Multiprogramming OS is an ability of an operating system that executes more than one program using a single processor machine.More than one task or program or jobs are present inside the main memory at one point of time.Buffering and spooling can overlap I/O and CPU tasks to improve the system performance but it has some limitations that a single user cannot always keep CPU or I/O busy all the time.To increase resource utilization, multiprogramming approaches.Operating SystemJob1Job2---Job NEmpty spaceThe OS could pick and start the execution of one of the jobs in memory, whenever the jobs does not need CPU that ... Read More

To improve the performance and to avoid the CPU idle time the operating system uses two approaches which are explained below in detail.BufferingIt is a method of overlapping input, output and processing of a single job.After reading the data, the CPU is about to start operating on it, the input device is instructed to begin the next input immediately. The CPU and the input device are both busy. By the time the CPU is ready for the next data item the input device will have to finish its reading.The CPU then begins processing the next data while the input device ... Read More

There are different operating systems for different kinds of computers and processors. They are divided into different categories.Single processor systemSingle CPU or processor that manages the computer and it runs on a different operating system, and performs a number of tasks using one processor called a single processor system.In a single processor system different types of tasks can be performed like disk control, user control, system control etc.The figure given below depicts the single processor system −Multiprocessor systemWhen one task can be performed by using multiple processors then it is called a multiprocessor system.In a multiprocessor system a number of ... Read More

The design of the operating system should be defined by the goals and specifications which are affected by hardware and systems. Thus there would be user goals and system goals for an OS.User GoalsThe user goals or requirements should be as follows −The OS usage should be convenientShould be easy to use and easy to learnShould be safe and secure to use and information handling, and security should be robustShould be fast in response to the user requestsSystem GoalsThe system design requirements should be as follows −The OS should be easy to design, maintain and also to implement.Updates should be ... Read More

System call provides an interface between user program and operating system. It is represented as follows −When the user wants to give an instruction to the OS then it will do it through system calls. Or a user program can access the kernel which is a part of the OS through system calls.It is a programmatic way in which a computer program requests a service from the kernel of the operating system.Program executes in two modes as follows −User mode − Cannot access any hardware resources, which perform only the user operations.Kernel mode − Can access hardware resources like RAM, ... Read More

The different system calls are as follows −System calls for Process managementSystem calls for File managementSystem calls for Directory managementLet us understand them one by one.System calls for Process managementA system is used to create a new process or a duplicate process called a fork. The duplicate process consists of all data in the file description and registers common. The original process is also called the parent process and the duplicate is called the child process.The fork call returns a value, which is zero in the child and equal to the child’s PID (Process Identifier) in the parent. The system ... Read More

System call mechanism is one of the techniques by which a user program requests for services from the kernel. System calls always provide an interface to the services that are available by an operating system.Let us see the step by step explanation of system call mechanism as given below −Step 1 − A process that is running a user program in user mode wants to execute a read instruction a file and it has to execute a trap instruction to transfer control to the operating system.Step 2 − The read operation in system call has three parameters which are as ... Read More

Locality of reference refers to the tendency of the computer program to access the same set of memory locations for a particular time period. The property of Locality of Reference is mainly shown by loops and subroutine calls in a program.On an abstract level there are two types of localities which are as follows −Temporal localitySpatial localityTemporal localityThis type of optimization includes bringing in the frequently accessed memory references to a nearby memory location for a short duration of time so that the future accesses are much faster.For example, if in an instruction set we have a variable declared that ... Read More

The Computer memory hierarchy looks like a pyramid structure which is used to describe the differences among memory types. It separates the computer storage based on hierarchy.Level 0: CPU registersLevel 1: Cache memoryLevel 2: Main memory or primary memoryLevel 3: Magnetic disks or secondary memoryLevel 4: Optical disks or magnetic types or tertiary MemoryIn Memory Hierarchy the cost of memory, capacity is inversely proportional to speed. Here the devices are arranged in a manner Fast to slow, that is form register to Tertiary memory.Let us discuss each level in detail:Level-0 − RegistersThe registers are present inside the CPU. As they ... Read More

Instruction cycle consists of fetch, execute and interrupt stage show in below diagram −If any interrupt occurs, it is indicated by an interrupt flag. The CPU will go to interrupt handler routine. Interrupt handler then checks the type of interrupt and executes the appropriate function. It involves overhead but still better than the CPU waiting for I/O completion or other activities.Interrupt handler activates most prior able activity first and later deferrable part will be handled.ExampleBlock of data arrives on the network line.Kernel marks the presence of data (urgent part) and gives the CPU back to the process that was running ... Read More