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

If a data transfer error occurs while reading aself-describing CCW, data can be placed into the'wrong core storage locations, causing destruction ofdata when the CCW is executed. If an error in aCCW is capable of causing subsequent commandchaining, it may cause chaining to a write command;the result can be alteration of data at the I/O device.SkipSkipping suppresses transfer of data to core storageduring a read or sense operation. Skipping is initiatedby turning on the skip flag in the CCW (bit 4 ofthe second word).The skip function affects only the handling ofdata by the channel. The operation at the controlunit and device proceeds normally, and data istransferred to the channel. The channel keeps updatingthe byte count, but does not transfer the datato core storage. If the chain data flag is on, anew CCW is fetched when the byte count reaches0. Normal operation is resumed if the skip flagin the new CCW is off.Skipping is normally used in conjunction withdata chaining to place only selected portions of datafrom an I/O device into core storage. To accomplishthis, the ability to data-chain within a datafield is required. Therefore, the skip function isnegated with devices, such as the 2311, with datarates that restrict data chaining within data fields.Channel Status WordThe channel status word (CSW) provides a meansfor the program to determine the status of an I/0device or the conditions under which an I/O operationhas been terminated. The CSW is formed, orparts of it replaced, in the process of I/O interruptsand during execution of an XIO initialize write (startI/O).The CSW consists of four 16-bit words:Word 1 -- Selector channel status.Word 2 -- Unit address-unit status.Word 3 -- Command address.Word 4 -- Byte count.Selector Channel Status (CSW Word 1)An XIO sense device with modifier bits 13 and 14both off reads the selector channel status portion ofthe CSW into the accumulator. Whether an indicatorbit will be on or off is determined by conditions existingat the channel, control unit, or device as aresult of processing an I/O instruction.Channel status is updated as it occurs and maybe sensed by the program at any time. Figure 109shows the various channel status indicators and denotesthose indicators that cause interrupts.NOT OPERATIONAL: This indicator turns on, causingan interrupt, if: (1) the control unit addressed byan XIO is not attached to the system, or (2) the deviceaddress specified by an XIO exceeds the numberthat the control unit is designed to handle.UNIT STATUS PENDING: This indicator turns on,causing an interrupt, when the channel acceptsstatus presented by a control unit or device. Theconditions causing status to be presented to the channeldepend on the control unit and device.Once status is accepted by the channel, pollingis suppressed (preventing overrun of status fromanother control unit or device) until it is reinitiatedby the program. This allows the program to (1)determine the cause of the interrupt, (2) sense theunit address-status, and (3) reset the unit statuspending indicator prior to reinitiation of polling.The unit status pending indicator can be resetonly by sensing the unit address-status portion of theCSW with an XIO sense device with reset (modifierbits 13, 14, and 15 equal 011).It should be noted that the unit address-statusportion of the CSW is valid only when sensed inresponse to a unit status pending interrupt.PROGRAM CONTROL INTERRUPT: This indicatorturns on, causing an interrupt, when the channelfetches a channel command word (CCW) in which theprogram control interruption flag is on.PROGRAM CHECK: This indicator turns on, causingan interrupt, if either of the following conditionsis detected:1. An input/output control command (IOCC) has afunction code that is invalid for the selectorchannel. Valid function codes are: control(halt I/O) 100, initialize write (start I/O) 101,sense device 111, and sense interrupt level 011.2. An IOCC or CCW has incorrect parity.If a program check occurs during initiation of anoperation, execution of the operation is suppressed.If a program check is detected after a device hasbeen started, the device is signaled to terminate theoperation.A program check condition causes commandchaining to be suppressed.CHANNEL DATA CHECK: This indicator turns onif:Selector Channel 205