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Encyclopedia > Channel command word

A channel command word (CCW) is a command used in the Channel I/O subsystem architecture of the ESA/390 IBM mainframe architecture to initiate a command on a channel-attached device. In computer architecture, 31-bit is an adjective used to describe integers, memory addresses or other data units that are at most 31 bits (just shy of 4 octets) wide, or to describe CPU and ALU architectures based on registers, address buses, or data buses of that size. ... An IBM mainframe is a large, high-performance computer made by International Business Machines (IBM). ...

In ESA/390 terminology, a channel is a parallel data connection inside the tree-like or hierachically organized I/O subsystem. In System/390 I/O cages, channels either directly connect to devices which are installed inside the cage (communication adapter such as ESCON, FICON, Open systems adapter or they run outside of the cage, below the raised floor as cables of the thickness of a thumb and directly connect to channel interfaces on bigger devices like tape subsystems, Direct access storage devices (DASDs), terminal concentrators and other ESA/390 systems. ESCON (or Enterprise Systems Connection) is a serial interface for connecting IBM mainframe storage and other devices. ... In computing, FICON (Fiber Connectivity) is an input and output protocol used in IBM mainframe computers and storage arrays. ... Raised floors are used in office buildings with a high requirement for servicing, such as IT data centers, and cary cables electrical supply and sometimes air conditioning server racks or desks in landscaped office areas. ... A direct access storage device, or DASD (IPA ) is a form of magnetic disk storage, historically used in the mainframe and minicomputer (mid-range) environments. ...

CCWs are used in channel programs which are created by ESA/390 standalone programs (e.g. diagnostic programs) and operating systems (e.g. MVS) for programming the I/O subsystem to do I/O with devices connected thru channels. The channel programs are then executed by the system assist processors SAPs and upon completion of the program, the SAP signals the initiator of the program. All data transfer is handled by the SAPs without requiring interruption of the application processor. This sounds similar to Direct Memory Access on Personal computers but goes much further, e.g. on Linux/390, for formatting a whole track of a DASD, only one channel program is issued. The program is executed by the SAP while the application processor is free for other work. MVS (Multiple Virtual Storage) was the most commonly used operating system on the System/370 and System/390 IBM mainframe computers. ... Sap is: Look up sap in Wiktionary, the free dictionary. ... Direct memory access (DMA) allows certain hardware subsystems within a computer to access system memory for reading and/or writing independently of the CPU. Many hardware systems use DMA including disk drive controllers, graphics cards, network cards, and sound cards. ... Linux on zSeries is the preferred term for the Linux operating system running on IBM mainframes, especially zSeries servers. ...

Since all Devices on the ESA/390 architecture are connected by channels, all I/O requires the use of channel programs.

Common device commands include SenseID or Basic Sense. A format 0 CCW can be used for 24 bit data addresses, else a format 1 CCW has to be used.

  Results from FactBites:
Method and system for predefined suspension and resumption control over I/O programs - Patent 5613163 (12341 words)
The suspend function is signaled to the channel subsystem during channel program execution by specifying a suspend flag in one of the channel command words within the channel program.
The command in this channel command word is not sent to the input/output device, and the device is signaled that the chain of commands is concluded and the input/output operation is complete.
Command chaining is suppressed unless the SLI flag in the CCW is one and the chain data flag is zero.
  More results at FactBites »



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