Cs238 Introduction to Operating Systems Dr. Alan R. Davis

Course Information Text: Applied Operating System Concepts. 1st Ed. Silbershatz, Galvin, and Gagne. John Wiley & Sons, Inc Grading: 2 Tests (25% each), a Final Exam (50%), and a Project (50%) Goals: be able to define and give examples of OS terminology; be able to describe, diagram, outline OS structures; be able to describe, implement, design OS algorithms

Operating Systems What is an operating system? An operating system is a program that makes the computer perform tasks for the computer user. An operating system controls and coordinates the use of the hardware among the various applications programs and users. An operating system is the one program always running on the computer. An operating system provides the environment within which programs are executed.

Operating Systems An operating system is a resource allocator And it is a control program CPU time Memory space File storage space I/O devices The OS manages these resources, allocating them to specific programs and users. It controls the execution of user programs. It controls the I/O devices

Examples of Operating Systems What operating systems are you familiar with? Windows? –Windows 98, Windows NT, Windows 2000 –Windows Me, Whistler Unix? – Linux Older operating systems? – Windows 3.1, Windows 95 – MS-DOS – Multics

Examples of Operating Systems cont’d What new operating systems have you heard of? Whistler JavaOS Microsoft Windows CE PalmOS AppForge vxWorks i-opener BEOS

Examples of Operating Systems cont’d Learn from the past Atlas, 1958, virtual memory, Univ Manchester XDS-940, 1965, time-shared, UC Berkeley THE, 1968, layer structure, concurrent processing RC4000, operating system kernel CTSS, compatible time-sharing system, MIT MULTICS, 1966, time-sharing, MIT OS/360, 1966 Mach

Introduction and Overview cont’d Computer architecture and operating systems influence each other. Change one, you need to change the other. Operating systems and applications programs are not always clearly separate. Efficient operating systems require efficient algorithms. Operating system features are developed to solve simple(?) problems. Complexity usually comes from the details.

Simple Batch Systems If computers are extremely expensive... ? Hire a professional to run them, and keep the users away. How do the users use the machine then? They give their programs to the professional operator. How does he know what the program is going to need to use? The programmer must supply the information. Job Control Language, JCL

Simple Batch Systems cont’d What information should be in the JCL? Accounting info... user name, dept, etc. What language is to be used. Where the program starts and stops. Where the data starts and stops.

Simple Batch Systems cont’d How do you reduce setup time? The operator must take the program from the user, look at the instructions, load the required compiler, compile the program, load the required assembler, assemble the program, load the data, run the program, check to see how the program stops (normally or abnormally), output the results, or output a dump of memory, etc. By placing all jobs requiring the same compiler together, some of this setup time can be eliminated. This is called batch processing.

Simple Batch Systems cont’d Programs used to be placed one instruction at a time onto punched cards, using a card punch machine. The cards were then placed into a card reader, and the information was sent to the CPU of the computer. Output would then go to a line printer. Card readers and line printers were very slow compared to CPU speeds. The CPU would often have to wait for more cards to be input, or for data to be printed.

Simple Batch Systems cont’d How do you decrease I/O time? Read and write the data to a buffer. Keep reading data to the buffer while the CPU is working on previous input. Keep printing data from the buffer while the CPU is working on subsequent instructions. This process is called spooling, or simultaneous peripheral operation on-line. A tape drive or disk drive could be used for spooling. The drive hardware was usually faster (and more expensive) than the card reader or line printer.

Simple Batch Systems cont’d What do you do with all those jobs on the disk? Spooling creates a set of jobs, called a job pool, on the external memory device. A human operator is too slow to organize them into proper batches. We need a program (operating system) on the CPU to handle job scheduling. This operating system has to decide which job to move from external memory to main memory for processing.

Multiprogrammed Batch Systems One user can’t keep the CPU busy enough... So we can let multiple users place their jobs into the job pool. If main memory is large enough (which by this point it is) the operating system can move several jobs from the job pool into main memory (job scheduling). Now the operating system must also decide which job to run next (only one job can run at a time). This is called CPU scheduling.

Multiprogrammed Batch Systems But the CPU is still waiting if Jobx is doing I/O... We can let the operating system halt Jobx briefly, and give the CPU resource to Jobz.

Multiprogrammed Batch Systems Won’t everything get mixed up? We need to make sure that Jobx does not interfere with or destroy Jobz or its data. Therefore we need the operating system to manage memory (memory management) and separate the multiple programs in memory (protection).

Time-Sharing Systems How do you reduce turn-around time? Multiprogrammed batched systems effectively used the CPU and other resources, except for the “programmer resource”. Users/programmers had no contact with their work until it was over. Errors could take many runs to correct. Programs could not interact with the user. Give the user access to the machine while the program is running.

Time-Sharing Systems cont’d Access requires hardware... keyboards, terminals, etc. Access requires software... the operating system must now control/allow the users to communicate with their programs. We are still multiprogramming, but the switches between programs occur much faster. We call this multitasking. Control statements are not read from the cards anymore, but from the user at the keyboard.

Time-Sharing Systems cont’d How do we access the data and the code conveniently? We now need an on-line file system. We have source files, object files, data files Data files can be numeric, alphabetic, alphanumeric Files can be free form or rigidly formatted The operating system implements the abstract concept of a file Files can be organized into directories They can have access rights

Time-Sharing Systems cont’d Time-sharing systems were developed to provide interactive use of a computer system at a reasonable cost. A process is a program that is loaded into memory and is executing. Time-sharing systems didn’t become popular until the early 1970s. Earlier batch systems were modified... IBM’s OS/360 gained the Time-Sharing Option (TSO)

Personal Computers Advances in hardware in the 1970s allowed the invention of personal computers Operating systems were key to the success of these machines, since they were designed to be easy to use. Many concepts of previous mainframe operating systems were adapted to personal computers. Some mainframe concepts were not adapted in the early PCs because the hardware support did not yet exist.

Personal Computers cont’d PCs have arguably been the driving force of change in both hardware and software in the last few decades. Apple’s operating system and Microsoft Windows have radically changed the way that users interact with computer systems. Because of the huge numbers of users of PCs operating system developments are now front page news.

Parallel Systems Multiprocessor systems, currently usually servers, have more than one CPU. Operating systems must be able to manage this hardware. Since these system often share the bus, the clock, and sometimes memory and peripheral devices, they are called tightly coupled systems.

Parallel Systems cont’d Parallel systems are designed to increase throughput. However, n CPUs do not accomplish n times the work because of the overhead of cooperation. Parallel systems are also designed to improve reliability. If one processor fails, then the system can continue with the remaining ones.

Parallel Systems cont’d In a symmetric multiprocessing system each processor runs an identical copy of the operating system. In an asymmetric multiprocessing system one processor (the master) controls the system, the others either have predefined tasks or look to the master processor for instructions. SunOS Version 4 provides asymmetric multiprocessing. SunOS Version 5 (Solaris 2) is symmetric.

Distributed Systems Multiple processors need not be inside the same system. A loosely coupled or distributed system is one in which each processor has its own memory and clock and communicates with the others through various means such as high-speed buses, telephone lines, network cables. The processors may also vary in size, type, and function.

Distributed Systems cont’d Distributed systems allow resource sharing, computation speedup, reliability, and communication.

Real-Time Systems Real-time systems are used when there are rigid time requirements on the operation of a processor or the flow of data. Often used in control devices hardware systems such as automobile engines, weapons systems, home appliances, or real-time stock quotes. Hard real-time systems guarantee that critical tasks complete on time. Soft real-time systems give priority to critical real- time tasks.