- What is an operating system?
- Early Systems
- Simple Batch Systems
- Multiprogramming Batched Systems
- Time-Sharing Systems
- Personal-Computer Systems
- Parallel Systems
- Distributed Systems
- Real-Time Systems
What is an Operating system
A program that acts as an intermediary between a user of a computer
and the computer hardware.
A systems program which controls all the computer's resources and provides
a base upon which application programs can be written.
Operating system goals:
- Execute user programs and make solving user problems easier.
- Make the computer system convenient to use.
- Use the computer hardware in an efficient manner.
Computer System Components
- Hardware - provides basic computing resources (CPU, memory, I/O devices).
- Operating system - controls and coordinates the use of the hardware
among the various application programs for the various users.
- Applications 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 pro grams).
- Users (people, machines, other computers).
Operating System Functions
- Resource allocator - manages and allocates resources.
- Control program - controls the execution of user programs and operation
of I/O devices.
- Kernel - the one program running at all times (all else being application
Early Systems - bare machine (early 1950s)
- First Generation.
- Large machines run from console
- Single user system
- Programmer/User as operator
- Paper tape or punched cards
- Early Software
- Libraries of common subroutines
- Device drivers
- Inefficient use of expensive resources
- Low CPU utilization
- Significant amount of setup time
Simple Batch Systems - Second Generation.
- Use an operator (somebody to work the machine)
- Add a card reader (a device to read programs written on punched
- Reduce setup time by batching similar jobs
- Automatic job sequencing - automatically transfers control from one
job to another. First rudimentary operating system.
- Resident monitor
- initial control in monitor
- control transfers to job
- when job completes control transfers back to monitor
- How does the monitor know about the nature of the job (e.g., Fortran
versus Assembly) or which program to execute?
- How does the monitor distinguish
a) job from job?
b) data from program?
Solution: introduce control cards
- Special cards that tell the resident monitor which programs to run.
- Parts of resident monitor
- 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.
- Problem: Slow Performance - since I/O and CPU could not overlap, and
card reader very slow.
- Solution: Off-line operation - speed up computation by loading jobs
into memory from tapes and card reading and line printing done off-line
using smaller machines.
- Advantage of off-line operation - main computer not constrained by
the speed of the card readers and line printers, but only by the speed
of faster magnetic tape units.
- No changes need to be made to the application programs to change from
direct to off-line I/O operation.
- Real gain - possibility of using multiple reader to-tape and tape-to-printer
systems for one CPU.
Spooling - overlap the I/O of one job
with the computation of another job.
(Simultaneous Peripheral Operation On Line) Simple Multiprogramming.
- While executing one job, the operating system:
- reads the next job from the card reader into a storage area on the
disk (job queue).
- outputs the printout of previous job from disk to the line printer.
- Job pool - data structure that allows the operating system to select
which job to run next, in order to increase CPU utilization.
Multiprogramming and Time Sharing- Third
- Several jobs are kept in main memory at the same time, and the CPU
is shared between them. Each job is called a process.
OS Features Needed for Multiprogramming
- I/O routine supplied by the system.
- Memory management - the system must allocate the memory to several
- CPU scheduling - the system must choose among several jobs ready to
- Allocation of devices.
Time-Sharing Systems- Interactive Computing
- Most efficient for many users to share a large computer.
- The CPU is shared between several processes.
- Each process belongs to a user and I/O is to/from a separate terminal
for each user.
- On-line file system must be available for users to access data and
Personal-Computer Systems - Fourth Generation
- Personal computers - computer system dedicated to a single user.
- I/O devices - keyboards, mice, display screens, small printers.
- User convenience and responsiveness.
- Can adopt technology developed for larger operating systems; often
individuals have sole use of computer and do not need advanced CPU utilization
or protection features.
Parallel Systems - multiprocessor systems
with 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 systems:
- Increased throughput
- Increased reliability
- Symmetric multiprocessing
- Each processor runs an identical copy of the operating system.
- Many processes can run at once without performance deterioration.
- Asymmetric multiprocessing
- Each processor is assigned a specific task; master processor schedules
and allocates work to slave processors.
- More common in extremely large systems.
Distributed Systems - distribute the computation
among several physical processors.
- Loosely coupled system - each processor has its own local memory; processors
communicate with one another through various communication lines, such
as high-speed networks.
- Advantages of distributed systems:
- Resource sharing
- Computation speed up - load sharing
- 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.
- Well-defined fixed-time constraints.
- OS must be able to respond very quickly.