Revision IT

Revision IT

1. Operating systems

   • A computer will not run without an operating system, as it is the program that controls the running of the computer.
     • Applications software is specific to a particular operating system and so cannot run under another one. For example, a Windows program cannot be run on a computer with a UNIX operating system.
       • Multi-access systems
         • Many users are able to work on the same computer at the same time. A large mainframe computer may have a number of workstations connected directly to it. For example, hundreds of workstations may be spread out in different rooms and floors of a large office block and the mainframe may be in the basement.
           • The workstations consist of input devices, such as a keyboard and a mouse, and a monitor but there is no computer unit. This is because the user of the workstation is actually using the mainframe computer. Networks are now replacing many multi-access systems.
           • Real-time transaction processing
             • Another type of real-time system involves transactions. Every time an item is sold or bought, or a booking is made, the data from the user is processed immediately. This may involve updating some data held in a record of a database. The transactions are processed in the order they are received and each transaction is completed before the next one is processed
             • An example of a real-time transaction processing system is a theatre-seat booking system.
   • An operation loads and runs programs, It maximises the use of memory (RAM) and It handles inputs and outputs If interrupts occur, then it deals with them It maintains the security of the system

2. Single-program & Batch-processing systems

   • Single-program systems – some simple computers, the operating system only runs one program at a time. When one program has completed its processing, then the next program is loaded up and run. This sort of operating system is slower than multi-programming systems it is not found much these days.
     • Batch-processing systems – a program, together with an data it needs, is called a job. In a batch-processing system, jobs that are waiting to be run are collected and stores in a job queue and then are all processed together. E.G an electricity bill program and the data about how many units of electricity a customer has used.
       • Batch-processing systems are often used in businesses where there is no urgency that jobs are processed. E.g. monthly payroll in a company – the jobs are collected together at the end of each month and run at night to calculate the wages of each employee and produce pay slips.
         • Distributed systems
           • In a distributed system, the processing and resources are shared between a number of different computers. It is the operating system that has to make sure that each of the computers in the system communicates properly.
             • Real-time systems
               • Sensors monitor processed by taking environmental measurements, sending them to the computer as input. The computer processes the data immediately, takes action if necessary. The system can react fast enough to be able to take actions in abnormal situations such as a machine breaking down or even a fire.
                 • Data is processed as soon as it is input into a real-time system. A process control system is an example of a real-time system.
                   • Another example of a real-time system is a flight simulator used to train pilots. Real-time operating systems can be found in embedded computers. The operating system is stored in a ROM chip and runs and manages a single program that operates such as a washing machine or a digital camera
           • The user feels that a single computer is being used but, in reality, processing may be shared between several computers in the system and the user is unaware of which ones are being used.
             • Parallel-processing systems
               • Some larger mainframe computers have more than one processor. These are large mainframes with hundreds or even thousands of processors. Large tasks are completed faster if they are shared between a number of processors.
                 • Special programming languages have been developed to write applications for parallel-processing systems and complex operating systems are needed to share out the tasks to the processors and coordinate their activities.
                   • Interactive systems
                     • A user inputs a command to a computer, which may then respond with a request for further input of data. This is interactive computing: it is similar to a conversation between a user and the computer. An example of interactive computing can be found when you take case out an ATM.
                   • System security
                     • One of the tasks of an operating system is to maintain security. This means protecting the system from malicious or accidental damage by users. Damage may involve changing a data file or even deleting it.
                       • Security on single-user systems is different from security on multi-user systems. Single-user security involves protecting files of data from accidental change or deletion and there is less need for protection from unauthorised users than there is on a multi-user system or network.
                 • Computers that process large quantities of data, such as weather-forecasting systems, use parallel processing.
   • The advantages of a batch-processing system are that: Processing can be done at off-peak teams. No human intervention is required once the batch has been started.

3. Multi-programming systems

   • The computer holds more than one program in its memory and gives each one a small amount of processing time in turn. This happens so fast that it seems to the user that the computer is running more than one program at a time.
     • It is the job of the operating system to share the resources, such as processing time, so that each program is eventually completed successfully. The operating system decided the order in which to run the programs and makes sure that no program is ignored completely.
       • The advantage of multi-programming systems is that more programs are processed in less time than in an operating system that only processes one program at a time.