1. xerox 560 computer system - The UK Mirror Service

I/o subchannel (i. e., data transfers between the deviceand data buffers, updati ng the memory byte address andbyte count, and functions as specified by the control flags)are performed in a normal manner.For input operations, the Skip flag (in conjunction withdata chaining) provides the capabi lity to selectively readportions of a record.For output operations, the lOP wi II generate and transmitzeros (XIOOI) unti I the byte count is reduced to zero. Thus,for example, if the 10CD contains a Punch Binary order, abyte count of 120, and the S flag is coded as.a 1, a blankcard may be punched without accessing main memoryfor data.WRITE KEYThis four-bit field (bit positions 40-43), if required, maybe coded with an appropriate write key. During input operationsand providing the Skip control flag is coded as aD,the lOP will access main memory and furnish a memory unitwith up to four bytes of data or information accompaniedwith a four-bit write key. If the write key matches thepreassigned write lock for the memory word location accessed,or if either the key or lock has a value of 0000,the memory unit accepts and stores the information. If thewrite key does not match the write lock, and neither thekey nor the lock has a value of 0000, the memory unit rejectsthe information, does not disturb the previous content,and transmits a Write Lock Violation (WLV) signal to thelOP. The write key/write lock relationship is comparedevery time a memory word location is accessed for storingdata or information. (Note: The write key/write lock relationshipmay change during an input operation when thebyte address is incremented (or decremented) across a memorypage boundary. )As long as the write key matches the write lock for eachmemory word location accessed, or the value of either thelock or the key is 0000, the input operation is performedcs specified by the other parameters within this IOCD; orthe input operation is terminated by an "unusual end" conditionwhich can not be inhibited (i. e., memory addresserror, control check fault, or lOP memory error).If the HTE control flag is coded as a 1 when a WLV signalis received, the I/O operation is terminated immediately.If either the ICE or IUE control flag is coded as a 1, anI/O interrupt is requested.If the HTE control flag is coded as a ° when a WLV signalis received, the I/O operation continues i.n a normal manner,even though the data or information may be rejectedby a memory unit.When the lOP receives a WLV signal, the WLV bit withinthe status information register is set to 1 and remains setuntil a new I/o operation is initiated within this I/O subchannelby an SIO instruction. Thus, after the first WLVsignal has been recorded, subsequent WL.V signals have nofurther effect upon the WLV bit. The status of the WLVbit is reported when the BP executes an SIO, no, TDV,HIO, or AIO instruction.The contents of the write key field is not required and maybe ignoredwhen the write key/write lock memory protectionfeature is not operative (i. e., during any output operationor during any input operation, if the Skip control flag ofthe current 10CD is coded as a 1).BYTE COUNTThis 16-bit field (bit positions 48-63), if required, may becoded to specify the total number of data or informationbytes that are to be transmitted by the current 10CD.The minimum number of bytes is 1 and the maximum is65,356 bytes (16,384 words). When the 10CD is fetched,the content of the byte count field is loaded into a bytecount register within the appropriate I/o subchannel.Thereafter, the content of the byte count register is decrementedby one for each byte transmitted and then testedfor a zero byte count condition. (Note: As a consequenceof decrementing before testing for a zero byte count condition,an initial byte count value of a is interpreted as65,356 bytes.) Unless the I/O operation is terminated(e. g., as the result of detecting an "unusual end"), datais transmitted until the byte count is reduced to zero. Atany time, the progress of the I/o operation may be ascertainedby evaluating the current byte count which isfurnished as status information when the BP executes anSIO, no, HIO, or TDV instruction. {That is, current bytecount is equal to the number of bytes remaining to be transmittedand initial byte count minus current byte count isequal to the number of bytes transmitted.} When the bytecount is reduced to zero, the MIOP may perform the followingfunctions:1. Transmit a "count done" signal to the device controller/device if data chaining is not specified.2. Request an I/o interrupt, if the IZC flag is codedas a i.3. Fetch the next IOCD, if the data chain flag is codedas a 1.Depending upon the characteristics of the I/o device,certain I/o orders (e. g., Rewind for magnetic tape units)may not require a byte count field. In such case, the bytecount fie Id is ignored. Refer to an appropriate Xerox peripheralequipment reference manual for further detai Is.CONTROL lOCOA control IOCD may contain either a Transfer in Channelor a Stop command.146 Input/Output Processor (lOP) Fundamentals