Patent Publication Number: US-7917813-B2

Title: Exception condition determination at a control unit in an I/O processing system

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present disclosure relates generally to input/output processing, and in particular, to providing feedback of exception conditions for input/output processing at a control unit to a channel subsystem. 
     2. Description of Background 
     Input/output (I/O) operations are used to transfer data between memory and I/O devices of an I/O processing system. Specifically, data is written from memory to one or more I/O devices, and data is read from one or more I/O devices to memory by executing I/O operations. 
     To facilitate processing of I/O operations, an I/O subsystem of the I/O processing system is employed. The I/O subsystem is coupled to main memory and the I/O devices of the I/O processing system and directs the flow of information between memory and the I/O devices. One example of an I/O subsystem is a channel subsystem. The channel subsystem uses channel paths as communications media. Each channel path includes a channel coupled to a control unit, the control unit being further coupled to one or more I/O devices. 
     The channel subsystem may employ channel command words (CCWs) to transfer data between the I/O devices and memory. A CCW specifies the command to be executed. For commands initiating certain I/O operations, the CCW designates the memory area associated with the operation, the action to be taken whenever a transfer to or from the area is completed, and other options. 
     During I/O processing, a list of CCWs is fetched from memory by a channel. The channel parses each command from the list of CCWs and forwards a number of the commands, each command in its own entity, to a control unit coupled to the channel. The control unit then processes the commands. The channel tracks the state of each command and controls when the next set of commands are to be sent to the control unit for processing. The channel ensures that each command is sent to the control unit in its own entity. Further, the channel infers certain information associated with processing the response from the control unit for each command. 
     Performing I/O processing on a per CCW basis may involve a large amount of processing overhead for the channel subsystem, as the channels parse CCWs, track state information, and react to responses from the control units. Therefore, it may be beneficial to shift much of the processing burden associated with interpreting and managing CCW and state information from the channel subsystem to the control units. Simplifying the role of channels in communicating between the control units and an operating system in the I/O processing system may increase communication throughput as less handshaking is performed. However, altering command sequences, as well as roles of the channel subsystem and the control units, can cause difficulties in detecting and reporting exception conditions associated with the I/O processing. When multiple commands are passed through the channel subsystem to the control units, the burden of detecting exception conditions, such as errors in the commands is placed on the control units. The control units must then provide feedback of any exception conditions to the channel subsystem to trigger exception handling for mitigating exception conditions. Accordingly, there is a need in the art for providing feedback of exception conditions for input/output processing at a control unit to a channel subsystem. 
     BRIEF SUMMARY OF THE INVENTION 
     Embodiments of the invention include a computer program product for providing exception condition feedback at a control unit to a channel subsystem in an I/O processing system. The computer program product includes a tangible storage medium readable by a processing circuit and storing instructions for execution by the processing circuit for performing a method. The method includes receiving a command message at the control unit from the channel subsystem, and detecting an exception condition in response to unsuccessful execution of at least one command in the command message. The method further includes identifying a termination reason code associated with the exception condition, writing the termination reason code to a response message, and sending the response message to the channel subsystem. 
     Additional embodiments include an apparatus for providing exception condition feedback in an I/O processing system. The apparatus includes a control unit in communication with a channel subsystem. The control unit performs a method that includes receiving a command message at the control unit from the channel subsystem, and detecting an exception condition in response to unsuccessful execution of at least one command in the command message. The method performed by the control unit also includes identifying a termination reason code associated with the exception condition, writing the termination reason code to a response message, and sending the response message to the channel subsystem. 
     Further embodiments include a method for providing exception condition feedback at a control unit to a channel subsystem in an I/O processing system. The method includes receiving a command message at the control unit from the channel subsystem, and detecting an exception condition in response to unsuccessful execution of at least one command in the command message. The method additionally includes identifying a termination reason code associated with the exception condition, writing the termination reason code to a response message, and sending the response message to the channel subsystem. 
     An additional embodiment includes a computer program product for providing exception condition feedback at a control unit to a channel subsystem in an I/O processing system. The computer program product includes a tangible storage medium readable by a processing circuit and storing instructions for execution by the processing circuit for performing a method. The method includes receiving a transport command information unit (IU) message at the control unit from the channel subsystem, and detecting an exception condition in response to unsuccessful execution of at least one command in the transport command IU message. The method also includes identifying a termination reason code and a reason code qualifier providing encoded meaning corresponding to the termination reason code associated with the exception condition. The method further includes writing the termination reason code and the reason code qualifier to an extended status section of a transport response IU message, and sending the transport response IU message to the channel subsystem. 
     A further embodiment includes an apparatus for providing exception condition feedback. The apparatus includes a control unit in communication with a channel subsystem. The control unit performs a method that includes receiving a transport command IU message at the control unit from the channel subsystem, and detecting an exception condition in response to unsuccessful execution of at least one command in the transport command IU message. The method performed by the control unit also includes identifying a termination reason code and a reason code qualifier providing encoded meaning corresponding to the termination reason code associated with the exception condition. The method performed by the control unit additionally includes writing the termination reason code and the reason code qualifier to an extended status section of a transport response IU message, and sending the transport response IU message to the channel subsystem. 
     Other computer program products, apparatuses, and/or methods according to embodiments will be or become apparent to one with skill in the art upon review of the following drawings and detailed description. It is intended that all such additional computer program products, apparatuses, and/or methods be included within this description, be within the scope of the present invention, and be protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  depicts one embodiment of an I/O processing system incorporating and using one or more aspects of the present invention; 
         FIG. 2   a  depicts one example of a prior art channel command word; 
         FIG. 2   b  depicts one example of a prior art channel command word channel program; 
         FIG. 3  depicts one embodiment of a prior art link protocol used in communicating between a channel and control unlit to execute the channel command word channel program of  FIG. 2   b;    
         FIG. 4  depicts one embodiment of a transport control word channel program, in accordance with an aspect of the present invention; 
         FIG. 5  depicts one embodiment of a link protocol used to communicate between a channel and control unit to execute the transport control word channel program of  FIG. 4 , in accordance with an aspect of the present invention; 
         FIG. 6  depicts one embodiment of a prior art link protocol used to communicate between a channel and control unlit in order to execute four read commands of a channel command word channel program; 
         FIG. 7  depicts one embodiment of a link protocol used to communicate between a channel and control unit to process the four read commands of a transport control word channel program, in accordance with an aspect of the present invention; 
         FIG. 8  depicts one embodiment of a control unit and a channel, in accordance with an aspect of the present invention; 
         FIG. 9  depicts one embodiment of a command message communicated from a channel subsystem to a control unit, in accordance with an aspect of the present invention; 
         FIG. 10  depicts one embodiment of a response message communicated from a control unit to a channel subsystem, in accordance with an aspect of the present invention; 
         FIG. 11  depicts one embodiment of a portion of an interrupt response block in a host system, in accordance with an aspect of the present invention; 
         FIG. 12  depicts one embodiment of a process for providing feedback of exception conditions for input/output processing at a control unit to a channel subsystem; and 
         FIG. 13  depicts one embodiment of a computer program product incorporating one or more aspects of the present invention. 
     
    
    
     The detailed description explains the preferred embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     In accordance with an aspect of the present invention, input/output (I/O) processing is facilitated. For instance, I/O processing is facilitated by readily enabling access to information, such as status and measurement data, associated with I/O processing. Further, I/O processing is facilitated, in one example, by reducing communications between components of an I/O processing system used to perform the I/O processing. For instance, the number of exchanges and sequences between an I/O communications adapter, such as a channel, and a control unit is reduced. This is accomplished by sending a plurality of commands from the I/O communications adapter to the control unit as a single entity for execution by the control unit, and by the control unit sending the data resulting from the commands, if any, as a single entity. 
     The plurality of commands are included in a block, referred to herein as a transport command control block (TCCB), an address of which is specified in a transport control word (TCW). The TCW is sent from an operating system or other application to the I/O communications adapter, which in turn forwards the TCCB in a command message to the control unit for processing. The control unit processes each of the commands absent a tracking of status relative to those individual commands by the I/O communications adapter. The plurality of commands is also referred to as a channel program, which is parsed and executed on the control unit rather than the I/O communications adapter. 
     In an exemplary embodiment, the control unit generates a response message including status and extended status information in response to executing the channel program. The control unit may also generate a response message without executing the channel program when an exception condition is detected, such as an error in the channel program that prevents execution. The control unit may include a number of elements to support communication between the I/O communications adapter and I/O devices, as well as in support of channel program execution. For example, the control unit can include control logic to parse and process messages, in addition to one or more queues, timers, and registers to facilitate communication and status monitoring. The I/O communications adapter parses the response message, extracting the status and extended status information, and provides feedback to processing elements of the I/O processing system. 
     One example of an I/O processing system incorporating and using one or more aspects of the present invention is described with reference to  FIG. 1 . I/O processing system  100  includes a host system  101 , which further includes for instance, a main memory  102 , one or more central processing units (CPUs)  104 , a storage control element  106 , and a channel subsystem  108 . The host system  101  may be a large scale computing system, such as a mainframe or server. The I/O processing system  100  also includes one or more control units  110  and one or more I/O devices  112 , each of which is described below. 
     Main memory  102  stores data and programs, which can be input from I/O devices  112 . For example, the main memory  102  may include one or more operating systems (OSs)  103  that are executed by one or more of the CPUs  104 . For example, one CPU  104  can execute a Linux® operating system  103  and a z/OS® operating system  103  as different virtual machine instances. The main memory  102  is directly addressable and provides for high-speed processing of data by the CPUs  104  and the channel subsystem  108 . 
     CPU  104  is the controlling center of the I/O processing system  100 . It contains sequencing and processing facilities for instruction execution, interruption action, timing functions, initial program loading, and other machine-related functions. CPU  104  is coupled to the storage control element  106  via a connection  114 , such as a bidirectional or unidirectional bus. 
     Storage control element  106  is coupled to the main memory  102  via a connection  116 , such as a bus; to CPUs  104  via connection  114 ; and to channel subsystem  108  via a connection  118 . Storage control element  106  controls, for example, queuing and execution of requests made by CPU  104  and channel subsystem  108 . 
     In an exemplary embodiment, channel subsystem  108  provides a communication interface between host system  101  and control units  110 . Channel subsystem  108  is coupled to storage control element  106 , as described above, and to each of the control units  110  via a connection  120 , such as a serial link. Connection  120  may be implemented as an optical link, employing single-mode or multi-mode waveguides in a Fibre Channel fabric. Channel subsystem  108  directs the flow of information between I/O devices  112  and main memory  102 . It relieves the CPUs  104  of the task of communicating directly with the I/O devices  112  and permits data processing to proceed concurrently with I/O processing. The channel subsystem  108  uses one or more channel paths  122  as the communication links in managing the flow of information to or from I/O devices  112 . As a part of the I/O processing, channel subsystem  108  also performs the path-management functions of testing for channel path availability, selecting an available channel path  122  and initiating execution of the operation with the I/O devices  112 . 
     Each channel path  122  includes a channel  124  (channels  124  are located within the channel subsystem  108 , in one example, as shown in  FIG. 1 ), one or more control units  110  and one or more connections  120 . In another example, it is also possible to have one or more dynamic switches (not depicted) as part of the channel path  122 . A dynamic switch is coupled to a channel  124  and a control unit  110  and provides the capability of physically interconnecting any two links that are attached to the switch. In another example, it is also possible to have multiple systems, and therefore multiple channel subsystems (not depicted) attached to control unit  110 . 
     Also located within channel subsystem  108  are subchannels (not shown). One subchannel is provided for and dedicated to each I/O device  112  accessible to a program through the channel subsystem  108 . A subchannel (e.g., a data structure, such as a table) provides the logical appearance of a device to the program. Each subchannel provides information concerning the associated I/O device  112  and its attachment to channel subsystem  108 . The subchannel also provides information concerning I/O operations and other functions involving the associated I/O device  112 . The subchannel is the means by which channel subsystem  108  provides information about associated I/O devices  112  to CPUs  104 , which obtain this information by executing I/O instructions. 
     Channel subsystem  108  is coupled to one or more control units  110 . Each control unit  110  provides logic to operate and control one or more I/O devices  112  and adapts, through the use of common facilities, the characteristics of each I/O device  112  to the link interface provided by the channel  124 . The common facilities provide for the execution of I/O operations, indications concerning the status of the I/O device  112  and control unit  110 , control of the timing of data transfers over the channel path  122  and certain levels of I/O device  112  control. 
     Each control unit  110  is attached via a connection  126  (e.g., a bus) to one or more I/O devices  112 . I/O devices  112  receive information or store information in main memory  102  and/or other memory. Examples of I/O devices  112  include card readers and punches, magnetic tape units, direct access storage devices, displays, keyboards, printers, pointing devices, teleprocessing devices, communication controllers and sensor based equipment, to name a few. 
     One or more of the above components of the I/O processing system  100  are further described in “IBM® z/Architecture Principles of Operation,” Publication No. SA22-7832-05, 6th Edition, April 2007; U.S. Pat. No. 5,461,721 entitled “System For Transferring Data Between I/O Devices And Main Or Expanded Storage Under Dynamic Control Of Independent Indirect Address Words (IDAWS),” Cormier et al., issued Oct. 24, 1995; and U.S. Pat. No. 5,526,484 entitled “Method And System For Pipelining The Processing Of Channel Command Words,” Casper et al., issued Jun. 11, 1996, each of which is hereby incorporated herein by reference in its entirety. IBM is a registered trademark of International Business Machines Corporation, Armonk, N.Y., USA. Other names used herein may be registered trademarks, trademarks or product names of International Business Machines Corporation or other companies. 
     In one embodiment, to transfer data between I/O devices  112  and memory  102 , channel command words (CCWs) are used. A CCW specifies the command to be executed, and includes other fields to control processing. One example of a CCW is described with reference to  FIG. 2   a . A CCW  200  includes, for instance, a command code  202  specifying the command to be executed (e.g., read, read backward, control, sense and write); a plurality of flags  204  used to control the I/O operation; for commands that specify the transfer of data, a count field  206  that specifies the number of bytes in the storage area designated by the CCW to be transferred; and a data address  208  that points to a location in main memory that includes data, when direct addressing is employed, or to a list (e.g., contiguous list) of modified indirect data address words (MIDAWs) to be processed, when modified indirect data addressing is employed. Modified indirect addressing is further described in U.S. application Ser. No. 11/464,613, entitled “Flexibly Controlling The Transfer Of Data Between Input/Output Devices And Memory,” Brice et al., filed Aug. 15, 2006, which is hereby incorporated herein by reference in its entirety. 
     One or more CCWs arranged for sequential execution form a channel program, also referred to herein as a CCW channel program. The CCW channel program is set up by, for instance, an operating system, or other software. The software sets up the CCWs and obtains the addresses of memory assigned to the channel program. An example of a CCW channel program is described with reference to  FIG. 2   b . A CCW channel program  210  includes, for instance, a define extent CCW  212  that has a pointer  214  to a location in memory of define extent data  216  to be used with the define extent command. In this example, a transfer in channel (TIC)  218  follows the define extent command that refers the channel program to another area in memory (e.g., an application area) that includes one or more other CCWs, such as a locate record  217  that has a pointer  219  to locate record data  220 , and one or more read CCWs  221 . Each read CCW  220  has a pointer  222  to a data area  224 . The data area includes an address to directly access the data or a list of data address words (e.g., MIDAWs or IDAWs) to indirectly access the data. Further, CCW channel program  210  includes a predetermined area in the channel subsystem defined by the device address called the subchannel for status  226  resulting from execution of the CCW channel program. 
     The processing of a CCW channel program is described with reference to  FIG. 3 , as well as with reference to  FIG. 2   b . In particular,  FIG. 3  shows an example of the various exchanges and sequences that occur between a channel and a control unit when a CCW channel program is executing. The link protocol used for the communications is FICON (Fibre Connectivity), in this example. Information regarding FICON is described in “Fibre Channel Single Byte Command Code Sets-3 Mapping Protocol (FC-SB-3), T11/Project 1357-D/Rev. 1.6, INCITS (March 2003), which is hereby incorporated herein by reference in its entirety. 
     Referring to  FIG. 3 , a channel  300  opens an exchange with a control unit  302  and sends a define extent command and data associated therewith  304  to control unit  302 . The command is fetched from define extent CCW  212  ( FIG. 2   b ) and the data is obtained from define extent data area  216 . The channel  300  uses TIC  218  to locate the locate record CCW and the read CCW. It fetches the locate record command  305  ( FIG. 3 ) from the locate record CCW  217  ( FIG. 2   b ) and obtains the data from locate record data  220 . The read command  306  ( FIG. 3 ) is fetched from read CCW  221  ( FIG. 2   b ). Each is sent to the control unit  302 . 
     The control unit  302  opens an exchange  308  with the channel  300 , in response to the open exchange of the channel  300 . This can occur before or after locate command  305  and/or read command  306 . Along with the open exchange, a response (CMR) is forwarded to the channel  300 . The CMR provides an indication to the channel  300  that the control unit  302  is active and operating. 
     The control unit  302  sends the requested data  310  to the channel  300 . Additionally, the control unit  302  provides the status to the channel  300  and closes the exchange  312 . In response thereto, the channel  300  stores the data, examines the status and closes the exchange  314 , which indicates to the control unit  302  that the status has been received. 
     The processing of the above CCW channel program to read 4k of data requires two exchanges to be opened and closed and seven sequences. The total number of exchanges and sequences between the channel and control unit is reduced through collapsing multiple commands of the channel program into a TCCB. The channel, e.g., channel  124  of  FIG. 1 , uses a TCW to identify the location of the TCCB, as well as locations for accessing and storing status and data associated with executing the channel program. The TCW is interpreted by the channel and is not sent or seen by the control unit. 
     One example of a channel program to read 4k of data, as in  FIG. 2   b , but includes a TCCB, instead of separate individual CCWs, is described with reference to  FIG. 4 . As shown, a channel program  400 , referred to herein as a TCW channel program, includes a TCW  402  specifying a location in memory of a TCCB  404 , as well as a location in memory of a data area  406  or a TIDAL  410  (i.e., a list of transfer mode indirect data address words (TIDAWs), similar to MIDAWs) that points to data area  406 , and a status area  408 . TCWs, TCCBs, and status are described in further detail below. 
     The processing of a TCW channel program is described with reference to  FIG. 5 . The link protocol used for these communications is, for instance, Fibre Channel Protocol (FCP). In particular, three phases of the FCP link protocol are used, allowing host bus adapters to be used that support FCP to perform data transfers controlled by CCWs. FCP and its phases are described further in “Information Technology—Fibre Channel Protocol for SCSI, Third Version (FCP-3),” T10 Project 1560-D, Revision 4, Sep. 13, 2005, which is hereby incorporated herein by reference in its entirety. 
     Referring to  FIG. 5 , a channel  500  opens an exchange with a control unit  502  and sends TCCB  504  to the control unit  502 . In one example, the TCCB  504  and sequence initiative are transferred to the control unit  502  in a FCP command, referred to as FCP_CMND information unit (IU) or a transport command IU. The control unit  502  executes the multiple commands of the TCCB  504  (e.g., define extent command, locate record command, read command as device control words (DCWs)) and forwards data  506  to the channel  500  via, for instance, a FCP_Data IU. It also provides status and closes the exchange  508 . As one example, final status is sent in a FCP status frame that has a bit active in, for instance, byte  10  or  11  of the payload of a FCP_RSP IU, also referred to as a transport response IU. The FCP_RSP IU payload may be used to transport FICON ending status along with additional status information, including parameters that support the calculation of extended measurement words and notify the channel  500  of the maximum number of open exchanges supported by the control unit  502 . 
     In a further example, to write 4k of customer data, the channel  500  uses the FCP link protocol phases, as follows: 
     1. Transfer a TCCB in the FCP_CMND IU. 
     2. Transfer the IU of data, and sequence initiative to the control unit  502 . 
     3. Final status is sent in a FCP status frame that has a bit active in, for instance, byte  10  or  11  of the FCP_RSP IU Payload. The FCP_RSP_INFO field or sense field is used to transport FICON ending status along with additional status information, including parameters that support the calculation of extended measurement words and notify the channel  500  of the maximum number of open exchanges supported by the control unit  502 . 
     By executing the TCW channel program of  FIG. 4 , there is only one exchange opened and closed (see also  FIG. 5 ), instead of two exchanges for the CCW channel program of  FIG. 2   b  (see also  FIG. 3 ). Further, for the TCW channel program, there are three communication sequences (see  FIGS. 4-5 ), as compared to seven sequences for the CCW channel program (see  FIGS. 2   b - 3 ). 
     The number of exchanges and sequences remain the same for a TCW channel program, even if additional commands are added to the program. Compare, for example, the communications of the CCW channel program of  FIG. 6  with the communications of the TCW channel program of  FIG. 7 . In the CCW channel program of  FIG. 6 , each of the commands (e.g., define extent command  600 , locate record command  601 , read command  602 , read command  604 , read command  606 , locate record command  607  and read command  608 ) are sent in separate sequences from channel  610  to control unit  612 . Further, each 4k block of data (e.g., data  614 - 620 ) is sent in separate sequences from the control unit  612  to the channel  610 . This CCW channel program requires two exchanges to be opened and closed (e.g., open exchanges  622 ,  624  and close exchanges  626 ,  628 ), and fourteen communications sequences. This is compared to the three sequences and one exchange for the TCW channel program of  FIG. 7 , which accomplishes the same task as the CCW channel program of  FIG. 6 . 
     As depicted in  FIG. 7 , a channel  700  opens an exchange with a control unit  702  and sends a TCCB  704  to the control unit  702 . The TCCB  704  includes the define extent command, the two locate record commands, and the four read commands in DCWs, as described above. In response to receiving the TCCB  704 , the control unit  702  executes the commands and sends, in a single sequence, the 16k of data  706  to the channel  700 . Additionally, the control unit  702  provides status to the channel  700  and closes the exchange  708 . Thus, the TCW channel program requires much less communications to transfer the same amount of data as the CCW channel program of  FIG. 6 . 
     Turning now to  FIG. 8 , one embodiment of the control unit  110  and the channel  124  of  FIG. 1  that support TCW channel program execution are depicted in greater detail. The control unit  110  includes CU control logic  802  to parse and process command messages containing a TCCB, such as the TCCB  704  of  FIG. 7 , received from the channel  124  via the connection  120 . The CU control logic  802  can extract DCWs and control data from the TCCB received at the control unit  110  to control a device, for instance, I/O device  112  via connection  126 . The CU control logic  802  sends device commands and data to the I/O device  112 , as well as receives status information and other feedback from the I/O device  112 . The CU control logic  802  uses check logic  804  to perform various checks of the command messages received at the control unit  110 . The check logic  804  may also determine termination reason codes for reporting exception conditions to the channel subsystem  108  as part of a response message. 
     The CU control logic  802  can access and control other elements within the control unit  110 , such as CU timers  806  and CU registers  808 . The CU timers  806  may include multiple timer functions to track how much time a sequence of I/O operations takes to complete. The CU timers  806  may further include one or more countdown timers to monitor and abort I/O operations and commands that do not complete within a predetermined period. The CU registers  808  can include fixed values that provide configuration and status information, as well as dynamic status information that is updated as commands are executed by the CU control logic  802 . The control unit  110  may further include other buffer or memory elements (not depicted) to store multiple messages or status information associated with communications between the channel  124  and the I/O device  112 . The CU registers  808  may include a maximum control unit exchange parameter that defines the maximum number of open control unit exchanges that the control unit  110  supports. 
     The channel  124  in the channel subsystem  108  includes multiple elements to support communication with the control milt  110 . For example, the channel  124  may include CHN control logic  810  that interfaces with CHN subsystem timers  812  and CHN subsystem registers  814 . In an exemplary embodiment, the CHN control logic  810  controls communication between the channel subsystem  108  and the control unit  110 . The CHN control logic  810  may directly interface to the CU control logic  802  via the connection  120  to send commands and receive responses, such as transport command and response IUs. Alternatively, messaging interfaces and/or buffers (not depicted) can be placed between the CHN control logic  810  and the CU control logic  802 . The CHN subsystem timers  812  may include multiple timer functions to track how much time a sequence of I/O operations takes to complete, in addition to the time tracked by the control unit  110 . The CHN subsystem timers  812  may further include one or more countdown timers to monitor and abort command sequences that do not complete within a predetermined period. The CHN subsystem registers  814  can include fixed values that provide configuration and status information, as well as dynamic status information, updated as commands are transported and responses are received. 
     One example of a command message  900 , e.g., a transport command IU, communicated from the channel subsystem  108  to the control unit  110  to execute a TCW channel program is depicted in  FIG. 9 . The command message  900  includes a header  902 , a transport command header (TCH)  904 , a transport command area header (TCAH)  906 , a transport command area (TCA)  908 , and a transport command area trailer (TCAT)  910 . In an exemplary embodiment, the TCCB  404  of  FIG. 4  includes the TCH  904 , TCAH  906 , TCA  908 , and TCAT  910 . 
     The header  902  may include multiple words as address header  912 , defining the highest level of header in the command message  900 . The header  902  may include information such as channel and control unit image IDs and a device address. 
     The TCH  904  includes a command reference number/task  914 , which may be set to a reserved value, e.g., zero, while operating in transport mode. The TCH  904  also includes L1 length  916  and read/write field  918 . The L1 length  916  defines the length of the TCA  908  in words+1. The L1 length  916  can be used to limit and define the size of the TCA  908 . The read/write field  918  defines whether read data, write data, or no data is being transferred in the command message  900 , where a read is a transfer from the control unit  110  to the channel subsystem  108 . 
     The TCAH  906  includes format field  920  and control field  922 . The format field  920  and control field  922  may be set to fixed values, such as 7F hexadecimal and zero respectively, to indicate that a variable length format is used, as defined by SPC-4. SPC-4 is further described in “SCSI Primary Commands-4 (SPC-4)”, Project T10/1731-D, Rev 11, INCITS (May 2007), which is hereby incorporated herein by reference in its entirety. The TCAH  906  additionally includes reserved fields  924  and  926 , as well as L2 length  928 . The L2 length  928  is also referred to as transport-command-area length (TCAL), and may represent the number of bytes after this position in the command message  900 . The L2 length  928  limits the size of the TCA  908 . The TCAH  906  further includes a service action code  930 , reserved field  932 , priority  934 , and reserved field  936 . The service action code  930  defines the type of DCWs used in the TCA  908 . The priority  934  can be set equivalent to a priority byte of a FICON command header as defined in FC-SB-3. 
     The TCA  908  includes DCW one and control data  940 , DCW two  942 , DCW three  944 , and DCW four  946 . The DCW one and control data  940  includes DCW fields such as a command  948 , flags field  950 , a reserved field  952 , control data (CD) count  954 , and data byte count  956 . The command  948  may be equivalent to a CCW command byte, but directly interpreted by the control unit  110  rather than the channel subsystem  108 . The flags field  950  includes reserved bits as well as one or more bits assigned to particular functions, such as indicating whether an additional DCW exists in the TCA  908  as part of a command chain. The CD count  954  is the byte count of control data  958 . The CD count  954  may be padded up to the next 4-byte boundary so that subsequent DCWs start on a 4-byte boundary. The data byte count  956  is a four-byte count of data without padding, e.g., customer data. The control data  958  exists when the CD count  954  is not zero. In the exemplary command message  900 , the DCW two  942 , DCW three  944 , and DCW four  946  contain substantially similar fields as the DCW one and control data  940 . For example, command  960 ,  970 , and  980  are formatted in a similar fashion as the command  948 . Furthermore, flags field  962 ,  972 , and  982  are formatted similar to the flags field  950 . Additionally, CD count  966 ,  976 , and  986  are formatted similar the CD count  954 , and data byte count  968 ,  978 , and  988  are similarly formatted to the data byte count  956 . Although only four DCWs, including one DCW with control data (i.e., DCW one and control data  940 ) are depicted in the command message  900 , it will be understood that a varying number of DCWs with and without control data can be included in the command message  900 , including a single DCW. 
     The TCAT  910  includes a longitudinal redundancy check (LRC) word  990  calculated on the entire command message  900 . The LRC word  990  can be generated through applying an exclusive-or operation to an initial seed value with each field included in the LRC calculation in succession. The TCAT  910  also includes a transport data byte count  992  indicating the total number of bytes transferred for a read or write I/O operation. 
     Upon sending the command message  900  to the control unit  110 , the control unit  110  may detect error or exception conditions with the contents of the command message  900 . The control unit  110  can also identify exception conditions that result in early termination of an I/O operation, including errors detected by the I/O device  112 . The control unit  110  reports reason code and qualifier information back to the channel subsystem  108  in a response message to assist in debugging and fault isolation. 
     One example of a response message  1000 , e.g., a transport response IU, communicated from the control unit  110  to the channel  124  of the channel subsystem  108  upon completion of a TCW channel program is depicted in  FIG. 10 . The response message  1000  provides status information to the channel  124  and may indicate that an open exchange between the channel  124  and the control unit  110  should be closed. The status information provided when a TCW channel program (e.g., as depicted in  FIGS. 5 and 7 ) is analyzed and/or executed includes additional information beyond the status information sent upon completion of a CCW channel program (e.g., as depicted in  FIGS. 3 and 6 ). The response message  1000  includes a status section  1002  and an extended status section  1004 . When the channel  124  receives the response message  1000 , it stores parts of status section  1002  in the subchannel for the device the TCW was operating with and the extended status section  1004  in status area  408  defined by the TCW  402  of  FIG. 4  associated with the TCW channel program that triggered the response message  1000 . For example, a TCW can designate a section of main memory  102  of  FIG. 1  for storage of the extended status section  1004 . 
     The status section  1002  of the response message  1000  can include multiple fields, such as an address header  1006 , status flags one  1008 , maximum control unit exchange parameter  1010 , response flags  1012 , response code  1014 , residual count  1016 , response length  1018 , reserved location  1020 , SPC-4 sense type  1022 , status flags two  1024 , status flags three  1026 , device status  1028 , and an LRC word  1030 . Each field in the status section  1002  is assigned to a particular byte address to support parsing of the response message  1000 . Although one arrangement of fields within the status section  1002  is depicted in  FIG. 10 , it will be understood that the order of fields can be rearranged to alternate ordering within the scope of the disclosure. Moreover, fields in the response message  1000  can be omitted or combined within the scope of the invention, e.g., combining status flags two  1024  and three  1026  into a single field. 
     In an exemplary embodiment, the address header  1006  is set to the same value as the value received by the control unit  110  in the TCCB that initiated the TCW channel program. Although the address header  1006  is not required, including the address header  1006  may support testing to trace command and response messages on an I/O device  112  while multiple I/O devices  112  are being accessed. 
     Status flags one  1008  may indicate information such as the success status of an I/O operation. Multiple bits within the status flags one  1008  can provide additional status information. In an exemplary embodiment, bits  0 - 3  of the status flags one  1008  are reserved, while bits  4  to  7  are encoded with the following definition: 
     1. Null. No exception condition was encountered with the operation. 
     2. Device level exception. The I/O device  112  was not available. 
     3. Link reject. A logical path was not established to the control unit  110 . 
     4. Resetting event. A special device status is included to indicate an event that occurred relative to the I/O device  112 . 
     5. Device requested a program check, which may possibly be escalated to an interface control check (IFCC). The control unit  110  sets this encode when certain conditions are identified in the extended status  1004  for the I/O device  112 , as described in greater detail herein. 
     6. Device requested a program check. The control unit  110  sets this encode when specific conditions are identified in the extended status  1004 , as described in greater detail herein. 
     7. to 15. Reserved. 
     The maximum control unit exchange parameter  1010  identifies the maximum number of exchanges that the control unit  110  allows the channel  124  to open to it. The maximum control unit exchange parameter  1010  may represent a base number to increment and/or scale to establish the maximum number of exchanges supported. 
     In an exemplary embodiment, the response flags field  1012  uses the standard definition as defined in FCP and can be set to a default value, e.g., two. The response code  1014  may be equivalent to a Small Computer System Interface (SCSI) status field and can be set to a default value, such as zero. The residual count  1016  for read or write commands indicates the difference between how many bytes were commanded to be read or written versus the number of bytes that actually were read or written. The channel  124  checks that the channel  124  received or sent the same amount of data that the control unit  110  sent or received using the residual count  1016 . If there is a disagreement the channel  123  terminates the operation with an IFCC set. The response length  1018  is an additional count of bytes of information in the response message  1000  after the reserved location  1020 . The response length  1018  supports variable sized response messages  1000 . The SPC-4 sense type  1022  can be assigned to a value of 7F hexadecimal which identifies this response IU as vender unique. 
     In one embodiment, the status flags two  1024  provides status for validity of the residual count  1016 , an initial status indication, and a request to generate a log record of an event. Invalidity of the residual count  1016  may result in an IFCC because of a link protocol error. Status flags three  1026  is set to a value of one to indicate that extended status  1004  is included as part of the response message  1000 . The device status  1028  relays status information generated by the I/O device  112 . The LRC word  1030  is a check word that covers the other fields in the status section  1002  of the response message  1000  to verify the integrity of the status section  1002 . The LRC word  1030  can be generated through applying an exclusive-or operation to an initial seed value with each field included in the LRC calculation in succession. 
     The extended status section  1004  provides information to the channel subsystem  108  and the OS  103  associated with operating the control unit  110  in a transport mode capable of running a TCW channel program. The extended status section  1004  may support configurable definitions with different type status definitions for each type. In an exemplary embodiment, the extended status section  1004  includes a transport status header (TSH)  1032 , a transport status area (TSA)  1034 , and an LRC word  1036  of the TSH  1032  and the TSA  1034 . The TSH  1032  may include extended status length  1040 , extended status flags  1042 , a DCW offset  1044 , a DCW residual count  1046 , and a reserved location  1048 . The TSH  1032  is common for the different formats, with each format defined by a type code in the extended status flags  1042 . The TSA  1034  may include a reserved value  1050 , a termination reason code  1052 , reason code qualifier (RCQ) words  1054 , and appended device sense data  1056 . Each of these fields is described in greater detail in turn. 
     The extended status length  1040  is the size of the extended status section  1004 . In an exemplary embodiment, the extended status flags  1042  has the following definition: 
     Bit  0 —The DCW offset  1044  is valid. 
     Bit  1 —The DCW residual count  1046  is valid. 
     Bit  2 —This bit set to a one informs the OS  103  of  FIG. 1  in a definitive manner when the control unit  110  had to access slow media for data, e.g., a cache miss. 
     Bit  3 —Time parameters are valid. The type code set to a one and this bit set to a one indicates that all of time parameters are valid when time parameters are included in the response message  1000 . 
     Bit  4 —Reserved. 
     Bits  5  to  7 —These three bits are the type code that defines the format of the TSA  1034  of the extended status section  1004 . The names of the encodes are:
         0. Reserved.   1. I/O Status. The extended status section  1004  contains valid ending status for the transport-mode I/O operation.   2. I/O Exception. The extended status section  1004  contains information regarding termination of the transport-mode I/O operation due to an exception condition.   3. Interrogate Status. The extended status section  1004  contains status for an interrogate operation.   4. to 7. Reserved.       

     The DCW offset  1044  indicates an offset in the TCCB of a failed DCW. Similarly, the DCW residual count  1046  indicates the residual byte count of a failed DCW (i.e., where execution of the DCWs was interrupted). 
     In an exemplary embodiment, the TSA  1034  definition when the type code of ES flags  1042  indicates a type of I/O Exception includes a reserved field  1050 , termination reason codes  1052 , reason code qualifier (RCQ) words  1054 , and optionally, appended device sense data  1056 . The termination reason codes  1052  indicate the reason for the termination of the I/O operation. The RCQ words  1054  include values encoded for meanings corresponding to specific termination reason codes  1052 . Exemplary termination reason codes  1052  include:
         0. Null value for no information.   1. Transport command IU transport failure. The I/O device  112  detected an invalid transport command IU, e.g., command message  900  of  FIG. 9 .   2. Invalid cyclic redundancy check (CRC) detected on output data. The control unit  110  detected an invalid CRC while receiving output data.   3. Incorrect transport command IU length specification.   4. TCAH specification error.   5. DCW specification error. There is an error with the DCW as designated by the DCW offset  1044 .   6. Transfer-direction specification error. The command specified by the DCW designated by the DCW offset  1044  specifies a direction of data transfer that disagrees with the transfer direction specified in the TCH  904 .   7. Transport-count specification error.   8. Two I/O operations active to the same device address. The I/O device  112  responds with this status to both I/O operations that are active for the device address. When this error is detected the control unit  110  also sets encode  4  “Device requested program check, possible IFCC” in status flags one  1008 . When encode  4  in status flags one  1008  is detected by the channel  124 , the channel  124  may notify the OS  103  of the device requested program check with a possible IFCC.   9. to 255. Reserved.       

     When termination reason codes  1052  indicate a transport command IU transport failure (i.e., a value of 1), the RCQ words  1054  can include the following information:
         0. No additional information.   1. The length of the transport command IU received (e.g., command message  900 ) does not match the L1 length  916 .   2. LRC error. The LRC  990  does not validate the transport command IU.   3. to 255. Reserved.       

     When this error is detected the control unit  110  also sets encode  4  “Device requested program check, possible IFCC” in status flags one  1008 . When encode  4  in status flags one  1008  is detected by the channel  124 , the channel  124  may notify the OS  103  of the device requested program check with a possible IFCC. 
     When termination reason codes  1052  indicates an invalid CRC is detected on output data (i.e., a value of 2), the RCQ words  1054  can include the following information that identifies the starting and ending byte of the unit of data that was detected as being corrupted: 
     Response message  1000 , word  12 , (RCQ word  0 ) contains a 32-bit unsigned integer offset of the first output data byte for which the invalid CRC was detected. 
     Response message  1000 , word  13 , (RCQ word  1 ) contains the 32-bit unsigned integer offset of the last output-data byte for which the invalid CRC was detected. When this error is detected the control unit  110  also sets encode  4  “Device requested program check, possible IFCC” in status flags one  1008 . When encode  4  in status flags one  1008  is detected by the channel  124 , the channel  124  may notify the OS  103  of the device requested program check with a possible IFCC. The OS  103  determines if this error is a program check or an IFCC. 
     When termination reason codes  1052  indicate an incorrect transport command IU length specification (i.e., a value of 3), the RCQ words  1054  can include the following: 
     0. No additional information. 
     1. The value specified by the L2 length  928  is not 8 greater than the value specified by the L1 length  916  in the transport command IU for this operation (command message  900 ). 
     2. The value specified by the L2 length  928  is less than 20 or greater than 252. 
     3. to 255 Reserved. 
     When this error is detected the control unit  110  also sets encode  5  “Device requested program check” in status flags one  1008 . When encode  5  in status flags one  1008  is detected by the channel  124 , the channel  124  may notify the OS  103  of the device requested program check. 
     When termination reason codes  1052  indicates a TCAH specification error (i.e., a value of 4), the RCQ words  1054  can include the following: 
     0. No additional information. 
     1. Format-field specification error. The format field  920  in the TCAH  906  contains an unrecognized value. 
     2. Reserved field specification error. A reserved field in the TCAH  906  does not contain zeros, e.g., reserved field  924 ,  926 ,  932  or  936 . 
     3. Service action code field specification error. The service action code field  930  contains an unrecognized value or a value that is incorrect for a command specified by the DCW designated by the DCW offset  1044 . 
     4. to 255 Reserved. 
     When this error is detected the control unit  110  also sets encode  5  “Device requested program check” in status flags one  1008 . When encode  5  in status flags one  1008  is detected by the channel  124 , the channel  124  may notify the OS  103  of the device requested program check. 
     When termination reason codes  1052  indicates a DCW specification error at DCW offset  1044  (i.e., a value of 5), the RCQ words  1054  can include the following: 
     0. No additional information. 
     1. Reserved field specification error. A reserved field in the DCW does not contain zeros, e.g., reserved field  952 ,  964 ,  974 , or  984 . 
     2. Flags field command chaining specification error. Either of the following is true: a command-chaining flag is one and the offset of the next DCW is such that all or part of the next DCW extends past the end of the TCA  908 , or a command-chaining flag is zero and more than 3 unused bytes remain in the TCA  908 . 
     3. Control data count field specification error. Either of the following is true: the command specified by the DCW requires control data and the CD count field (e.g., CD count  954 ,  966 ,  976 , or  986 ) contains zeros, or the CD count field (e.g., CD count  954 ,  966 ,  976 , or  986 ) specifies control data past the end of the TSA  908 . 
     4. to 255 Reserved. 
     When this error is detected the control unit  110  also sets encode  5  “Device requested program check” in status flags one  1008 . When encode  5  in status flags one  1008  is detected by the channel  124 , the channel  124  may notify the OS  103  of the device requested program check. 
     When termination reason codes  1052  indicates a transfer-direction specification error (i.e., a value of 6), the RCQ words  1054  can include the following: 
     0. No additional information. 
     1. Read-direction specification error. The DCW specifies an input operation, but the R-bit in the read/write field  918  is zero. 
     2. Write-direction specification error. The DCW specifies an output operation, but the W-bit in the read/write field  918  is zero. 
     3. to 255 Reserved. 
     When this error is detected the control unit  110  also sets encode  5  “Device requested program check” in status flags one  1008 . When encode  5  in status flags one  1008  is detected by the channel  124 , the channel  124  may notify the OS  103  of the device requested program check. 
     When termination reason codes  1052  indicates a transport-count specification error (i.e., a value of 7), the RCQ words  1054  can include the following: 
     0. No additional information. 
     1. Read count specification error. The transport data byte count  992  specifies a value that is not equivalent to the total count of data bytes specified by the DCWs in the TCA  908 . 
     2. Write count specification error. The transport data byte count  992  specifies a value that is not equivalent to the total count of data bytes specified by the DCWs in the TCA  908 . 
     3. to 255 Reserved. 
     When this error is detected the control unit  110  also sets encode  5  “Device requested program check” in status flags one  1008 . When encode  5  in status flags one  1008  is detected by the channel  124 , the channel  124  may notify the OS  103  of the device requested program check. 
     The appended device sense data  1056  is supplemental status that the control unit  110  provides conditionally in response to an active unit check (UC) bit in the device status  1028 . The amount of data in the appended device sense data  1056  can be determined by subtracting a fixed amount (e.g., 32 bytes) from the ES length field  1040 . 
     The LRC word  1036  is a longitudinal redundancy check word of the TSH  1032  and the TSA  1034 , calculated in a similar fashion as the LRC word  1030  in the status  1002  section of the response message  1000 . The LRC word  1036  can be calculated on a variable number of words, depending upon the number of words included in the appended device sense data  1056 . 
     In response to an exception condition detected at the channel subsystem  108 , an I/O interrupt is communicated to one or more of the CPUs  104 . The I/O interrupt includes an interrupt response block (IRB)  1100 , an example portion (words  0 - 3 ) of which is depicted in  FIG. 11 . The IRB  1100  includes a key  1102 , a reserved field (R)  1104 , an extended status word (ESW) format field (L)  1106 , and a deferred condition code field (CC)  1108 . The IRB  1100  also includes IRB format fields F 0   1110 , F 1   1112 , F 2   1114 , and IRB format control field (X)  1116 . The IRB  1100  further includes interrogate complete field (Q)  1118 , a reserved field (R)  1120 , an extended control field (E)  1122 , a path not operational field (N)  1124 , a reserved field (R)  1126 , a function control field (FC)  1128 , an activity control field (AC)  1130 , and a status control field (SC)  1132 . The IRB  1100  additionally includes a TCW address  1134 , a device status  1136 , a sub-channel status  1138 , a FICON-extended (FCX) status  1140 , and a sub-channel extended status  1142 . 
     In an exemplary embodiment, the key  1102 , L  1106 , CC  1108 , E  1122 , N  1124 , FC  1128 , and SC  1132  are unchanged from the IRB format as defined in “IBM® z/Architecture Principles of Operation,” Publication No. SA22-7832-05, 6th Edition, April 2007. When the IRB format control field X  1116  is set to a one, the IRB format fields F 0   1110 , F 1   1112 , and F 2   1114  are reserved for FCX use. The Q  1118  indicates completion of an interrogate operation. The AC  1130  provides activity status information, such as pending status, sub-channel active status, and device active status. The TCW address  1134  indicates the TCW being executed when the interrupted occurred. 
     The device status  1136  is copied from the device status  1028  of the response message  1000  of  FIG. 10 . The sub-channel status  1138  includes various checks and reserved values, e.g., program, protect, data, and control checks. The FCX status  1140  is copied from the status flags three  1026  of the response message  1000  of  FIG. 10 . The sub-channel extended status  1142  provides an extension to the sub-channel status  1138 , adding details as to why a particular check condition occurred. For example, bit  0  of the sub-channel extended status  1142  can be set to indicate that a program check, protect check, or IFCC was the result of an interrogate operation. A program check may be the result of a TCW channel program error detected by the control unit  110 . When a program check occurs, encoded values in bits  1  to  7  of the sub-channel extended status  1142  can convey the following information: 
     0. Null value used for program check conditions that do not require a value in the sub-channel extended status  1142 . 
     1. Storage-Request limit exceeded. A model-dependent number of storage requests have been exceeded for the requested block of data because software programming built an impossible to execute channel program. 
     2. Program check when the count in the transport command IU did not match the count the device expected. 
     3. Transport mode (i.e., TCW channel programs) is not supported in the control unit  110 . Execution of transport mode I/O was attempted to a device that does not support transport mode. 
     4. Fibre Channel Extension (FCX) is not supported in the channel  124 . Execution of transport mode I/O was attempted to a channel that does not support transport mode. 
     5. Reserved. 
     6. Program check on the TCW. The channel  124  detected an invalid TCW. 
     7. Device detected program check, possible IFCC. This encode is set if the channel  124  received encode  4  in status flags one  1008 . This error may be caused either by invalid control block structures in memory  102  or the information was corrupted on its way to the I/O device  112 . The OS  103  may escalate this to an IFCC, if all of the parameters and control blocks are correct in memory  102  for the operation. The channel  124  may create a log on this error. 
     8. Device detected program check. This encode is set if the channel  124  received encode  5  in status flags one  1008 . This error may be caused by invalid control block structures that were detected by the I/O device  112 . 
     9. to 31. Reserved. 
     Any one of the following encodes may be set in bits  1  to  7  as result of a protect check, invalid address (program check) or uncorrectable error (channel control check or channel data check) received as a response to a storage operation: 
     32. Storage exception on a TCW fetch. The following errors can cause this:
         a. Invalid address on a TCW fetch. Program check is set in the sub-channel status  1138 .   b. Protected address on a TCW fetch. Protect check will be set in the sub-channel status  1138 .   c. An uncorrectable error on a TCW fetch. Channel control check is set in the sub-channel status  1138 .       

     33. Storage exception on a TSB store. The following errors can cause this:
         a. Invalid address on a TSB store. Program check is set in the sub-channel status  1138     b. Protected address on a TSB store. Protect check is set in the sub-channel status  1138 .       

     34. Storage exception on a transport command IU fetch. The following errors can cause this:
         a. Invalid address on a TCCB fetch. Program check is set in the sub-channel status  1138 .   b. Protected address on a TCCB fetch. Protect check is set in the sub-channel status  1138 .   c. An uncorrectable error on a TCCB fetch. Channel control check is set in the sub-channel status  1138 .       

     35. Storage exception on a TIDAL fetch. The following errors can cause this:
         a. Invalid address on a TIDAL fetch. Program check is set in the sub-channel status  1138 .   b. Protected address on a TIDAL fetch. Protect check is set in the sub-channel status  1138 .   c. An uncorrectable error on a TIDAL fetch. Channel control check is set in the sub-channel status  1138 .       

     36. Storage exception on a data access. The following errors can cause this:
         a. Invalid address on a data access. Program check is set in the sub-channel status  1138 .   b. Protected address on a data access. Protect check is set in the sub-channel status  1138 .   c. An uncorrectable error on a data access. Channel data check is set in the sub-channel status  1138 .       

     37. to 63 reserved. 
     Any one of the following encodes may be set in bits  1  to  7  as result of an IFCC. 
     64. IFCC because of a CRC error detected by the channel  124  on data being received from the control unit  110 . 
     65. Reserved. 
     66. IFCC because of a Fibre Channel link protocol error. 
     67. IFCC occurred because a purge path command did not complete. 
     68. IFCC occurred on a purge path command because of an abort. 
     69. IFCC because a TCW residual count did not match the residual count  1016 . This can occur because the channel  124  or control unit  110  did not receive all of data IUs. 
     70. Invalid LRC  1030 . 
     71. Invalid LRC  1036 . 
     72. to 127 reserved. 
     Turning now to  FIG. 12 , a process  1200  for providing exception condition feedback at a control unit to a channel subsystem in an I/O processing system will now be described in accordance with exemplary embodiments, and in reference to the I/O processing system  100  of  FIG. 1 . At block  1202 , the control unit  110  receives a command message from channel  124  in the channel subsystem  108 . The command message may be a transport command IU, including a TCCB with multiple DCWs as part of a TCW channel program, e.g., command message  900  of  FIG. 9 . Communication between the channel subsystem  108  and the control unit  110  may be managed by the CU control logic  802  and the CHN control logic  810  of  FIG. 8  for a specific channel  124  of the channel subsystem  108 . 
     At block  1204 , the control unit  110  detects an exception condition in response to unsuccessful execution of at least one command in the command message. The exception condition may be relative to the command message format, I/O device  112  with which the control unit  110  interfaces, or other conditions as previously described in reference to  FIGS. 10 and 11 . 
     At block  1206 , the control unit  110  identifies a termination reason code associated with the exception condition. Other status flags, DCW offset and RCQ words associated with the exception condition can also be set in response to the exception condition as previously described. 
     At block  1208 , the control unit  110  writes the termination reason code to a transport response IU message (e.g., response message  1000  of  FIG. 10 , including a status section  1002  and an extended status section  1004 ). The transport response IU message includes exception condition feedback identifying a termination reason code (e.g., termination reason codes  1052  of  FIG. 10 ) in response to unsuccessful execution of at least one command in the command message. Additional status flags, DCW offset and RCQ words can also be included in the transport response IU message. 
     At block  1210 , the control unit  110  sends the transport response IU message to the channel subsystem  108 . The channel subsystem  108  stores the extended status  1004  at status area  408  referenced by TCW  402  of  FIG. 4 . The channel subsystem  108  can then use the exception condition feedback information in the transport response IU message to interrupt CPU  104  to provide exception condition information in IRB  1100  of  FIG. 11  and the extended status at location  104  of  FIG. 4 . 
     Technical effects of exemplary embodiments include providing of exception condition feedback to a channel subsystem from a control unit in an I/O processing system. The control unit provides the channel subsystem with notice of exception conditions, including termination reason codes, DCW offset and RCQ words, in a response message from a control unit. The channel subsystem can trigger an interrupt to a CPU in the I/O processing system, and provide both status and extended status information related to the exception conditions. Advantages include enabling control units to execute multiple commands unless/until an exception condition is encountered. Further advantages include providing enhanced reporting of exception conditions to a CPU via a channel subsystem interrupt, where a control unit identifies the exception conditions. Detecting exception conditions at the control unit may reduce the processing burden on the channel subsystem, as the channel subsystem can act as an information conduit without performing an additional layer of detailed checks of commands sent to the control unit. 
     As described above, embodiments can be embodied in the form of computer-implemented processes and apparatuses for practicing those processes. In exemplary embodiments, the invention is embodied in computer program code executed by one or more network elements. Embodiments include a computer program product  1300  as depicted in  FIG. 13  on a computer usable medium  1302  with computer program code logic  1304  containing instructions embodied in tangible media as an article of manufacture. Exemplary articles of manufacture for computer usable medium  1302  may include floppy diskettes, CD-ROMs, hard drives, universal serial bus (USB) flash drives, or any other computer-readable storage medium, wherein, when the computer program code logic  1304  is loaded into and executed by a computer, the computer becomes an apparatus for practicing the invention. Embodiments include computer program code logic  1304 , for example, whether stored in a storage medium, loaded into and/or executed by a computer, or transmitted over some transmission medium, such as over electrical wiring or cabling, through fiber optics, or via electromagnetic radiation, wherein, when the computer program code logic  1304  is loaded into and executed by a computer, the computer becomes an apparatus for practicing the invention. When implemented on a general-purpose microprocessor, the computer program code logic  1304  segments configure the microprocessor to create specific logic circuits. 
     While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another. Furthermore, the use of the terms a, an, etc. do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item.