Operating Systems Principles (Introduction)

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The user did not interact directly with the computer systems. Rather, the user prepared a job-which consisted of the program, the data, and some control information about the nature of the job (control cards)-and submitted it to the computer operator. The job was usually in the form of punch cards. At some later time (after minutes, hours, or days), the output appeared. The output consisted of the result of the program, as well as a dump of the final memory and register contents for debugging. The operating system in these early computers was fairly simple. Its major task was to transfer control automatically from one job to the next. The operating system was always resident in memory. To speed up processing, operators batched together jobs with similar needs and ran them through the computer as a group.

Memory layout for batch system

Even a slow CPU works in the microsecond range, with thousands of instructions executed per second. A fast card reader, on the other hand, might read 1200 cards per minute (or 20 cards per second). Thus, the difference in speed between the CPU and its I/O devices may be three orders of magnitude or more.

The most important aspect of job scheduling is the ability to multiprogram. A single user cannot, in general, keep either the CPU or the I/O devices busy at all times.

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memory

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Job Scheduling

Job pool on Disk

Memory management Resource Protection

The idea is as follows: The operating system keeps several jobs in memory simultaneously. This set of jobs is a subset of the jobs kept in the job pool-since the number of jobs that can be kept simultaneously in memory is usually much smaller than the number of jobs that can be in the job pool. The operating system picks and begins to execute one of the jobs in the memory. Eventually, the job may have to wait for some task, such as an I/O operation, to complete. In a non-multiprogrammed system, the CPU would sit idle. In a multiprogramming system, the operating system simply switches to, and executes, another job. When that job needs to wait, the CPU is switched to another job, and so on. Eventually, the first job finishes waiting and gets the CPU back. As long as at least one job needs to execute, the CPU is never idle.

Memory layout for multiprogramming system

Multiprogramming is the first instance where the operating system must make decisions for the users. Multiprogrammed operating systems are therefore fairly sophisticated. All the jobs that enter the system are kept in the job pool. This pool consists of all processes residing on disk awaiting allocation of main memory. If several jobs are ready to be brought into memory, and if there is not enough room for all of them, then the system must choose among them. Making this decision is job scheduling, which is discussed later.

When the operating system selects a job from the job pool, it loads that job into memory for execution. Having several programs in memory at the same time requires some form of memory management.

In addition, if several jobs are ready to run at the same time, the system must choose among them. Making this decision is CPU scheduling.

Finally, multiple jobs running concurrently require that their ability to affect one another be limited in all phases of the operating system, including process scheduling, disk storage, and memory management.


Disadvantage: Multiprogrammed, batched systems provided an environment where the various system resources (for example, CPU, memory, peripheral devices) were utilized effectively, but it did not provide for user interaction with the computer system.



Problems: Job and CPU scheduling, Memory management, different tasks resources protection issues should be taken in account to keep the system effective.

1.2.1.3. Time Sharing Systems
Time sharing (or multitasking) is a logical extension of multiprogramming.


Advantage: The CPU executes multiple jobs by switching among them, but the switches occur so frequently that the users can interact with each program while it is running.


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Process concurrent execution

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Users

Virtual Memory

Paging

Memory management and protection



Disadvantage: Time-sharing operating systems are even more complex (difficult, expensive) than multiprogrammed operating systems.


Problems: Job and CPU scheduling, Memory management, User and Security management, Protection issues should be taken in account to keep the system effective.

An interactive (or hands-on) computer system provides direct communication between the user and the system. The user gives instructions to the operating system or to a program directly, using a keyboard or a mouse, and waits for immediate results. Accordingly, the response time should be short typically within 1 second or so.

A time-shared operating system allows many users to share the computer simultaneously. Since each action or command in a time-shared system tends to be short, only a little CPU time is needed for each user. As the system switches rapidly from one user to the next, each user is given the impression that the entire computer system is dedicated to her use, even though it is being shared among many users.
A time-shared operating system uses CPU scheduling and multiprogramming to provide each user with a small portion of a time-shared computer.

Each user has at least one separate program in memory. A program loaded into memory and executing is commonly referred to as a process. When a process executes, it typically executes for only a short time before it either finishes or needs to perform I/O. I/O may be interactive; that is, output is to a display for the user and input is from a user keyboard, mouse, or other device. Since interactive I/O typically runs at "people speeds," it may take a long time to complete. Input, for example, may be bounded by the user's typing speed; seven characters per second is fast for people, but incredibly slow for computers.

Rather than let the CPU sit idle when this interactive input takes place, the operating system will rapidly switch the CPU to the program of some other user.
In both, several jobs must be kept simultaneously in memory, so the system must have memory management and protection.

To obtain a reasonable response time, jobs may have to be swapped in and out of main memory to the disk that now serves as a backing store for main memory. A common method for achieving this goal is virtual memory, which is a technique that allows the execution of a job that may not be completely in memory. The main advantage of the virtual-memory scheme is that programs can be larger than physical memory. Further, it abstracts main memory into a large, uniform array of storage, separating logical memory as viewed by the user from physical memory. This arrangement frees programmers from concern over memory-storage limitations.

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  CPU utilization: the time the CPU is busy during some time period. We want to keep the CPU as busy as possible. CPU utilization may range from 0 to 100 percent. In a real system, it should range from 40 percent (for a lightly loaded system) to 90 percent (for a heavily used system).
  
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  Throughput: If the CPU is busy executing processes, then work is being done. One measure of work is the number of processes completed per time unit, called throughput. For long processes, this rate may be 1 process per hour; for short transactions, throughput might be 10 processes per second.
  
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  Turnaround time: From the point of view of a particular process, the important criterion is how long it takes to execute that process. The interval from the time of submission of a process to the time of completion is the turnaround time. Turnaround time is the sum of the periods spent waiting to get into memory, waiting in the ready queue, executing on the CPU, and doing I/O.