1. xerox 560 computer system - The UK Mirror Service

Otherwise, the first word of the next lOCO is fetched andloaded as described above, and the second word is fetchedand loaded as described below.Since the Transfer in Channel command permits lOCOs tobe fetched from nonconsecutive locations, lOCOs containingTransfer in Channel commands may be included withina command list either to achieve command list continuityfrom one segment of a command list to another segment orto c?nstruct reiterative loops.For all lOCOs, except a control lOCO containing a Transferin Channel command, the lOP will automatically accessmain memory at the appropriate time, as determined by thepriority that prevails for accessing main memory, and fetchthe second word of the lOCO from the next consecutiveascending (odd) memory word location of the command listand increment the command address register by one. Thus,in all cases, after a fetching operation is completed, thecontent of the command address register wi II be an even(or doubleword) address.The contents of the second word are stored in appropriateregisters within the I/O subchannel. Depending upon theI/o order, as described under IIExecution Phase ll , the contentsof the various fields are either used or ignored.In addition to the lOP Control Error (IOPCE), the followingtypes of lIunusual end ll conditions may be detected duringthe fetching phase of an I/O operation: Memory AddressError (MAE), Control Check Fault (CCF), lOP Memory Error(IOPME), Bus Check Fault (BCF), and Memory InterfaceError (MIE). The detection of any of these errors causes theI/O operation to be terminated and if the IUE flag is set toa 1, an "unusual end" interrupt is requested.Depending upon the control function performed, certainControl orders may be a part of an I/o operationwhich may be continued after the Control order isexecuted. For example, an I/o operation involvinga magnetic tape unit may contain a Rewind order toreposition the tape prior to reading (or writing) one ormore records.Note: Within the context of the above explanation,the Control order is defined to be one thatdoes not transfer any information; thus, datachaining is precluded within the lOCO containingthe Control order; however, commandchaining may be specified. Control orders thatinvolve information transfers when executedare described below (see paragraphs 2 and 4).2. If the order specifies an input operation (e. g., Read,Read Backward, or Sense) and the Skip flag is codedas a 0, all parameters of the current lOCO may beapplicable. As a result of receiving an appropriateinput order, the devi ce transmits data (Read, or ReadBackward order) or information from special registers(Sense order) into data buffers of the associated I/Osubchannel within the lOP.Depending upon the priority that prevai Is for accessingmain memory, the lOP accesses a memory word location(as specified by the current memory byte address),transfers up to four bytes of data or i nformati on fromthe data buffers to a memory unit, provides a writekey, and increments (or decrements, if Read Backward",,.,.10,.' _._-'J +ho ...- ... mom",r" _..._., -,.- h.,+o ,..",1,.1roc:" --_._-- nn,.l -..- ,.Iorromon+c: --_._..._.. -- +ho ----byte count by one for each byte transferred out of thedata buffers.EXECUTION PHASEAlthough the services of the BP are not required duringthis phase, the BP may at any time execute either a TIO,TDV, or POL instruction without interfering with the I/ooperation. However, excessive testing may cause a dataoverrun condition. The BP may also execute either anHIO or P.IO instruction and stop the I/o operation. Afterthe second word of an lOCO is fetched and providing nolIunusual end ll condition was detected, the lOCO is executedas prescribed by the parameters contained therein. As afunction of the order and the status of the Skip flag, ifapplicable, an lOCO may be executed in one of five ways,as described below:1. Certain Control orders (e. g., Stop) may be executedby the device whi Ie the lOP monitors the operation inaccordance with the applicable control flags. Sinceno memory accesses and data (or information) transfersoccur, the contents of the memory byte address register,write key register, and byte count register maybe ignored. Other Control orders (e. g., Rewind for amagnetic tape unit) are listed and described in applicableXerox peripheral equipment reference manuals.The write key is evaluated against the preassignedwrite lock for the memory word location accessed.If the write key is valid for each memory word locationaccessed, the input operation continues, as describedabove, unti I it is completed or terminatedby an "unusual end" condition, other than Write LockViolation. If the write key is not valid, the memoryunit (1) generates and transmits a Write Lock Violation(WLV) signal to the lOP, (2) rejects the new data,and (3) does not disturb the previous contents of thememory word location accessed.If the write key is invalid for any memory word locationaccessed and the HTE flag is coded as a 1, the inputoperation is terminated immediately upon receipt of aWLV signal (see "Termination Phase ll ).If the HTE flag is coded as a 0, the memory unit mayaccept or reject the data or information, based on thewrite key/write lock evaluation for each memory wordlocation accessed, without affecting the operationswithin the lOP, device controller, or device. Theinput operation continues unti I either completed orterminated by an "unusual end" condition, other thana Write Lock Violation.I/O Operation Phases 149