Systems Reference Library - All about the IBM 1130 Computing ...

Page of GA26-5918-8Revised July 14, 1971By TNL: GN26-0269Sense DeviceInitialize Write (Start I/O)15 0 4 8 II 12 13 14 151 0 0 1 0 1 1 1 10 = Al low pol I ing1 = Suppress polling15 0 4 II 12 15CCW Address [1.0.0.1.011. 0.11Control unitaddress00 = Channel status word 101 = Channel status word 21 0 = Channel status word 31 1 = Channel status word 40 = No indicator reset1 = Indicator resetDevice addressI13019001130198.41This command causes one of the four words comprisingthe selector channel status word (CSW) to beread into the accumulator. Modifier bits 13 and 14specify the word to be read, and modifier bit 15 controlsreset of the program resettable indicators in theCSW. If modifier bit 15 is on, the reset function isperformed. If modifier bit 15 is off, the reset functionis not performed. (The individual words of theCSW, interrupt indicators, and program resettableindicators are described under "Channel StatusWord. ")Modifier bit 12 of an XIO sense device controlsthe channel polling function. Polling is the ability ofthe selector channel to acknowledge a request forservice from a control unit. If modifier bit 12 is on,polling is suppressed. If modifier bit 12 is off,polling is allowed.Aside from XIO sense device with modifier bit12 on, polling also is suppressed by: (1) initiationof a start I/0 operation (XIO initialize write), or(2) the selector channel accepting a request forservice from a control unit.Regardless of the cause of the suppression, pollingremains suppressed until: (1) an XIO sense devicewith modifier bit 12 off is given, (2) a halt I/O(XIO control) is given, or (3) RESET on the P-Cconsole is pressed.A request for service from a control unit isusually the result of device end or attention turningon in the unit status portion of the CSW. If polling isnot suppressed and a control unit requests service,the unit address-status portion of the CSW is transferredfrom the control unit to the selector channel,and a unit status pending interrupt is given to theP-C. The selector channel suppresses further pollinguntil the program reads the status word andreinitiates polling.Polling should be suppressed when any operationis in progress in the channel or control unit. Pollingshould always be enabled after the last interrupthas been serviced, unless another operation is beingstarted. In that case, polling must be suppressed.This allows the selector channel to acknowledge arequest from the control unit, such as attention or adevice becoming ready.Polling must also be enabled after receiving abusy in response to a start I/O. This allows theselector channel to acknowledge the request (fromthe control unit) that indicates when the device orcontrol unit is no longer busy.To prevent losing the interrupts that occur atthe end of busy, a delay of approximately 30 tisshould be provided between the XIO sense that suppressespoll and the XIO sense that checks for aninterrupting condition before a start I/O. The selectorchannel may have started to acknowledge arequest from a control unit at the time polling wassuppressed. If a start I/O is performed beforesensing for this interrupting condition, the unitstatus, device, and control unit address will be lost.Polling should be reinitiated following completionof a start I/0 operation by issuing an XIO sensedevice with modifier bit 12 equal to 0.This command is used as a start I/0 to initiate allselector channel I/0 operations. The address fieldcontains the core storage address of a channel commandword (CCW). The CCW specifies the operationto be performed by the channel, control unit, anddevice as well as the core storage area associatedwith the operation. IOCC modifier bits 8 through 11select the control unit, and modifier bits 12 through15 select the device to which the operation pertains.Once the IOCC area code, function, and modifiershave been sent to the selector channel and the CCWaddress has been loaded into the data channel addressregister (CAR), the P-C is released. The selectorchannel continues the operation by means of datachannel (cycle steal) operations by fetching theaddressed CCW from core storage.If an XIO initialize write (start I/0) is givenwhile the selector channel is busy, it is ignored andno indication is given to the program.Selector Channel 199