Chapter 1 – Introduction to OS
What is an Operating System?
§ Operating System is a Resource Manager.
– Handles multiple computer resources: CPU, Internal/External memory, Processes, Tasks, Applications, Users, etc…
– Manages
and allocates resources to multiple users or multiple jobs running at
the same time (e.g., processor time, memory space, I/O devices)
– Arranges to use the computer hardware in an efficient manner (maximize throughput, minimize response time) and in a fair manner.
§ It is a Control Program.
– Manages all the components of a complex computer system in an integrated manner.
– Controls the execution of user programs and I/O devices to prevent errors and improper use of the computer resources.
– Looks over and protects the computer.
§ It is an extended/virtual machine
- An interface between the user and hardware that hides the details of the hardware (e.g., I/O).
- Constructs higher-level (virtual) resources out of lower-level (physical) resources (e.g., files).
- Definition:
Is a collection of software enhancements, executed on the bare
hardware, culminating in a high-level virtual machine that serves as an
advanced programming environment
Why Operating System?
§ Computer hardware is developed to execute user programs and make solving user problems easier.
§ An operating system makes a computer more convenient to use.
- It
acts as an interface between user and computer hardware. Therefore, the
end-users are not particularly concerned with the computer’s
architecture, and they view the computer system in terms of an
application.
- To
programmers, it provides some basic utilities to assist him in creating
programs, the management of files, and the control of I/O devices.
Operating System Objectives
§ Convenience
- Makes the computer more convenient to use
§ Efficiency
- Allows computer system resources to be used in an efficient manner
§ Ability to evolve
- Permit effective development, testing, and introduction of new system functions without interfering with service
Services Provided by Operating Systems
§ Facilities for program creation
- Editors, compilers, linkers, debuggers, etc.
§ Program execution
- Loading in memory, I/O and file initialization.
§ Access to I/O and files
- Deals with the specifics of I/O and file formats.
§ System access
- Resolves conflicts for resource contention.
- Protection in access to resources and data.
§ Error detection and response
- internal and external hardware errors
§ memory error
§ device failure
- software errors
§ arithmetic overflow
§ access forbidden memory locations
- operating system cannot grant request of application
§ Accounting
- collect statistics
- monitor performance
- used to anticipate future enhancements
- used for billing users
Computer System Components
§ A computer system can be divided in to four components.
- The Hardware: Provides basic computing resources (CPU, memory, I/O devices).
- The Operating System: Controls and coordinates the use of the hardware among the various application programs for the various users.
- The Application Programs:
Define the ways in which the system resources are used to solve the
computing problems of the users (compilers, database systems, video
games, business programs).
- The Users: Users (people, machines, other computers).
§ These components can be viewed as layers, where each layer uses the services provided by the layer beneath it.
A Static View of System Components
Dynamic View of System Components
Another view of computer system components
History of Operating Systems
§ Let’s see how operating systems evolve over time.
§ This
will help us to identify some common features of operating systems and
how and why these systems have been developed as they are.
Evolution of Operating Systems
§ Early Systems (1950)
§ Simple Batch Systems (1960)
§ Multiprogrammed Batch Systems (1970)
§ Time-Sharing and Real-Time Systems (1970)
§ Personal/Desktop Systems (1980)
§ Multiprocessor Systems (1980)
§ Networked/Distributed Systems (1980)
§ Handheld Systems (1990)
Early Systems
§ 
Structure
Structure
- Single user system.
- Large machines run from console.
- 
Programmer/User as operator.
Programmer/User as operator.
- Paper Tape or Punched cards.
- No tapes/disks in computer.
§ Early software: Assemblers, Libraries of common subroutines, Device Drivers, Compilers, Linkers.
§ Significant amount of setup time.
§ Low CPU utilization.
§ But very secure.
Simple Batch Systems
§ Mainframe machines. Input devices were card readers. Output devices were line printer, tape drives, and card punch.
§ A job (a single program+ associated data + control information) usually on the punch cards submitted to the operator.
§ The output consisted of the results of the program or memory dump in case of error.
§ The operator used to batch together similar programs and run as a group to reduce setup time.
§ No user interaction while the job is executing.
§ Current examples include .bat files under Dos – Windows and shell files under Unix/Linux.
Example of card deck of a job
§ The operating systems (called resident monitor) manages the execution of each program in the batch.
- Monitor utilities are loaded when needed.
- Resident monitor is always in main memory and available for execution.
- The resident monitor usually has the following part.
§ Control card interpreter – responsible for reading and carrying out instructions on the cards.
§ Loader – loads systems programs and applications programs into memory.
§ Device drivers – know special characteristics and properties for each of the system’s I/O devices.
§ In batch systems:
- Initial control is in monitor.
- Load next program and transfer control to it.
- When a job completes, the control transfers back to monitor.
- Automatically transfer control from one job to another (Automatic job sequencing).
Problems
§ Slow Performance – I/O and CPU could not overlap; card reader very slow.
§ CPU was often idle.
Solutions
1. Off-line Operation
- Speed up computation by loading jobs into memory from tapes while card reading and line printing is done off-line using smaller machines.
2. Use spooling (Simultaneous Peripheral Operation On Line).
- Cards are read directly from the card reader onto a disk and location of card images are kept in a table by the operating system.
- The output is sent to the disk and when the job is completed then the output was actually printed.
- I/O and computations were overlapped. While executing one job, the OS:
- Reads next job from card reader into a storage area on the disk (job queue).
Outputs printout of previous job from disk to printer.
§
Uniprogramming Until Now
§ I/O operations are exceedingly slow (compared to instruction execution).
§ A program containing even a very small number of I/O operations will spend most of its time waiting for them.
§ Hence: poor CPU usage when only one program is present in memory.
Memory Layout of Uniprogramming
 |
Memory layout of a simple batch processing system
Multiprogrammed Batch Systems
§ Several jobs are kept in main memory at the same time, and the CPU is multiplexed among them.
 |
§ If
memory can hold several programs, then CPU can switch to another one
whenever a program is waiting for an I/O to complete – This is
multiprogramming.
OS Features Needed 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.
Time Sharing Systems (Interactive Systems)
§ TSS extends Batch multiprogramming to handle multiple interactive jobs – It’s Interactive Multiprogramming.
§ Multiple users simultaneously access the system through terminals.
§ Processor’s time is shared among multiple users, that is, the
CPU is multiplexed among several jobs that are kept in memory and on
disk (the CPU is allocated to a job only if the job is in memory).
§ On-line
communication between the user and the system is provided; when the
operating system finishes the execution of one command, it seeks the
next “control statement” from the user’s keyboard.
§ TS system provides each user with her/her own virtual machine.
Multitasking
§ TS eventually supports multitasking.
§ A time share system that supports multiple processes (program in execution) per user is called a multitasking system.
Why Does Time Sharing Work?
§ Because of slow human reaction time, a typical user needs 2 seconds of processing time per minute.
§ Then many users should be able to share the same system without noticeable delay in the computer reaction time.
Batch Multiprogramming
Vs. Time Sharing
Batch Multi-Prog.
|
Time Sharing
| |
Principle obj.
|
Max. Processor use
|
Min. response time
|
Source of inst. To OS
|
JCL provided with the job
|
Commands entered at the terminal
|
OS Features Needed for Time Sharing Systems
§ On-line file system must be available for users to access data and code.
§ Should do memory management
§ Should do CPU scheduling
§ Should do job synchronization and have communication facilities.
§ Should ensure that dead lock and indefinite waiting does not occur.
§ Should allow sharing of computer resources.
Personal Computer Systems
§ Personal computers – computer system dedicated to a single user.
§ Have a wide variety of I/O devices – keyboards, mice, display screens, small printers.
§ User convenience and responsiveness are of prime importance.
§ Can adopt technology developed for larger operating system.
§ Often individuals have sole use of computer and do not need advanced CPU utilization of protection features.
§ May run several different types of operating systems (Windows, MacOS, UNIX, Linux)
Two Categories of Computer Systems
§ Single Instruction Single Data (SISD)
- Single processor executes a single instruction sequence to operate on data stored in a single memory.
- This is a Uniprocessor.
§ Multiple Instruction Multiple Data (MIMD)
- A set of processors simultaneously execute different instruction sequences on different data sets.
- This is a Multiprocessor.
Multiprocessor Systems
§ Multiprocessor systems have more than one CPU in close communication.
- Tightly coupled system – processors share memory and a clock; communication usually takes place through the shared memory.
§ Advantages of parallel system:
- Increased throughput
- Economical
- Increased reliability
§ Graceful degradation
Multiprocessor architecture
Symmetric Multiprocessing (SMP)
§ Each processor runs an identical copy of the operating system.
§ Each processor can perform the same functions and share same main memory and I/O facilities (symmetric).
§ The OS schedules processes/threads across all the processors (real parallelism).
§ Existence of multiple processors is transparent to the user.
§ Incremental growth: just add another CPU!
§ Robustness: a single CPU failure does not halt the system, only the performance is reduced.
§ Many processes can run at once without performance deterioration.
§ Most modern operating systems support SMP
Asymmetric multiprocessing
§ Each processor is assigned a specific task; master processor schedules and allocated work to slave processors.
§ More common in extremely large systems
Distributed Systems
§ Distribute the computation among several physically separated processors.
- Loosely coupled system
– each processor has its own local memory; processors communicate with
one another through various communications lines, such as high-speed
buses or telephone lines.
§ Advantages of distributed systems.
- Resources Sharing
- Computation speed up – load sharing
- Reliability and fault tolerance
- Communications
§ Requires networking infrastructure - Local area networks (LAN) or Wide area networks (WAN)
§ May be either client-server or peer-to-peer systems.
General structure of client-server
Peer-to-peer systems
Network Operating System
§ Provides file sharing
§ Provides communication scheme
§ Runs independently from other computers on the network
Distributed Operating System
§ Less autonomy between computers
§ Gives the impression there is a single operating system controlling the network.
Clustered Systems
§ Clustering allows two or more systems to share external storage and balance CPU load.
§ Asymmetric clustering: one server runs the application while other servers standby.
§ Symmetric clustering: all N hosts are running the application.
Real-Time Systems
§ Note that not all Operating Systems are general-purpose systems.
§ Real-Time (RT) systems are dedicated systems that need to adhere to deadlines, i.e., time constraints.
§ Correctness of the computation depends not only on the logical result but also on the time at which the results are produced.
§ Often
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 may be either hard or soft real-time.
Hard Real-Time System
§ Must meet its deadline.
§ Conflicts with time-sharing systems, not supported by general-purpose operating systems.
§ Often used as a control device in a dedicated application such as industrial control and robotics
§ Secondary storage limited or absent, data stored in short term memory, or read-only memory (ROM).
Soft Real-Time System
§ A critical real-time task gets priority over the other tasks (Deadline desirable but not mandatory).
§ Limited utility in industrial control of robotics
§ Useful in applications (multimedia, virtual reality) requiring advanced operating-system features.
Hand Held Systems
§ Personal Digital Assistants (PDAs)
§ Cellular telephones
§ Issues:
- Limited memory
- Slow processors
- Small display screens.
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