A formal description of SYSTEM/360

Table 11Device status bytes stored in CSWdevice status bits channel activity CH andline on which storedcontrolatten- status unit channel device unit unit intertionmodifier end busy end end check exception ruption TIO SIO HIOtion status from a device, SIO or TIO to a device that is workingor holding status information, or a rejected SIO. The status bytessubsequently stored in line 158 or line 159 will indicate thesituation: no device status at this point should contain channelend; a busy device will return the busy bit to SIO or TIO; a deviceholding status information will include the busy bit in responseto SIO but not TIO (in both cases the device will be cleared, as ininterruption service); and the status for a rejected SIO will containa unit check or unit exception.All possible configurations of device status bytes that mayappear in a CSW are shown in Table 11. As usuaI, the symbol 2 denotesthe possibility of a zero or one in that position; the first line,for example, represents 8 possible status bytes. Altogether, 94possibilities are represented.At line 161 an HIO from line 90 may be present as well as SIOor TIO from line 156. In all cases a condition code of zero is specified.HIO leaves immediately at line 162, TIO leaves at line 164 aftersignalling on the interface. If this is SIO, a command has been successfullyinitiated and the channel and subchannel state variablesare set in line 165 to reflect this. For a multiplexor channel thetimer is started. If the status byte is all zero (line 129), service-outis signalled on the interface (line 130) and the data transfer phaseis entered at line 59. A non-zero status byte for SIO at line 129indicates an immediate command, usually a control operationthat can be executed without help from the channel. It also meansthat command chaining was indicated by the bits in c". Bothof these factors were checked in line 156, which strongly resemblesline 67, and the branch from line 129 therefore goes to line 68to prepare for command chaining.If commandchaining had been in progress at line 127, thebranch to line 128 would have been followed. Here, either a nonzerostatus byte or a program check will cause a branch to line 67where the choice is made between termination and further com-234 A. D. FALKOFF, K. E. IVERSON, AND E. H. SUSSENGUTH