Systems Reference Library IBM System/360 Principles of Operation

efficient switching from one program to another, as well as for the relocation <strong>of</strong> programs in storage. To the prob1em programmer, the supervisory program and the equipment are indistinguishable. <strong>System</strong> Alerts The interruption system permits the CPU automatically to change state as a result <strong>of</strong> conditions arising outside <strong>of</strong> the system, in 1/0 units, or in the CPU itself. Interruption switches the CPU from one program to another by changing not only the instruction address but all essential machine-status information. A storage protection feature permits one program to be preserved when another program erroneously attempts to store information in the area assigned to the first program. Protection does not cause any loss <strong>of</strong> performance. Storage operations initiated from the CPU, as well as those initiated from a channel, are subject to the protection procedure. Programs are checked for correct instructions and data as they are executed. This policing-action identifies and separates program errors and machine errors. Thus, program errors cannot create machine checks since each type <strong>of</strong> error causes a unique interruption. In addition to an interruption due to machine malfunction, the information necessary to identify the error is recorded automatically in a predetermined storage location. This procedure appreciably reduces the mean-time to repair a machine fault. Moreover, operator errors are reduced by minimizing the active manual controls. To reduce accidental operator errors, operator consoles are 110 devices and function under control <strong>of</strong> the system program. Multisystem <strong>Operation</strong> Several models <strong>of</strong> <strong>System</strong>/<strong>360</strong> can be combined into one multisystem configuration. Three levels <strong>of</strong> communication between CPu's are available. Largest in capacity, and moderately fast in response, is communications by means <strong>of</strong> shared 1/0 device, for example, 6 a disk file. Faster transmission is obtained by direct connection between the channels <strong>of</strong> two individual systems. Finally, storage may be shared on some models between two cpu's, making information exchange possible at storage speeds. These modes <strong>of</strong> communication are supplemented by allowing one CPU to be interrupted by another CPU and by making direct status information available from one CPU to another. Input/Output Channels provide the data path and control for 1/0 devices as they communicate with the CPU. In general, channels operate asynchronously with the CPU and, in some cases, a single data path is made up <strong>of</strong> several sub channels. When this is the case, the single data path is shared by several low-speed devices, for example, card readers, punches, printers, and terminals. This channel is called a multiplexor channel. Channels that are not made up <strong>of</strong> several such subchannels can operate at higher speed than the multiplexor channels and are called selector channels. In every case, the amount <strong>of</strong> data that comes into the channel in parallel from an 110 device is a byte. All channels or subchannels operate the same and respond to the same 110 instructions and commands. Each 110 device is connected to one or more channels by an 110 interface. This 110 interface allows attachment <strong>of</strong> present and future 110 devices without altering the instruction set or channel function. Control units are used where necessary to match the internal connections <strong>of</strong> the 110 device to the interface. Flexibility is enhanced by optional access to a control unit or dcvice from either <strong>of</strong> two channels. Technology <strong>System</strong>/<strong>360</strong> employs solid-logic integrated components, which in themselves provide advanced equipment reliability. These components are also faster and smaller than previous components and lend themselves to automated fabrication.