Time-Sharing Systems

Time-Sharing Systems

Multiprogrammed batched systems provide an environment where the various system resources (for example, CPU, memory, peripheral devices) are utilized effectively. There are some difficulties with a batch system from the point of view of the user, however. Since the user cannot interact with the job when it is executing, the user must set up the control cards to handle all possible outcomes. In a multistep job, subsequent steps may depend on the result of earlier ones. The running of a program, for example, may depend on successful compilation. It can be difficult to define completely what to do in all cases.

Another difficulty is that programs must be debugged statically, from snapshot dumps. A programmer cannot modify a program as it executes to study its behavior. A long turnaround time inhibits experimentation with a program. (Conversely, this situation may instill a certain amount of discipline into the writing and testing of programs.)

Time sharing, or multitasking, is a logical extension of multiprogramming. Multiple jobs are executed by the CPU switching between them, but the switches occur so frequently that the users may interact with each program while it is running.

An interactive, or hands-on, computer system provides on-line communi­cation between the user and the system. The user gives instructions to the operating system or to a program directly, and receives an immediate response. Usually, a keyboard is used to provide input, and a display screen (such as a cathode - ray tube (CRT) or monitor is used to provide output. When the oper­ating system finishes the execution of one command, it seeks the next "control statement" not from a card reader, but rather from the user's keyboard. The user gives a command, waits for the response, and decides on the next com­mand, based on the result of the previous one. The user can easily experiment, and can see results immediately. Most systems have an interactive text editor for entering programs, and an interactive debugger for assisting in debugging programs.)

If users are to be able to access both data and code conveniently, an on-line file system must be available. A file is a collection of related information defined by its creator. Commonly, files represent programs (both source and object forms) and data. Data files may be numeric, alphabetic, or alphanumeric. Files may be free-form, such as text files, or may be rigidly formatted. In general, a file is a sequence of bits, bytes, lines, or records whose meaning is defined by its creator and user. The operating system implements the abstract concept of a file by managing mass-storage devices, such as tapes and disks. Files are normally organized into logical clusters, or directories, which make them easier to locate and access. Since multiple users have access to files, it is desirable to control by whom and in what ways files may be accessed.

Batch systems are appropriate for executing large jobs that need little interaction. The user can submit jobs and return later for the results; it is not necessary for the user to wait while the job is processed. Interactive jobs tend to be composed of many short actions, where the results of the next command may be unpredictable. The user submits the command and then waits for the results. Accordingly, the response time should be short—on the order of seconds at most. An interactive system is used when a short response time is required.