Operating System Services

Operating System Services

• Operating System (OS) services are designed for users and user programs.
• These services include user interface, program execution, I/O operations, file system manipulation, communications, and error detection and handling.

User Interface

• The User interface takes several forms including:
  • Graphical User Interface (GUI)
  • Touch-Screen Interface
  • Command-Line Interface (CLI)

Program Execution

• The OS provides execution of programs
• It involves loading a program into memory to run
• Programs must be able to end their execution.

I/O Operations

• A running program may require I/O such as a file or I/O device
• Users can't directly control I/O, so the OS provides the means to do so.

File System Manipulation

• Programs can read, write, create, delete, and search through files and directories.
• The OS provides permission management for these tasks based on file ownership.

Communication

• Processes need to exchange information
• Processes do this together whether they are on the same computer, or a different computer system.
• This can be implemented via shared memory to read and write sections of memory.
• It can also be accomplised by message passing: where packets of information are moved between processes.

Error Detection

• The OS constantly detects and corrects errors.
• Error can come from:
  • Error
  • CPU
  • Memory
  • I/O
  • Program

Resource Management

• Multiple processes can run concurrently and they each need resources
• Resources include:
  • CPU cycle
  • main memory
  • file storage

Accounting/Logging

• Keeping track of what programs are using what kind and how much of resources.

Protection and Security

• Information stored in network or multi user system, want to control use of information.
• Protection involves secure access to resources, and ensures concurrently running processes should not interfere with each other or the OS itself.
• Security provides protection from outsiders.

OS Kernel

• The OS Kernel comprises users, applications, operating system API, operating system kernel, and computer hardware.
• The real work gets done in the kernel.

User vs Kernel Mode

• The modes are user process and kernel process
• User mode executes user applications
• Kernel mode exectutes system operations to carry out user processes

System Calls

• These prevent user processes from gaining open access to the kernel code.
• A system call is triggered by any user or application request that involves access to any system resource, and they must be handled by the kernel code.
• System calls provide an interface to the services.
• These calls are functions within C and C++ and also use assembly language instructions for certain low-level tasks.

Application Programming Interface (API)

• An API is a set of specific functions with parameters that are passed to each function, and the return values to programers.
• Commonly used in Windows API, POSIX API, and Java API
• The benefits of using an API over the actual system calls are:
  • Portability (can compile/run on any system supporting the same API)
  • System calls can be more detailed and are difficult to work with.

Run-Time Environment (RTE)

• RTE provides an interface that serves as a link to system calls. Includes:
  • A system-call interface, which intercepts function calls in the API and invokes the necessary system calls.
  • A number is associated with each system call and the interface maintains a table indexed according to that number.
  • The interface invokes system calls in the OS and returns.

Standard C Library Example

• In C programming, the printf() is invoke
• Is invkedd by library call which calls write() system call

System Call Execution

• The following steps occur during execution:
  • A user program calls a library function.
  • The library routine puts appropriate parameters in a known location (registers, stack, or memory table).
  • A "trap" instruction changes the mode from user to kernel.
  • Control goes to the operating system.
  • The OS determines the system call to be carried out.

Semantics of System Call Execution

• Involves these steps:
  • The kernel indexes the dispatch table, which contains pointers to service routines for system calls.
  • Service routine gets executed and returns a parameter or error code in a register.
  • Control is given back to user program.
  • Library function executes the instruction following the trap.

Types of System Calls

• System calss are divided by:
  • Process control
  • File management
  • Device management
  • Information maintenance
  • Communications

Process Control

• create process, terminate process
• load, execute
• get process attributes, set process attributes
• wait event, signal event
• allocate and free memory

File Management

• create file, delete file
• open, close
• read, write, reposition
• get file attributes, set file attributes

Device Management

• request device, release device
• read, write, reposition
• get device attributes, set device attributes
• logically attach or detach devices

Information Maintenance

• get time or date, set time or date
• get system data, set system data
• get process, file, or device attributes
• set process, file, or device attributes

Communications

• create, delete communication connection
• send, receive messages
• transfer status information
• attach or detach remote devices

Operating system design

• Design goals involve defining goals and specifications, including choice of hardware and types of systems.
• Key requirements are user goals vs systems goals.
  • User wants the system to be convenient, easy to use, reliable, safe, and fast.
  • Engineer wants the system to be easy to design, implement, maintain, operate, flexible, and efficient.

Mechanism and Policy

• Mechanism determines how to do something.
• Policy determines what will be done.
• One important idea is separating policy from mechanism.

Implementation of Operating System

• A system must be implemented after being designed.
• Traditionally, OS were written in assembly language.
• OS are now commonoly written in higher-level langues, such as C and C++.
• The advantages of high level languages are the code is compact, the code can be written faster, easier to understand, debug, and is easier to port.
• A disadvange would be that certain languates (like MS DOS written) are specific to processor familys (like Intel 8088 assembly language, Consequently, it is available on only the Intel family of CPUs.)

Structure of OS

• The structural components invlove:
  • Simple structure
  • Monolithic Structures
  • Layered Approach

Simple Structures

• MS-DOS is an example of simple layer structure
• MS-DOS was written to be the simplest funtion, that was not divided into modules
• Although MS-DOS has some structure, its interfaces and levels of functionality aren't well separated
• Other DOS OS dont have well defined structures
• Due to this fails in application program, the whole system crashes

Monolithic Structures

• UNIX consists of two separable parts: the kernel and the system programs.
• The kernel is everything below the system call interface and above the physical hardware.
• The UNIX structure can make it difficult to improve because an enormous combination of functionality at one level in UNIX can cause changes in one section to adversely affect other areas.

Layered Approach

• The OS is broken up into layers
• The bottom is harware, and the atop (Layer N) is the user interface
• Each layer consists of data structures, and routines to service the layer above it
• All together comprise the kernel
• This approach has modularity, using functions from lower layers, simplifies debugging and system verification.
• Major difficulty: carefully defining the layers, because a layer can only use the layers below it.

OS Structure: Modules

• Most modern operating systems implement kernel modules
• Uses object-oriented approach
• Each core component is separate
• Components interact through known interfaces
• Each is loadable as needed
• An Example is Solaris

Virtual Machine

• A virtual machine (VM) is a virtual environment which functions as a virtual computer system; with its own CPU, memory, network interface, and storage; created on a physical hardware system.
• VMs are isolated from the rest of the system, and multiple VMs can exist on a single piece of hardware, such as a server.
• A VM is a simulated image of application software and operating system, which is executed on a host computer or a server.
• A virtual machine software runs on kernel mode
• Virtual machine itself runs on user mode

Operating System Generation and System Boot

• Operating systems are designed to run on various machines and sites with a variety of peripheral configurations.
• The system must be configured or generated for each specific computer site using system generation (SYSGEN).
• SYSGEN Program determines the CPU to be used, memory available, available devices, and operating-system options desired.

System Boot

• System boot is the process of starting a computer by loading the kernel.
• A piece of code known as the bootstrap program or bootstrap loader locates the kernel.
• This is usually stored as read-only memory (ROM), because the RAM is in an unknown state at system startup, and ROM doesn't need initialization and is immune to computer viruses.