Operating System Objectives and Services

Operating System Objectives

• An OS controls the execution of application programs
• An OS acts as the primary interface between all applications and computer hardware
• Objectives include convenience, efficiency, and ability to evolve
• Convenience refers to how easily a computer can be operated via the OS
• Efficiency refers to using computer system resources in the most effective way possible
• The ability to evolve refers to how an OS should permit effective development, testing and introduction of new system functions without interfering with service

Services Provided by the Operation System

• Program development: Editors and debuggers
• Program execution: Scheduling duties.
• I/O Device Access: Simple reads and writes
• File Access Control: Protection mechanisms to control file access
• System Access: OS controls access to the system as a whole and other specific system resources.
• Error detection and response includes internal/external hardware errors, software errors, and error recovery

System Services Continued

• Accounting involves collecting statistics, monitoring performance, anticipating future enhancements, and user billing
• Instruction Set Architecture (ISA) defines the repertoire of machine language instructions
• Application Binary Interface (ABI) defines a standard for binary portability across programs
• Application Programming Interface (API) gives a program access to hardware resources/services, and enables application software to be ported easily

Operating System Evolution

• OS evolve over time due to hardware upgrades, new services, and various fixes

Serial Processing

• From the late 1940s to the mid 1950s, programmers interacted directly with computer hardware
• There was no operating system at this time
• Machines ran from a console with display lights, toggle switches and a printer
• Programs were loaded via an input device, such as a card reader
• Two main problems: scheduling and setup time

Simple Batch Systems

• Developed Mid 1950s
• Improved utilization by use of a monitor
• Users no longer had direct access to the processor as the Monitor controlled the running programs
• Batched jobs run together
• Programs branches back to the monitor when finished
• Resident monitor resides in main memory and is available for execution

Job Control Language (JCL)

• Programming language of special type
• Provides instructions to the monitor that specifies: what compiler to use; what data to use

Hardware Features/Modes of Operation

• Memory Protection: Memory containing the monitor cannot be altered.
• Timer: Prevents a job from monopolizing the system.
• Privileged Instructions: Machine level instructions, executed only by the Monitor,
• Interrupts: Early computer models lacked this capability
• User Mode: Program executes in User Mode; memory areas are protected; certain instructions cannot be executed
• Kernel Mode (System Mode): Portion of OS resident in main memory; machine-independent part (code for system calls) and machine-dependent part (device drivers); maintains the OS state; executes in privileged/supervisor mode

Multi-programming And Time Sharing Considerations

• Automatic job sequencing occurs with simple batch OS, but the processor often idles.
• I/O devices are slow, further contributing to processor idle time
• Using Multiprogramming, when one job needs to wait for I/O, the processor can switch to the other job.
• In 1961, Time-Sharing Systems were developed
• Time sharing uses multiprogramming to handle multiple interactive jobs
• Processor time can be shared among multiple users
• Multiple users simultaneously access the system through terminals
• Batch processing and time-sharing use multiprogramming

Theoretical Advances

• There have been four major theoretical advances in OS development
• Processes relate to programs in execution
• Memory management relates to automatic allocation and support for modular systems
• Information protection and security is the integrity of data
• Scheduling & Resource Management is fairness and efficiency

Process Information

• A process serves as a program in execution
• All processes are instances of programs running on a computer
• A process can be assigned to and executed on a processor; it encapsulates a single sequential thread of execution, current state, and the set of system resources

Potential Design Difficulties

• Software can experience difficulties when designing due to improper synchronization, failed mutual exclusion, non-determinate program operations, and deadlocks
• Process Components are executable programs
• Processes rely on associated data (variables, workspace, buffers etc)
• The execution context of a program
• Memory Management handles the needs of users via modular programming
• The OS handles key storage management responsibilities like processing isolation, automatic allocation/management, modular programming support, protection/access control, and long-term storage

Virtual Memory and File Systems

• Virtual memory and file systems meet the discussed requirements
• The Files System implements long-term storage of information stored in named objects
• A file system can be a unit of access control and protection, per the OS
• Virtual memory allows programs to address memory from a logical point of view
• Virtual memory was designed to meet the requirement of having multiple user jobs concurrently reside in main memory
• Paging systems were introduced, if the processor switches between processes, it is difficult to pack them compactly into main memory.
• Paging enables processes to be comprised of several fixed-size blocks, called pages
• The Virtual address is a page number and an offset within the page
• Pages can be located anywhere in main memory
• Translation between a virtual address and a real address can occur

Information Protection and Security

• Availability is concerned with protecting a system against interruption
• Confidentiality assures users cannot read data for which access is unauthorized
• Data Integrity protects against data modification from unauthorized sources.
• Authenticity ensures proper verification of identity and data validity

Scheduling & Resource Management

• Fairness means giving all processes equal/fair access
• Differential responsiveness means distinguishing between different classes of jobs/users
• Efficiency includes maximizing throughput, minimizing response time, and accommodating many users

Evolution of OS

• There has been a gradual evolution of OS structures/capabilities
• Modern OS respond to new developments in hardware, new applications and new security threats
• Key hardware drivers are multiprocessor systems, greatly increased processor speed, high-speed network attachments, and increasing size and variety of memory storage devices.
• Application Drivers Include multimedia apps, Internet/Web Access, Client-Server

Modern OS

• Microkernel systems assign only a few essential functions to the kernel: address space management, interprocess communication (IPC), and basic scheduling.
• Multithreading divides processes into threads that run simultaneously
• A thread is dispatchable unit of work and is interruptible
• Processes are collections of one or more threads and associated system resources
• Symmetric multiprocessing presents multiple processors that share memory and can perform the same functions
• Distributed operating systems give the illusion of a single main memory and single secondary memory space and are used for distributed file systems
• Object-oriented design lends discipline to the process of adding modular extensions to a small kernel and eases development of distributed tools/operating systems

Fault Tolerance

• Fault tolerance is the ability of a system/component to continue operating despite hardware/software faults; it increases system reliability
• Fault tolerance can come with a cost, either in financial terms or performance
• Fault tolerance measures must be determined by how critical the resource is
• Three basic measures relate to the operation of a system: reliability, mean time to failure (MTTF), and availability
• Availability of a system/service is the fraction of time the system is available to service user requests
• Faults can be permanent, temporary, transient, or intermittent
• Spatial redundancy leverages multiple components; temporal redundancy repeats an operation upon error detection; information redundancy replicates or codes data so bit errors can be detected and/or corrected

Operating System Mechanisms

• Include process isolation, concurrency controls, virtual machines, checkpoints & rollbacks
• In 1985, Microsoft initially used the name Windows for personal computers
• Windows NT in 1993 was released for laptop and desktop systems
• Microsoft Windows contains an Executive, a Kernel, and a Hardware Abstraction Layer (HAL)
• System elements include: device drivers, windowing/graphics system, I/O manager, cache manager, object manager, plug-and-play manager, power manager, security reference monitor, virtual memory manager, process/thread manager, configuration manager, and an Advanced Local Procedure Call (ALPC) facility

Microsoft Windows Architecture

• Includes special system processes, service processes, environment subsystems, and other user applications
• The Windows OS services, environment subsystems and applications use the client/server computing model
• This model is designed to simplify the executive, improve reliability, provide uniform means for client communication
• SMP and threads enable OS routines to be used on any available processor
• Windows support the use of multiple threads of execution within a single process/multiple clients simultaneously
• Windows manages objects using encapsulation, class/instance objects and inheritance

Traditional Unix System

• Developed at Bell Labs and became operational on a PDP-7 in 1970.
• Rewritten in the C programming language.
• Licensing to commercial/educational institutions
• Version 7, released in 1978, served as inspiration for the modern system
• Linux served as a Unix variant for the IBM PC
• Linus Torvalds wrote the initial version of the kernel.
• Since 1991, many collaborators have contributed to the development.
• Linux code is free and available

Linux Systems

• Linux has a modular structure, modules collected and automatically loaded/unloaded on demand
• Independent blocks are loadable modules
• A module is an object file whose code can be linked/unlinked to the kernel at runtime, using dynamic linking and stacked modules
• The Android operating system is a Linux-based system originally designed for mobile phones
• Initial Android OS development was coordinated by Android, Inc.
• Google subsequently acquired Android in 2005.
• It is currently maintained as an Android Open-Source Project (AOSP).