussion will follow in the subsequent lectures.

Common System Components

§ Due to the complex nature of the modern operating systems, it is partitioned into smaller component. Each component performs a well-defined function w

§ ith well-defined inputs and outputs.

§ Many modern operating systems have the following components.

- Process Management

- Main Memory Management

- File Management

- I/O System Management

- Secondary Management

- Networking

- Protection System

- Command-Interpreter System

Basic OS Organization

Process Management

§ A process is a program in execution. For example

- A batch job is a process

- A time-shared user program is a process

- A system task (e.g. spooling output to printer) is a process.

§ Remember a program itself is not a process rather it is a passive entity.

§ A process needs certain resources, including CPU time, memory, files, and I/O devices, to accomplish its task. These resources are either given to the process when it is created or when it is running. When the process completes, the OS reclaims all the resources.

§ The operating system is responsible for the following activities in connection with process management.

- Process creation and deletion.

- Process suspension and resumption.

- Provision of mechanisms for:

o Process synchronization

o Process communication

Main

Memory Management

§ Memory is a large array of words or bytes, each with its own address. It is a repository of quickly accessible data shared by the CPU and I/O devices.

§ Main memory is a volatile storage device. It loses its contents in the case of system failure.

§ The operating system is responsible for the following activities in connections with memory management:

- Keep track of which parts of memory are currently being used and by whom.

- Decide which processes to load when memory space becomes available.

- Allocate and deallocate memory space as needed.

File Management

§ Most visible component of OS. Computers can store information on several different types of physical media (e.g. magnetic tap,

§ magnetic disk, CD etc).

§ For convenient use of the computer system, the OS provides a uniform logical view of information storage.

§ A file a logical storage unit, which abstract away the physical properties of its storage device.

§ A file is a collection of related information defined by its creator. Commonly, files represent programs (both source and object forms) and data.

§ The operating system is responsible for the following activities in connections with file management:

- File creation and deletion.

- Directory creation and deletion.

- Support of primitives for manipulating files and directories.

- Mapping files onto secondary storage.

- File backup on stable (nonvolatile) storage media.

I/O System Management

§ The I/O system consists of:

- A buffer-caching system

- A general device-driver interface

- Drivers for specific hardware devices

Secondary Storage management

§ Since main memory (primary storage) is volatile and too small to accommodate all data and programs permanently, the computer system must provide secondary storage to back up main memory.

§ Most modern computer systems use disks as the principle on-line storage medium, for both programs and data.

§ The operating system is responsible for the following activities in connection with disk management:

- Free space management

- Storage allocation

- Disk scheduling

Networking (Distributed Systems)

§ A distributed system is a collection processors that do not share memory or a clock. Each processor has its own local memory.

§ The processors in the system are connected through a communication network.

§ Communication takes place using a protocol.

§ A distributed system provides user access to various system resources.

§ Access to a shared resource allows:

- Computation speed-up

- Increased data availability

- Enhanced reliability

Protection System

§ Protection refers to a mechanism for controlling access by programs, processes, or users to both system and user resources.

§ The protection mechanism must:

- Distinguish between authorized and unauthorized usage.

- Specify the controls to be imposed.

- Provide a means of enforcement.

Command-Interpreter System

§ Many commands are given to the operating system by control statements which deal with:

- Process creation and management

- I/O handling

- Secondary-storage management

- Main-memory management

- File-system access

- Protection

- Networking

§ The program that reads and interprets control statements is called variously:

- Command-line interpreter

- Shell (in UNIX)

§ Its function is to get and execute the next command statement.

Operating System Services

§ Program execution – system capability to load a program into memory and to run it.

§ I/O operations – since user programs cannot execute I/O operations directly, the operating system must provide some means to perform I/O.

§ File-system manipulation – program capability to read, write, create, and delete files.

§ Communications – exchange of information between processes executing either on the same computer or on different systems tied together by a network. Implemented via shared memory or message passing.

§ Error detection – ensure correct computing by detecting errors in the CPU and memory hardware, in I/O devices, or in user programs.

Ø Additional functions exist not for helping the user, but rather for ensuring efficient system operations.

§ Resource allocation – allocating resources to multiple users or multiple jobs running at the same time.

§ Accounting – keep track of and record which users use how much and what kinds of computer resources for account billing or for accumulating usage statistics.

§ Protection – ensuring that all access to system resources is controlled

Requesting Services from OS

§ System Call

- Process traps to OS Interrupt Handler

- Supervisor mode set

- Desired function executed

- Returns to application

§ Message Passing

- User process constructs message indicating function (service needed)

- Invokes send to pass message to OS

- Process blocks

- ……

- OS receives message

- OS initiates function execution

- Upon function completion, OS returns “OK”

- Process unblock…

System Calls

§ System calls provide the interface between a process and the operating system. These calls are generally available as assembly language instructions

§ . Some systems also allow to make system calls from a high level language, such as C.

§ Three general methods are used to pass parameters between a running program and the operating system.

- Pass parameters in registers.

- Store the parameters in a table in memory, and the table address is passed as a parameter in a register.

- Push (store) the parameters onto the stack by the program, and pop off the stack by operating system.

Types of System Calls

§ Process control – load, execute, abort, end, create process, allocate and free memory, wait event etc.

§ File management – Create file, delete file, open, close, read, write, get file attribute etc.

§ Device management – Request device, release device, read, write, logically attach or detach device etc.

§ Information maintenance – Get time and date, set time and date, get process attribute etc.

§ Communications – create, close communication connection, send, receive messages, etc.

System Programs

§ System programs provide a convenient environment for program development and execution. The can be divided into:

- File manipulation – create, delete, copy, rename, print, list etc.

- Status information – Display date, time, disk space, memory size, etc.

- File modification – Create and modify files using text editors.

- Programming language support – Compilers, assemblers, and interpreters.

- Program loading and execution – Loaders, linkers.

- Communications – remote login, send and receive messages.

§ Most users’ view of the operation system is defined by system programs, not the actual system calls.

System Structure

§ Modern OS should be developed carefully due to their size and complexity.

§ A common approach is to divide the systems into small components.

MS-DOS System Structure

§ MS-DOS – written to provide the most functionality in the least space

- Not divided into modules

- Although MS-DOS has some structure, its interfaces and levels of functionality are not well separated

Unix System Structure

§ UNIX – limited by hardware functionality, the original UNIX operating system had limited structuring. The UNIX OS consists of two separable parts.

o Systems programs – use kernel supported system calls to provide useful functions such as compilation and file manipulation.

o The kernel

- Consists of everything below the system-call interface and above the physical hardware

- Provides the file system, CPU scheduling, memory management, and other operating-system functions; a large number of functions for one level.

Layered Approach

§ The operating system is divided into a number of layers (levels), each built on top of lower layers. The bottom layer (layer 0), is the hardware; the highest (layer N) is the user interface.

§ An OS layer is an implementation of an abstract object that is the encapsulation of data and operations that can manipulate those data. These operations (routines) can be invoked by higher-level layers. The layer itself can invoke operations on lower-level layers.

§ Layered approach provides modularity. With modularity, layers are selected such that each layer uses functions (operations) and services of only lower-level layers.

§ Each layer is implemented by using only those operations that are provided lower level layers.

§ The major difficulty is appropriate definition of various layers.

Microkernel System Structure

§ Moves as much from the kernel into “user” space.

§ Communication takes place between user modules using message passing.

§ Benefits:

- Easier to extend a microkernel

- Easier to port the operating system to new architectures

- More reliable (less code is running in kernel mode)

- More secure