1. Operating Systems

2. What is an operating system
an extended machine, easier to program for the user
a resource manager, with more multiple users using the system efficient and correct use of the available resources is conducted by the operating systems.

3. Typical Services Provided by an O.S.
CPU scheduling: how to serve the program execution (batch, multiprogramming, timesharing, real time)
Memory management: how to allocate the memory
Swapping: how to move data between the main memory and secondary storage
I/O device drivers: how to operate I/O devices

4. File system: organize mass storage (disk) into files and directories
Utilities: date/time, accounting, file copy, etc.
Command interpreter: allow users to enter commands interactively
System calls: allow user programs access to OS services
Protection: keep processes (and users) from interfering with each other and system
Communication & Resource sharing: allow users/ processes to communicate (within a computer or over networks) and share resources (e.g. laser printers, disks, etc.)
Security: protect machines from intruders and unauthorized people.

5. Evolution of the Operating Systems
It is better tie up the evolution of the operating systems to the hardware, to give a true picture, as done by Tanenbaum.

6. First generation (1945-1955): characterized by vacuum tubes and plug-boards:
First Computers
All the programming was in machine language, often by wiring up plug-boards to control machine's basic functions. All the problems programmed were numeric calculations such as tables of sines and cosines. Punch-cards were introduced in 1950 which replaced plug-boards…

7. Second generation: Characterized by Transistors (1955-1965)
Batch systems,
Commercial use of computers has become possible. They were small enough and reliable enough to be manufactured and sold. They were kept in special dust-free air conditioned rooms and attended by operators only.
Use of high level languages such as FORTRAN has become possible, other than the assembly language.
Batch systems were order of the day as they allowed the best utilization of the equipment under the present condition (see Figure 1.2 and 1.3)
Examples: IBM 1401 and 7094.

8. Simple Batch Systems in more details
Are the first operating systems
The user submit a job (written on card or tape) to a computer operator
The computer operator place a batch of several jobs on a input device
A special program, the monitor, manages the execution of each program in the batch
Resident monitor is in main memory and available for execution
Monitor utilities are loaded when needed

9. The Monitor
Monitor reads jobs one at a time from the input device
Monitor places a job in the user program area
A monitor instruction branches to the start of the user program
Execution of user pgm continues until:
end-of-pgm occurs or error occurs
Causes the CPU to fetch its next instruction from Monitor

10. Job Control Language (JCL)
Is the language to provide instructions to the monitor
what compiler to use and what data to use
Each read instruction (in user pgm)
causes one line of input to be read
causes (OS) input routine to be invoked, to
check for not reading a JCL line
skip to the next JCL line at completion of user program

11. Batch OS
Alternates execution between user program and the monitor program
Relies on available hardware to effectively alternate execution from various parts of memory

12. Desirable Hardware Features
Memory protection
do not allow the memory area containing the monitor to be altered by user programs
Timer
prevents a job from monopolizing the system
an interrupt occurs when time expires
Privileged instructions
can be executed only by the monitor
an interrupt occurs if a program tries these instructions
Interrupts
provides flexibility for relinquishing control to and regaining control from user programs

13. Third generation computers, Characterized by Ics (1965-1980): Multiprogramming
The most representative machine of this generation is IBM 360, which has combined the numeric and non-numeric applications in one machine.
The operating systems were written in assembly language and have soon become very bulky as they have to work both on newer as well as older systems.
One important break from the second generation was “multiprogramming” which was very important for the utilization of the machine. The CPU could be multiplexed between the programs residing in the memory. When one tied up with IO, the other would be tied up with execution.
Another important characteristic of the third generation is “SPOOLING”, which comes from “simultaneous peripheral operation on line.” The job flow from card readers to disk, IO from/to disk, print-out from disk to printers could take place concurrent with CPU operation.
The desire for quick response time speeded up the birth of “timesharing” systems. This has made interactive on-line of systems order of the day.
Examples: IBM 360, 370, 43xx, and 30xx series.

14. Multiprogrammed Batch Systems
I/O operations are exceedingly slow (compared to instruction execution)
A program containing even a very small number of I/O ops, will spend most of its time waiting for them
Hence: poor CPU usage when only one program is present in memory
If memory can hold several programs, then CPU can switch to another one whenever a program is awaiting for an I/O to complete
This is multitasking (multiprogramming)

15. Requirements for Multiprogramming
Hardware support:
I/O interrupts and (possibly) DMA
in order to execute instructions while I/O device is busy
Memory management: several ready-to-run jobs must be kept in memory
Memory protection (data and programs)
Software support from the OS:
Scheduling (which program is to be run next)
To manage resource contention

16. MULTICS
Multics project was initiated to integrate all the novelty of multiprogramming, timesharing, and user requirements into one operating system. Unfortunately the project was never fully complete, initiated by AT&T
Minicomputers have allowed computers to enter small business and scientific institutions: PDP, Interdata, VAX, Prime, etc.
People who have taken part in MULTICS were able to come up MULTICS like but far simpler (far less ambitious) operating system- UNIX for minicomputers, with no written objectives.

17. Time Sharing Systems (TSS) in summary
Batch multiprogramming does not support interaction with users
TSS extends multiprogramming to handle multiple interactive jobs
Processor’s time is shared among multiple users
Multiple users simultaneously access the system through terminals
Because of slow human reaction time, a typical user needs 2 sec of processing time per minute
Then (about) 30 users should be able to share the same system without noticeable delay in the computer reaction time
The file system must be protected (multiple users…)

18. Fourth generation (1980- ): Personal Computers, Workstations, and Distributed Systems
Chip technology (development in LSI) allowed mass production, thus the present state of cheap but powerful computers finding their way to homes as well as offices.
The dominant operating systems: UNIX, NT, WindowsXX, etc.
Networking (TCP/IP) allowed operating systems like UNIX to evolve into network operating systems, in network environment.
Distributed operating systems: Multiple processors will appear one single system to the user.

19. Fifth generation (1990- ): Intelligent systems
(still to come!), distributed computing, parallel computing, very high speed communication (order of many giga bits per second), www, mobile systems