A formal description of SYSTEM/360

one device and the initiation of a new sequence, the channel willservice requests from devices that have outstanding status informationto transmit.A selector channel always works in the “burst” mode, and thecharacteristic operation of a multiplexor channel is the “nmltiplex”mode. However, a multiplexor channel is said to operatein the burst mode if a particular device monopolizes its facilitiesfor more than approxinmtely 100 lnicroseconds. This does notimply a difference in channel operation, but relates only to thequestion of availability, since efficient operation requires that theCPU be able to distinguish between a current operation thatis likely to keep the channel busy for a relatively long time andone that will soon be over. In term of the variables used here,it is a question of how long B: remains equal to I.The interface between a channel and its attached control unitsor devices is represented by thc variables U arid P‘. The suffix09/ U is a 1)us that carries a byte of infornlatiorl (ox/Bc) and abit ( e) for odd parity. The prefix ax/ U comprises three tagbits which specify the type of information on the bus. When theinterplay bit Ui = 1, information on the bus is outgoing (fromchannel to control units) and thc tags are, in order, command-outservice-out, and address-out; when = 0, information is ingoingand the tags are status-in, seruice-i’n, and address-in. In operationonly one tag may be set at a time, and its significance depends,in part, on the state of the channel. The remaining elelnents of Uare concerned with establishing and holding a logical connectionbetween the channel and a particular control unit. They aresuppress-out ( U:) and operational-out (e,) which are set onlyby the channel, and operational-in (&), which is set only by thecontrol unit. The polling line P is a vector that has a positionfor each control unit on the interface, in the order in which theyare connected. Pi is called select-out and Pwc, where w, has a valueequal to the number of control units connected, is called select-in.’Control units may be physically separate from or integralwith their associated devices; a multiplicity of similar devicesmay be connected to a single control unit or, conversely, a devicemay communicate with more than one control unit. Although thepresent formal description does not include details of this sideof the interface, it will sometimes be necessary to refer to it inthe text. The term “device” will usually be used when speakingof specific tasks that devices perform once an active connectionhas been established between device and channel, and “controlunit” will usually be used when emphasis is on the establishmentof such a connection.The fornml description shows the generation and processingof all results of channel operation to which a programmer hasaccess, including the data transferred to or from memory, a conditioncode setting (p,,,,,7) for 1/0 operations, an interruptioncode setting (p,18(,6,), and a channel status word (CSW) whichcomprises CAW“, a device status byte a*//, a channel status