Patent Publication Number: US-7904606-B2

Title: Transport control channel program chain linked branching

Description:
BACKGROUND 
     1. Field of Invention 
     The present disclosure relates generally to input/output (I/O) processing, and in particular, to transport control channel program chain linked branching in an I/O processing system. 
     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 I/O 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. 
     SUMMARY 
     An exemplary embodiment includes a computer program product for processing a transport control channel program with chain linked branching at a control unit configured for communication with an I/O 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 I/O subsystem to perform an I/O operation. The method further includes reading a chain linked flag in the command message, the chain linked flag indicating that a subsequent command message for the I/O operation follows the command message. The method also includes reading a serialization flag in the command message, the serialization flag requesting that device status be returned to the I/O subsystem in order to select the subsequent command message. The method additionally includes executing one or more commands in the command message, and transmitting the device status to the I/O subsystem in response to executing the one or more commands in combination with the serialization flag. 
     Another exemplary embodiment includes an apparatus for processing a transport control channel program with chain linked branching at a control unit in an I/O processing system. The apparatus includes a control unit configured for communication with an I/O subsystem of the I/O processing system. The control unit receives a command message from the I/O subsystem to perform an I/O operation, and reads a chain linked flag in the command message. The chain linked flag indicates that a subsequent command message for the I/O operation follows the command message. The control unit reads a serialization flag in the command message requesting that device status be returned to the I/O subsystem in order to select the subsequent command message. The control unit executes one or more commands in the command message, and transmits the device status to the I/O subsystem in response to executing the one or more commands in combination with the serialization flag. 
     A further exemplary embodiment includes method for processing a transport control channel program with chain linked branching at a control unit configured for communication with an I/O subsystem in an I/O processing system. The method includes receiving a command message at the control unit from the I/O subsystem to perform an I/O operation. The method further includes reading a chain linked flag in the command message, the chain linked flag indicating that a subsequent command message for the I/O operation follows the command message. The method also includes reading a serialization flag in the command message, the serialization flag requesting that device status be returned to the I/O subsystem in order to select the subsequent command message. The method additionally includes executing one or more commands in the command message, and transmitting the device status to the I/O subsystem in response to executing the one or more commands in combination with the serialization flag. 
     An additional exemplary embodiment includes a computer program product for processing a transport control channel program with chain linked branching at a channel subsystem configured for communication with a control unit 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 configuring a chain linked flag in a command message to indicate that a subsequent command message follows the command message to perform an I/O operation. The method further includes configuring a serialization flag in the command message to request that device status be returned to the channel subsystem in order to select the subsequent command message. The method also includes transmitting the command message from the channel subsystem to the control unit. 
     A further exemplary embodiment includes an apparatus for processing a transport control channel program with chain linked branching at a channel subsystem in an I/O processing system. The apparatus includes a channel subsystem configured for communication with a control unit of the I/O processing system. The channel subsystem configures a chain linked flag in a command message to indicate that a subsequent command message follows the command message to perform an I/O operation. The channel subsystem also configures a serialization flag in the command message to request that device status be returned to the channel subsystem in order to select the subsequent command message. Further, the channel subsystem transmits the command message from the channel subsystem to the control unit. 
     Additional features and advantages are realized through the techniques of the present invention. Other embodiments and aspects of the invention are described in detail herein and are considered a part of the claimed invention. For a better understanding of the invention with the advantages and the features, refer to the description and to the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS 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 forgoing and other 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  depicts one embodiment of a control unit and a channel subsystem, in accordance with an aspect of the present invention; 
         FIG. 3  depicts one embodiment of a transport control word (TCW) channel program with chain linked branching, in accordance with an aspect of the present invention; 
         FIG. 4  depicts one embodiment of a link protocol used to identify a compatible control unit of an I/O processing system, in accordance with an aspect of the present invention; 
         FIG. 5  depicts one embodiment of a request message of the link protocol of  FIG. 4 ; 
         FIG. 6  depicts one embodiment of an accept message of the link protocol of  FIG. 4 ; 
         FIG. 7  depicts one embodiment of an anchor control block in accordance with an aspect of the present invention; 
         FIG. 8  depicts one embodiment of a TCW 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 link protocol used to communicate between a channel and control unit to execute the TCW channel program with chain linked branching of  FIG. 3 , in accordance with an aspect of the present invention; 
         FIG. 11  depicts one embodiment of a process for providing TCW channel program chain linked branching at a channel subsystem in accordance with an aspect of the present invention; 
         FIG. 12  depicts one embodiment of a process for providing TCW channel program chain linked branching at a control unit in accordance with an aspect of the present invention; and 
         FIG. 13  depicts one embodiment of an article of manufacture incorporating one or more aspects of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In accordance with an aspect of the present invention, input/output (I/O) is facilitated with chain linked branching to enable conditional execution of transport control channel program portions. Support for program controlled interrupts between portions of the transport control channel program may also be provided. A transport control channel program facilitates I/O processing 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 through sending multiple commands and/or data to the control unit grouped in blocks for execution at the control unit rather than sending individual channel command words (CCWs). 
     Channel programs implemented with CCWs (also referred to as “CCW channel programs”) involve a large degree of handshaking to perform tasks. For example, writing a 4 kilobyte block of data using a CCW channel program typically requires an exchange to be opened, transmission of a define extent command with data, transmission of a locate record command with data, and transmission of a write command with data from the channel to the control unit. The control unit typically responds by opening an exchange and sending a response to acknowledge the write command, sending a status message upon completing the write command, and closing the exchange it opened. The channel may then respond by closing the exchange that it opened. Using a TCW channel program, a transport command control block (TCCB) can be sent from the channel to the control unit as a block of commands, avoiding many of the messages between the channel and the control unit that would otherwise be performed using a CCW channel program. For example, the TCW channel program can avoid opening an exchange to respond that the control unit received the write command. The cumulative effect over multiple command sequences can result in a large time savings when running a TCW channel program instead of a CCW channel program, and thus overall I/O processing system throughput is increased. In an exemplary embodiment, an I/O processing system can support CCW channel programs in command mode and TCW channel programs in transport mode. Transport mode indicates that the channel transports commands and data to the control unit without interpreting or distinguishing between the commands and data transported. 
     In an exemplary embodiment, the link protocol used for command mode communications is FICON (Fibre Connectivity). 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. The link protocol used for transport mode communications may be, for instance, Fibre Channel Protocol (FCP). In particular, three phases of the FCP link protocol can be used, allowing use of host bus adapters that support FCP to perform data transfers. 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. It will be understood that other versions of these protocols and/or similar protocols can be used within the scope of the invention. 
     A plurality of commands (e.g., device command words or “DCWs”) can be included in a TCCB, the contents of which are located via one or more address references (indirect or direct) in a transport control word (TCW). In an exemplary embodiment, the TCW is sent from an operating system (OS) 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. 
     A single TCCB may be constrained in size as a function of a link protocol or buffer size constraints, which can in turn limit the number of commands and/or amount of data associated with the TCCB. Some I/O operations can include a greater number of commands or volume of data than may be incorporated in a single TCCB. In an exemplary embodiment, chain linking of multiple TCWs with associated TCCBs is employed to create larger TCW channel programs, allowing a single I/O operation to span multiple TCWs and TCCBs. A program control interrupt (PCI) is also supported to send an intermediate notification of progress of a chain linked TCW channel program from the channel to the OS, enabling the OS to release or reuse resources that had previously been allocated for one or more commands of a TCCB prior to the PCI. The PCI serves as a compact status indicator without requiring a full extended status message after each TCCB. Chain linking of a TCW channel program with PCI support can enhance conversion of a lengthy CCW channel program into a chain linked TCW channel program that would otherwise include a larger number of commands than a single TCCB can hold. Furthermore, a chain linked TCW channel program may be more efficient than running a series of separate TCW channel programs in that an extended status message can be held off until completion of the full chain linked TCW channel program, rather than sending it for each separate TCW channel program. Moreover, overhead involved in configuring and managing communications may be further reduced when running a chain linked TCW channel program in contrast to a series of separate TCW channel programs, where each separate TCW channel program accomplishes a portion of an I/O operation. 
     Some I/O operations may be more efficient if looping or branching is employed. For example, in order to locate data on an I/O device, a search command with a search argument can be repeatedly executed until the search argument is found. In an exemplary embodiment, TCWs and TCCBs are modified to include chain linked branching support with conditional branching between TCCBs to perform an I/O operation. A serialize bit may be defined in a TCW and TCCB to present jump status between DCWs. The I/O communications adapter sends a first set of TCCBs containing “n” DCWs to the control unit. In response to the control unit presenting status to the I/O communications adapter for the first set of n DCWs, the I/O communications adapter examines the status and determines which of multiple TCCBs to fetch and send to the control unit. 
     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 (e.g., a fibre channel network). 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  1   10 . 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. 
     Turning now to  FIG. 2 , one embodiment of the control unit  110  and the channel  124  of  FIG. 1  that support chain linked branching TCW channel program execution is depicted in greater detail. The control unit  110  includes CU control logic  202  to parse and process command messages containing one or more TCCBs, received from the channel  124  via the connection  120 . The CU control logic  202  can extract DCWs and control data from the TCCB(s) received at the control unit  110  to control a device, for instance, I/O device  112  via connection  126 . The CU control logic  202  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  202  may use CU chain logic  204  to perform various checks of the command messages received at the control unit  110 , as well as determine an appropriate response. For example, the CU chain logic  204  can inform the channel  124  of the maximum number of linked commands that are supported. The CU chain logic  204  may also handle padding, incorrect length suppression, chain linking, and chain linked branching at the DCW level. While the CU chain logic  204  is depicted separately from the CU control logic  202 , it will be understood that the CU chain logic  204  can be incorporated as part of the CU control logic  202 . 
     The CU control logic  202  can access and control other elements within the control unit  110 , such as CU timers  206  and CU registers  208 . The CU timers  206  may include multiple timer functions to track how much time a sequence of I/O operations or a single I/O operation takes to complete. The CU timers  206  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. In an exemplary embodiment, the CU timers  206  continue to run between chained TCCBs until the chain completes as an I/O operation spanning multiple TCCBs. The CU registers  208  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  202 . 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  208  may include a maximum linked commands parameter that defines the maximum number of streamed command messages for one I/O operation that the control unit  110  supports. 
     The channel  124  in the channel subsystem  108  includes multiple elements to support communication with the control unit  110 . For example, the channel  124  may include CHN control logic  210  that interfaces with CHN subsystem timers  212  and CHN subsystem registers  214 . In an exemplary embodiment, the CHN control logic  210  controls communication between the channel subsystem  108  and the control unit  110 . The CHN control logic  210  may directly interface to the CU control logic  202  via the connection  120  to send commands and receive responses, such as transport command information units (TC_IUs) and response IUs. Alternatively, messaging interfaces and/or buffers (not depicted) can be placed between the CHN control logic  210  and the CU control logic  202 . The CHN subsystem timers  212  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  212  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  214  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. 
     In an exemplary embodiment, the channel subsystem  108  further includes CHN chain logic  216 . The CHN chain logic  216  can manage chain linking, chain linked branching, and PCI generation for the channel  124 . Although the CHN chain logic  216  is depicted separately from the CHN control logic  210 , it will be understood that the CHN chain logic  216  can be incorporated as part of the CHN control logic  210 . 
       FIG. 3  depicts an embodiment of a TCW channel program  300  with chain linked branching that includes an anchor control block (ACB)  302 , and TCWs  304 ,  306 ,  308 , and  310 . In an exemplary embodiment, the ACB  302  serves as an initial control block when the first TCW in the chain (TCW  304 ) includes two next TCW addresses for branching. In order to support common formatting and sizing constraints across TCWs  304 - 310 , the TCW field that would otherwise be used to hold an interrogate address location can hold a second next TCW address for branching. The ACB  302  may be the same size as the TCWs  304 - 310 , as a TCW format variation, and contains the interrogate address location, which frees space in the TCW  304  to support branching. The ACB  302  is chain linked to TCW  304 , and TCW  304  is chain linked to both TCW  306  and TCW  310 . TCW  306  is chain linked to TCW  308 . During execution of the TCW channel program  300 , the channel decides whether to branch to TCW  306  or TCW  310  based on status received from the control unit  110  in response to executing DCWs in TCCB  312 . 
     The TCW  304  also includes links to the TCCB  312  and a transport status block (TSB)  314 . The TCW  306  includes links to TCCB  316 , TSB  314 , and data area  318 . The TCW  308  includes links to TCCB  320 , TSB  314 , and data area  322 . The TCW  310  includes links to TCCB  324 , TSB  314 , and data area  326 . The various links to TCCBs, TSBs and data areas, such as TCCB  312 ,  316 ,  320 , and  324 , can be direct or indirect references to areas of memory. For example, transport blocks and data areas  312  and  316 - 326  can be further subdivided into smaller blocks (contiguous or non-contiguous) and managed using indirect lists pointing to the smaller blocks (e.g., lists of transport mode indirect data address words (TIDALs)). In an exemplary embodiment, the TCCB  312  is sent from channel subsystem  108  of  FIG. 1  to a targeted control unit  110  that parses and executes DCWs in the TCCB  312 . The control unit  110  reports conditions associated with the execution of the DCWs in TCCB  312  to the channel  124  in a status message. The CHN chain logic  216  in the channel  124  may select to send either TCCB  316  or TCCB  324  to the control unit  110  based on the conditions in the status message. If the channel selects TCCB  316 , then TCCB  320  is also sent to the control unit  110 . 
     The TSB  314  may remain at the channel subsystem  108  to hold status information associated with the execution of the TCCBs  312 ,  316 ,  320 , and/or  324  at the control unit  110 , enabling OSs  103  to access status information. The data areas  318 ,  322 , and  326  can be used to hold write data to send to the control unit  110  or read data received from the control unit  110 . 
     In an exemplary embodiment, the TCW channel program  300  with chain linked branching represents a single I/O operation that includes multiple commands chained across the TCWs  304 - 310  and TCCBs  312 ,  316 ,  320 , and  324 . The TCWs  304 - 310  each include a TSB address pointing to the same TSB  314 . If the I/O operation ends successfully, only the TSB address in the last TCW (TCW  308  or  310 ) is used by the channel  124 ; however, if the I/O operation ends early for whatever reason, the channel  124  can uses the TSB address in any TCW that the channel  124  may be working with, to obtain the memory address to store ending status in the TSB  314 . 
     The TCWs of  FIG. 3  may also include PCI support to generate a PCI upon completion of commands in the associated TCCB at the control unit  110  executing the TCCB. It will be understood that the configuration of and number of TCWs  304 - 310  and ACB  302  merely represents an embodiment, and is not limiting in scope, as there could be any number of TCWs chain linked with branching, including multiple or no PCIs as part of the TCW channel program  300 . Additionally, other branching configurations can be implemented in exemplary embodiments. For example, a TCW can branch back to its own address to loop on the same command set until a condition is met, such as a search. Branching can also be used to skip over or loop back to any TCW in the TCW channel program  300 . 
     In order to determine whether a control unit can support chain linked TCW channel programs, a compatibility link protocol may be employed prior to sending chain linked TCCBs to the control unit. An example of a compatibility link protocol is depicted in  FIG. 4 . Channel  400  sends a process login (PRLI) request  404  to the control unit  402  in a default communication format. The control unit  402  responds with a PRLI accept  406 , which may include information defining communication parameters that are acceptable to the control unit  402 . In response to receiving the PRLI accept  406 , the channel may proceed with sending chain linked TCCBs to the control unit  402  for execution, such as chain linked TCCBs  312 ,  316 ,  320 , and  324 . Other messages may also be exchanged between the channel  400  and the control unit  402  as part of link initialization and configuration. The channel  400  and the control unit  402  represent embodiments of the channel  124  and control unit  110  of  FIG. 1 . 
       FIG. 5  depicts an example of a PRLI Request message  500 , which represents an embodiment of the PRLI request  404  of  FIG. 4 . The payload of the PRLI Request message  500  may include a service parameter page, which includes service parameters for one or all image pairs. 
     The service parameter page of the PRLI Request message  500  may include multiple fields, such as type code  502 , type extension  504 , maximum initiation delay time  506 , flags  508 , and max linked commands  510 . Each field in the page of the PRLI Request message  500  is assigned to a particular byte address. Although one arrangement of fields within the page of the PRLI Request message  500  is depicted in  FIG. 5 , 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 page of the PRLI Request message  500  can be omitted or combined within the scope of the invention. 
     The type code field  502 , located at word  0 , byte  0 , represents the protocol type code, such as the Fibre Channel Single Byte Protocol type code. For example, a value of “1B” hexadecimal in this byte indicates that this service parameter page  500  is defined in the selected protocol (e.g., Fiber Channel single byte). The type extension  504 , located at word  0 , byte  1 , may further supplement the type code field  502 . 
     The maximum initiation delay time field  506 , located at word  3 , byte  0 , provides the maximum time (e.g., in seconds) that the channel  124  of  FIG. 1  can allow in the Initiation Delay Time field in a process Logout (PRLO) from the control unit  110 . 
     Flags  508 , in an exemplary embodiment, has the following definition: 
     Bit  0 —Transport Mode/Command Mode. A value of this bit set to one (1) means that the sender supports both Command Mode and Transport Mode. If the bit is set to zero (0), the sender only supports Command Mode. If the channel  124  sets this bit to a one, then the control unit  110  may respond with this bit set to one if it supports Transport Mode. 
     Bits  1 - 6 —Reserved. 
     Bit  7 —First Transfer Ready for Data Disabled. If both the channel  124  and control unit  110  choose to disable the first write transfer ready information unit (XFER_RDY IU), then the first TC_IU of all I/O operations performing writes between the channel  124  and control unit  110  operate without using the XFER_RDY IU before the first data information unit (Data IU) is transmitted for the first TC_IU of an I/O operation. The XFER_RDY IU is transmitted to request each additional Data IU, if any for the current TC_IU and any following TC_IUs for the channel program if any. 
     The max linked commands field  510  indicates the maximum count of additional Transport Command information units (TC_IUs) that the channel  124  supports for streaming to the control unit  110  as chain linked commands for the same I/O device  112  after the first TC_IU has been sent to the control unit  110 . Values may range from 0 to 15, with a value of zero meaning that the channel  124  does not support chain linking of TC_IUs. A value of X equal to one to fifteen indicates that the channel  124  will send out X TC_IUs after the first TC_IU for the same I/O device  112  (if there are X TCWs chain linked together) and then send out one new TC_IU for each previous TC_IU that completed until the channel program is completely executed. 
     In one exemplary embodiment, the remaining fields in the page of the PRLI Request message  500  are reserved and/or set to zero (0). For example, bytes  2  and  3  of word  0 , and words  1  and  2  are set to zero. Byte  1  and a portion of byte  2  of word  3  may also be reserved. 
     Turning now to  FIG. 6 , an example of a PRLI Accept message  600  is depicted, which represents an embodiment of the PRLI accept  406  of  FIG. 4 . The payload of the PRLI Accept message  600  may include a service parameter page. The service parameter page of the PRLI Accept message  600  may include multiple fields, such as type code  602 , type extension  604 , response code  606 , first burst size  608 , flags  610 , and max linked commands  612 . Each field in the page of the PRLI Accept message  600  is assigned to a particular byte address. Although one arrangement of fields within the page of the PRLI Accept message  600  is depicted in  FIG. 6 , it will be understood that the order of fields can be rearranged to alternate ordering, or can be omitted or combined, within the scope of the disclosure. 
     The type code field  602 , located at word  0 , byte  0 , is the protocol type code, and is similar to the type code field  502  of  FIG. 5 . The type extension field  604 , located at word  0 , byte  1 , corresponds to the type extension field  504  of  FIG. 5 . 
     The response code field  606 , located at word  0 , byte  2 , bits  4 - 7 , is defined by its corresponding protocol, such as the Fibre Channel Framing and Signaling protocol (FC-FS), which is described further in “ANSI INCITS 433-2007, Information Technology Fibre Channel Link Services (FC-LS)”, July 2007, which is hereby incorporated herein by reference in its entirety. 
     The First Burst Size field  608 , located at word  3 , bytes  0 - 1 , bits  0 - 15 , provides the maximum amount of data (e.g., the maximum number of 4k byte blocks of data) allowed in the first Data IU that is sent immediately after the first TC_IU, when the First Transfer Ready for Data Disabled flag bit (word  3 , byte  3 , bit  7 ) is set to one. A value of zero in this field indicates that there is no specified first burst size. 
     Flags  610  are similar to the flags  508  of  FIG. 5  described in conjunction with the PRLI Request message  500 . The control unit  110  sets values to these flags that correspond to the mode of operation it will run with the channel  124 . 
     In an exemplary embodiment, the max linked commands field  612  is the maximum count of streamed TC_IUs that the control unit  110  supports for one I/O operation. The control unit  110  responds with a count equal to or less than the value the channel  124  sent to the control unit  110  in the service parameter page for the PRLI Request message  500 . The channel  124  uses the count received from the control unit  110  as the maximum number of linked TC_IUs queued at the control unit  110 . If the control unit  110  responds with a count of zero, this means the control unit  110  does not support chain linking of TC_IUs. 
     In one exemplary embodiment, the remaining fields in the page of the PRLI Accept message  600  are reserved and/or set to zero (0). For example, bits  1 - 3  of word  0 , byte  2 , and words  1  and  2  are set to zero. Byte  3  of word  0  is reserved and set to zero. A portion of byte  2  of word  3  may also be reserved. 
     An exemplary embodiment of an anchor control block (ACB)  700  is depicted in  FIG. 7 , as a type of TCW. The ACB  700  may be utilized by the channel  124  of  FIG. 1  to link to the first TCW in a chain, such as ACB  302  linking to TCW  304  of  FIG. 3 , when the first TCW in the chain includes two TCW address pointers. The ACB  700  is the first control block, of a list of TCWs used when multiple TCWs that branch are used by one start subchannel command. The ACB  700  does not drive a TCCB to the control unit  110 . The channel  124  retains the address of the ACB  700  to fetch interrogate TCW address  712  if and when the channel  124  receives initiative to interrogate I/O device  112 . 
     In the exemplary ACB  700  depicted in  FIG. 7 , a format field  702  equal to “01” binary indicates that what follows is the ACB  700 , rather than a standard TCW with a value of “00” binary. The format field  702  with values of “10” and “11” binary may be reserved for future TCW/ACB formats. The ACB  700  may include reserved locations  704 ,  706 , and  708  for possible future use. First TCW address field  710  is the address of the first TCW in a chain for execution (e.g., TCW  304 ). OS  103  may configure the first TCW address field  710  when the ACB  700  is built. The interrogate-TCW address field  712  contains the address of another TCW and is used by the channel  124  to interrogate the state of an operation under the initiative of a cancel sub-channel I/O instruction. 
     The ACB  700  depicted in  FIG. 7  is one example of how an ACB can be configured. Other configurations are possible where additional fields are included and/or fields depicted in  FIG. 7  are not included. 
     An exemplary embodiment of a transport control word (TCW)  800  is depicted in  FIG. 8 . The TCW  800  may be utilized by the channel  124  of  FIG. 1  to set up the I/O operation and is not sent to the control unit  110 . The TCW depicted in  FIG. 8  provides for both input and output data within a single I/O operation. The TCW  800  illustrates formatting that can be used for TCWs that employ chain linked branching, such as TCW  304  of  FIG. 3 . 
     In the exemplary TCW  800  depicted in  FIG. 8 , a format field  802  equal to “00” binary indicates that what follows is a standard TCW  800 , with other values (e.g., 01, 10, 11) equating to TCW format variations. The TCW  800  may include reserved bits  804  for possible future use. 
     The TCW  800  also includes a flags field  806 . Reserved flags in the flags field  806  may be set to zero. Examples of flags bits that are mapped to the flags field  806  include a chain linked flag bit, a serialize flag bit, a PCI flag bit, a jump status supported flag bit, a TIDAL read flag, a TCCB TIDAL flag, and a TIDAL write flag. 
     When the chain linked flag bit set to a one, this informs the channel  124  that the next TCW address field  828  is to be used as the next TCW to be executed for the continuation of the I/O program. Counters, timers, and status tracking (e.g., CU timers  206  and/or CHN subsystem timers  212  of  FIG. 2 ) can continue from one TCCB to the next TCCB when the chain linked flag is set to a one, such as between TCCBs  312  and  316  or  324 . If the serialize flag bit is set to zero, exchanges may be closed by the control unit  110  for the intermediate TCCBs that were executed successfully with an equivalent of FCP zero status in the associated transport response IU. If the serialize flag bit is set to one, a status response is sent in a transport response IU. A full transport response IU with extended status is not transferred until the last TCCB of the chain linked channel program is executed or until the control unit  110  encounters an early end condition. Since the TCW  800  remains local to the channel  124 , the state of the chain linked flag can be sent to the control unit  110  as a chain linked TCCB flag in a TCCB as part of a TC_IU. 
     If the chain linked flag bit is set to a one and the serialize flag bit is set to a one, the channel  124  waits until the current TCW has completed before fetching the next TCW and transmitting the next TCCB to the control unit  110  (e.g., TCW  304  to TCW  306  or  310 ). In an exemplary embodiment, the serialize flag bit is set to a one if software is appending another TCW to the current TCW or if common data addresses exist for the current TCW and the following TCWs. Also, the serialize flag bit may be set to a one if the next TCW/TCCB to be executed is dependent on the ending device status from the I/O device  112 . A serialization required TCCB flag bit in a TCCB is also set to a one, when the serialize flag bit is set to one, informing the control unit  110  to send the device status to the channel  124  in the transport response IU. 
     If the chain linked flag and the PCI flag are set, the channel  124  generates an intermediate status interrupt when the TCW  800  is completed. This may result in marking the associated sub-channel as Sub-channel Active, Device Active and intermediate status pending. 
     The jump status supported flag bit indicates whether jump status is supported for the TCW  800 . The control unit  110  may send jump status encoded as a combination of a channel end (CE), a device end (DE), and a status modifier (SM) set in the transport response IU in response to determining that non-sequential execution between DCWs in TCCBs is desired. SM indicates that the control unit  110  detected that a status-modifying condition has occurred, and that a non-sequential instruction should be executed, rather than continuing to the next sequential instruction. When jump status is received, as determined by the combination of CE, DE and SM, and the chain linked, serialize, and jump status supported flag bits are all set to a one, then words  14  and  15  of TCW  800  are used to fetch the next TCW to execute (next TCW address for CE, DE, and SM status field  830 ). If the jump status supported flag bit is set to a zero and jump status is received, the channel  124  generates a program check. 
     In an exemplary embodiment, the TIDAL read flag is set to one when input-data address field  818  contains an address of a TIDAL. If the TIDAL read flag is set to zero, then the input-data address field  818  contains a data address. In an exemplary embodiment, the TCCB TIDAL flag is set to one when TCCB address field  822  contains an address of a TIDAL. If the TCCB TIDAL flag is set to zero, then the TCCB address field  822  directly addresses the TCCB. The TCCB TIDAL flag allows the operating system software or hyper-visor to layer function and prefix user channel programs. In an exemplary embodiment, the TIDAL write flag is set to one when output-data address field  816  contains an address of a TIDAL. If the TIDAL write flag is set to zero, then the output-data address field  816  contains a data address. 
     The TCW  800  also includes a TCCB length field  810  which indirectly represents the length of the TCCB and may be utilized to determine the actual length of the TCCB. 
     Read/write bits  812  in the TCW  800  are utilized to indicate whether data is being read and/or written as a result of executing the TCW  800 . In an exemplary embodiment, the read bit in the read/write  812  bits is set to one to indicate that input data is being transferred from an I/O device  112  to system storage (e.g., main memory  102 ) in the host system  101  as a result of executing the TCW  800 . The write bit in the read/write bits  812  is set to one to indicate that output data is being transferred from system storage (e.g., main memory  102 ) in the host system  101  to an I/O device as a result of executing the TCW  800 . 
     The output-data address field  816  includes the address for the output data (if any). As described previously, the contents of the output-data address field  816  may be an address of a TIDAL for output data (e.g., an indirect address) or the actual address of the output data (e.g., a direct address). The input-data address field  818  includes the address for the input data (if any). As described previously, the contents of the input-data address field  818  may be an address of a TIDAL for input data or the actual address of the input data. 
     The TCW  800  also includes a transport-status-block address field  820 . A portion (e.g., the extended status part) of a completion status in a transport response IU for an I/O operation is stored at this address. The TCCB address field  822  in the TCW  800  includes an address where the TCCB is located in system storage. As described previously, the TCCB is the control block where the DCWs to be executed for the TCW  800  reside. Also as described previously, the contents of the TCCB address field  822  may be an address of a TIDAL for the TCCB or the actual address of the TCCB. 
     The output count field  824  in the TCW  800  indicates the amount of output data to be transferred by the TCW/TCCB for an output operation. In an exemplary embodiment, the output count field  824  specifies the number of bytes in the output storage area designed by the TCW (the output-data address  816 ) to be transferred. The input count field  826  in the TCW  800  indicates the amount of input data to be transferred by the TCW/TCCB for an input operation. In an exemplary embodiment, the input count field  826  specifies the number of bytes in the input storage area designed by the TCW (the input-data address  818 ) to be transferred. 
     In an exemplary embodiment, words  12  and  13  of TCW  800  are used as a next TCW address for CE, DE status field  828 , holding the address of the next TCW to be executed when the status received from the I/O device  112  is CE and DE, and the chain linked and the serialize flag bits are set to a one. For example, in the TCW channel program  300  with chain linked branching of  FIG. 3 , the next TCW address for CE, DE status is field  828  of TCW  304  may be the address of TCW  306 . 
     As previously described, words  14  and  15  of TCW  800  may be the next TCW address for CE, DE, and SM status and field  830  is holding the address of the next TCW to be executed when the status received from the I/O device  112  is jump status (CE, DE, SM), and the chain linked, serialize, and jump status supported flag bits are set to a one. For example, in the TCW channel program  300  with chain linked branching of  FIG. 3 , the next TCW address for CE, DE, and SM status is field  830  of TCW  304  and may be the address of TCW  310 . Thus, depending upon status returned from the control unit  110 , the CHN chain logic  216  can select between at least two TCWs to determine the next TCW for execution. The next TCW address fields  828  and  830  may point to any TCW that is part of the channel program, including looping back to the same TCW to continuously execute a sequence of commands. 
     The TCW  800  depicted in  FIG. 8  is one example of how a TCW can be configured. Other configurations are possible where additional fields are included and/or fields depicted in  FIG. 8  are not included. 
     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  illustrates formatting that can be used for a variety of TC_IUs. 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 TCCBs  312 ,  316 ,  320 , and  324  of  FIG. 3  utilize formatting as depicted in the 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 sequence number  913 . The sequence number  913  informs the control unit  110  of the order to execute multiple command messages  900  that are all part of the same channel I/O operation targeting an I/O device (e.g., I/O device  112 ). The sequence number  913  starts at (01h) in the first TC_IU for each start to the I/O device  112  independent of the value it ended on for the last start to the same I/O device  112 . If an I/O operation only contains one TCW/TCCB, then the value of the sequence number  913  is set to zero. The TC_IUs chain linked together are executed in the order of the sequence numbers, even if the TC_IUs are received at the control unit  110  out of order. 
     The TCH  904  includes task information  914 , which may be set to a reserved value, e.g., zero, while operating in transport mode. The TCH  904  also includes L 1  length  916  and read/write field  918 . The L 1  length  916  defines the length of the TCA  908  in words +1. The L 1  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 , TCCB-flags  927 , as well as L 2  length  928 . 
     The TCCB-flags  927  inform the control unit  110  about the characteristics of the command message  900  (the current TC_IU). The TCCB-flags  927  may include a chain linked TCCB flag bit and a serialization required flag bit. The chain linked TCCB flag set to a one informs the control unit  110  that there is another TC_IU following the current TC_IU that is part of the same I/O operation. Counters, timers, and status tracking (e.g., CU timers  206  and/or CHN subsystem timers  212  of  FIG. 2 ) can continue from one TCCB to the next TCCB when the chain linked TCCB flag is set to a one, and a CC bit is set to a one in the last DCW (e.g., DCW  946 ) in the TCA  908  for this TC_IU. If the serialization required flag bit is not set to a one, the exchange may be closed when the TC_IU is executed successfully with an equivalent of FCP zero status, which equates to channel end (CE), device end (DE) only status. If the serialization required flag bit is set to a one, then device status is sent in the response IU as further described herein. No extended status is transferred until the last TC_IU for the TCW channel program is executed or for the TC_IU that ended the TCW channel program. The channel  124  sends the next TC_IU to the control unit  110  based on the serialize flag bit in the TCW  800  and the value of a TC_IU streaming count, which can be tracked in the CHN subsystem registers  214  of  FIG. 2 . If the serialize flag is set to a zero, the channel  124  sends TC_IUs up to the max linked commands (e.g., max linked commands  612 ), and then sends the subsequent TC_IUs as each previous TC_IU is completed. If the serialize flag bit is set to a one, the channel  124  waits until status is received for the last TC_IU sent to the control unit  110  before sending the next TC_IU. 
     The serialization required flag bit has no meaning if the chain linked TCCB flag bit is not set to a one. The serialization required flag bit informs the control unit  110  that even though the chain linked TCCB flag bit is set to a one, the next TC_IU will not be seen by the control unit  110  until device status is sent to the channel  124  in an 8-word transport response IU. However, extended status is not sent in the transport response IU for this case. 
     The L 2  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 L 2  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 flags field  950  may also include a command chain (CC) flag bit. 
     The CC flag bit indicates a command chain to the next DCW in the TCA  908 . The CC flag bit set to zero means that the associated DCW is the last DCW of the program. The CC flag bit can be set in the last DCW of the TCA  908  if the chain linked TCCB flag is set in the TCCB-flags field  927  and the chain linked flag bit is set in the flags field  806  in the TCW  800 . 
     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. If both the read and write bits are active in read/write field  918 , then the transport data byte count  992  is for the write data, and bidirectional read data length  994  in TCAT  910  is the read transport data byte count. 
     Unusual ending conditions may be handled as follows when multiple TCWs/TCCBs are chained to form a chain-linked TCW channel program. For chain linked TCWs channel programs, a halt subchannel command causes all active exchanges to be aborted for the I/O device  112  and the subchannel to be returned to the OS  103  with primary, secondary and alert status. A clear subchannel command for chain-linked TCW channel programs may cause all active exchanges to be aborted for the I/O device  112 , followed by sending a selective reset command to the I/O device  112 . 
     For the case where the channel  124  is sending multiple TCCBs (in TC_IUs) chain linked together to the control unit  110 , if the control unit  110  cannot execute any one of the TCCBs, the control unit  110  can send terminating ending status, busy status (can only be sent in response to the first TC_IU of a channel program) or retry status, with a status confirm, on the exchange for the TCCB that is ending early. The control unit  110  also closes other outstanding exchanges for the same I/O operation, which have a sequence number greater than the sequence number of the exchange on which the terminating status was sent. When the channel  124  detects a terminating ending status IU with the request for a confirm request, the channel  124  stops sending new TCCBs to the control unit  110  for that operation. All other exchanges for that I/O operation that are not closed after a timeout period (for example, 100 milliseconds) are aborted by the channel  124 . When all of the other exchanges are closed for the I/O operation, the channel  124  sends the confirm message, which closes the final exchange. 
     If one of the exchanges, out of many that were opened to send TCCBs chain linked together to the control unit  110  is lost, the channel  124  times out that exchange and send a Read Exchange Concise (REC) to the control unit  110  inquiring about the exchange. If the control unit  110  informs the channel  124  that it does not have the exchange, the channel  124  aborts outstanding exchanges to the control unit  110  for the I/O operation. 
       FIG. 10  depicts one embodiment of a link protocol used to communicate between a channel  1000  and control unit  1002  to execute the TCW channel program with chain linked branching of  FIG. 3 , where the channel  1000  and control unit  1002  are embodiments of the channel  124  and control unit  110  of  FIG. 1 . Once the channel  124  and the control unit  110  establish that Transport Mode is supported and the maximum number of linked commands is established, the TCW channel program with chain linking can be executed. An OS, such as OS  103  of  FIG. 1 , builds the ACB  302  and TCWs  304 ,  306 ,  308  and  310  associated control blocks TCCB  312 ,  316 ,  320  and  324  shown in  FIG. 3  and executes a start subchannel command with an address in an operation request block that points to ACB  302 . The ACB  302  provides a pointer to the first TCW in the chain (e.g., first TCW address field  710  pointing to TCW  304 ), and is also the control block that the channel  1000  re-fetches to acquire the interrogate address (e.g., interrogate-TCW address field  712 ) when the channel  1000  is given initiative to perform an interrogate. Assume in this example that the chain linked flag bit is set to a one in TCWs  304  and  306  and the serialize flag bit is set to a one in TCW  304 . 
     Upon attempting to fetch the first TCW, the channel  1000  discovers that it is actually an ACB (ACB  302 ), based on format field  702 . The channel  1000  remembers the address of the ACB  302  and uses the first TCW address field  710  in the ACB  302  to fetch TCW  304  and the associated TCCB  312 . 
     The channel  1000  sends TCCB  312  in TC_IU  1004 , opening exchange A with a sequence number of one to the control unit  1002 . Because the serialize and chain linked flag bits are set to a one in TCW  304 , the channel  1000  will not fetch the next TCW until status is received from the control unit  1002  (e.g., status of I/O device  112 ) for TC_IU  1004 . In TCW  304  and in TCCB  312  for this example, both the read and write bits are set to a zero, which tells the channel  1000  and the control unit  1002  that no customer data will be transferred for TCW  304 /TCCB  312 . 
     At the control unit  1002 , the chain linked TCCB flag bit set to a one in TC_IU  1004  informs the control unit  1002  that another TC_IU with a sequence number one greater than the sequence number of the TC_IU  1004  is part of this I/O operation. The serialization required flag bit is also set to a one, informing the control unit  1002  that the next TC_IU will not be sent by the channel  1000  until device status is sent to the channel  1000  for TC_IU  1004 . 
     When the control unit  1002  completes the execution of the TC_IU  1004  at I/O device  112 , it sends a 32-byte Transport Response IU  1006  that includes the device status that also closes exchange A and informs the channel  1000  that TC_IU  1004  has completed. For this example, assume the device status is CE, DE and SM (jump status), which causes the channel  1000  to fetch TCW  310  of  FIG. 3  using the next TCW address for CE, DE, and SM status field  830  in TCW  304 . 
     The channel  1000  sends TCCB  324  in TC_IU  1008  opening exchange B with a sequence number of  2  to the control unit  1002 . The control unit executes TC_IU  1008  at the I/O device  112  and sends the data read from the I/O device  112 , by read DCW commands in TC_IU  1008 , to the channel  1000  as data IUs  1010  on exchange B. When the control unit  1002  completes TC_IU  1008 , it sends a complete Transport Response IU  1012  that includes the extended status that includes  48  to  64  (or more) bytes and closes exchange B and informs the channel  1000  that the entire I/O operation has completed. The channel  1000  presents primary status to the OS (e.g., OS  103 ), informing the OS that the I/O operation has completed. 
     In an exemplary embodiment, extended status includes various timing parameters that can be continued between TC_IUs, such as TC_IUs  1004  and  1008 , as calculated using CU timers  206  of  FIG. 2 . For example, extended status can include a total device time parameter, defer time parameter, queue time parameter, device busy time parameter, device active only time parameter, and appended device sense data. The total device time parameter is the elapsed time from when the control unit  1002  received the TC_IU  1004  until sending the transport response IU  1012  for the I/O operation. The defer time parameter indicates control unit defer time. This is the time accumulated by the control unit  1002  working with the I/O device (e.g., I/O device  112 ) when no communication with the channel  1000  is performed. The queue time parameter is the time that an I/O operation is queued at the control unit  1002 , but does not include queue time for device busy time where the I/O device is reserved by another channel  1000  under control of a different OS (e.g., OS  103 ) on the same system or on another system. The device busy time parameter is the time that a TC_IU is queued at the control unit  1002  waiting on a device busy caused by the I/O device being reserved by another channel  1000  under control of a different OS on the same system or on another system. The device active only time parameter is the elapsed time between a CE and a DE at the control unit  1002 , when the control unit  1002  holds the CE until DE is available. The appended device sense data is supplemental status that the control unit  1002  provides conditionally in response to an active unit check (UC) bit in the device status. 
       FIG. 11  depicts a process  1100  for providing TCW channel program chain linked branching at a channel subsystem in accordance with an exemplary embodiment, and is described in reference to the I/O processing system  100  of  FIG. 1  and subsequent figures. The process  1100  is also described in conjunction with process  1200  as depicted in  FIG. 12  for providing TCW channel program chain linked branching at a control unit, such as between channel  124  of channel subsystem  108  and control unit  110  of  FIG. 1 . In an exemplary embodiment, the CHN chain logic  216  of  FIG. 2  manages processing associated with chain linked branching for the channel  124 , and the CU chain logic  204  manages processing associated with chain linked branching for the control unit  110 . At block  1102 , the channel  124  of channel subsystem  108  configures a chain linked flag in a command message to indicate that a subsequent command message follows the command message to perform an I/O operation. At block  1104 , the channel  124  of channel subsystem  108  configures a serialization flag in the command message to request that device status be returned in order to select the subsequent command message. At block  1106 , the channel  124  of channel subsystem  108  transmits the command message from the channel  124  of channel subsystem  108  to the control unit  110 . 
     At block  1202 , the control unit  110  receives the command message from the channel  124  of channel subsystem  108  to perform the I/O operation. For example, the command message may be TC_IU  1004  of  FIG. 10  with formatting as depicted in  FIG. 9 . 
     At block  1204 , the control unit  110  reads the chain linked TCCB flag in the command message. The chain linked TCCB flag indicates that the subsequent command message for the I/O operation follows the command message, such as TC_IU  1008  following TC_IU  1004  of  FIG. 10 . 
     At block  1206 , the control unit  110  reads the serialization flag in the command message. The serialization flag requests that device status be returned to the channel  124  of channel subsystem  108  in order to select the subsequent command message. 
     At block  1208 , the control unit  110  executes one or more commands in the command message. The one or more commands may be DCWs, such as DCWs  940 - 946  of  FIG. 9 , requesting to read or write data to the I/O device  112 . The control unit  110  receives device status in response to executing the one or more commands. The device status can include CE, DE, and/or SM, where the combination of CE, DE, and SM is a jump status. 
     At block  1210 , the control unit  110  transmits the device status to the channel  124  of channel subsystem  108  in response to executing the one or more commands in combination with the serialization flag. The device status may be transmitted in a transport response IU without extended status, such as transport response IU  1006  of  FIG. 10 . 
     Returning to  FIG. 11 , at block  1108 , the channel  124  of channel subsystem  108  receives the device status in response to transmitting the command message. At block  1110 , the channel  124  of channel subsystem  108  selects the subsequent command message in response to the received device status. The channel  124  examines device status received in the transport response IU and determines which TCW to access for a TCCB holding the subsequent commands. If the device status received includes CE and DE, then the channel  124  reads the next TCW address for CE, DE status field  828  of the current TCW to determine the next TCW/TCCB as the subsequent command message including a subsequent set of one or more commands sequential to the current commands. If the device status received includes CE, DE and SM, then the channel  124  reads the next TCW address for CE, DE, and SM status field  830  of the current TCW to determine the next TCW/TCCB as the subsequent command message including a subsequent set of one or more commands non-sequential to the current commands. For example, if CE, DE, and SM are set after executing commands in TCCB  312 , then the channel  124  can select TCW  310  and TCCB  324  as the next TCW/TCCB; however, if SM is not set after executing commands in TCCB  312 , then the channel  124  can select TCW  306  and TCCB  316  as the next TCW/TCCB. At block  1112 , the channel  124  of channel subsystem  108  transmits the selected subsequent command message from the channel  124  of channel subsystem  108  to the control unit  110 . 
     Additional subsequent messages can be received at the control unit  110  as part of the chain linked channel program. The control unit  110  can read a chain linked flag (e.g., chain linked TCCB flag in TCCB-flags  927  of  FIG. 9 ) in the command message to determine whether subsequent command messages are expected to follow the first command message as part of the I/O operation. In response to determining that the subsequent command message is expected, and upon executing the one or more commands received, the control unit  110  may continue to run counters associated with the I/O operation to span multiple command messages (e.g., CU timers  206 ), and transmit a transport response message without extended status. In similar fashion, each command message received can be analyzed to determine whether additional command messages are expected as part of the chained I/O operation. The control unit  110  receives the subsequent command message including a subsequent set of one or more commands, and examines command chain flags associated with each command (e.g., CC bits of the DCW flags  950 ,  962 ,  972 , and/or  982 ) in the subsequent one or more commands received to locate a final chain linked command. The control unit  110  transmits a response message with extended status for the I/O operation in response to locating and executing the final chain linked command, in which case the subsequent command message is the final command message of the chain. Shorter status messages communicated as transport response messages without extended status can provide the channel  124  with intermediate status after each command message is executed prior to the final command message. Upon executing the commands of the final command message, extended status is transmitted that provides additional information and status for the full I/O operation. 
     The control unit  110  can also handle other error conditions. For example, the control unit  110  may determine that one or more commands associated with a communication exchange cannot execute. The control unit  110  can respond sending a termination status message to the channel  124  of the channel subsystem  108  indicating an inability to execute. The control unit  110  closes open communication exchanges with sequence numbers greater than the sequence number associated with the one or more non-executable commands. For example, if the control unit  110  has received sequence numbers 1, 2, 3, and 4 on exchanges A, B, C, and D, and an error occurs in executing commands associated with sequence number 2, the control unit  110  can notify the channel  124  of the error on exchange B with extended status, with a request for a confirm, on exchanges B, and closes exchanges C, and D. The channel  124  will close exchange B with a confirm on exchange B after it has seen that exchanges A, C and D have been closed (assuming A closes after successful completion of sequence 1 commands). 
     Technical effects of exemplary embodiments include chaining of multiple TCW and TCCBs together with conditional links to form a transport control channel program with chain linked branching that spans multiple TCWs and TCCBs for an I/O operation. The channel reads TCW contents for formatting constraints to determine whether a TCW is actually an ACB, and to further determine which addresses to access in response to status returned from the control unit. A channel may inform a control unit that a subsequent command message is chain linked to a current command message and that the channel will select a specific subsequent command message in response to the control unit returning device status associated with the current command message. If the channel receives a jump status, it can fetch a non-sequential TCW and TCCB, rather than proceeding to the next immediate TCW and TCCB in the chain. Non-sequential TCWs/TCCBs can be a branch back to the current TCW, a jump ahead in the same chain path as would be reached sequentially or a redirection to a different chain path that would not otherwise be accessible during sequential progression of the channel program. The channel may also enable periodic status interrupts to be sent while a transport control channel program is executing but not fully complete. Periodic status enables the host to confirm that a number of commands have executed and thus buffers associated with the commands that have completed can be released or reused without waiting for the full program to complete. 
     The capabilities of the present invention can be implemented in software, firmware, hardware or some combination thereof 
     As will be appreciated by one skilled in the art, the present invention may be embodied as a system, method or computer program product. Accordingly, the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a “circuit,” “module” or “system.” Furthermore, the present invention may take the form of a computer program product embodied in any tangible medium of expression having computer usable program code embodied in the medium. An example includes 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. There may be multiple computer program products  1300 , with each directed to implement functional processes on separate processing circuitry. For example, the processes  1100  and  1200  of  FIGS. 11 and 12  can be embodied as computer program code logic  1304  on separate computer program products  1300 , with one executable on the host system  101  of  FIG. 1  and the other executable at one or more control units  110  of  FIG. 1 . Alternatively, the processes  1100  and  1200  can be stored as computer executable code on a single computer program product  1300 . 
     Any combination of one or more computer usable or computer readable medium(s) may be utilized. The computer-usable or computer-readable medium may be, for example but not limited to, an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system, apparatus, device, or propagation medium. More specific examples (a non-exhaustive list) of the computer-readable medium would include the following: an electrical connection having one or more wires, a portable computer diskette, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or Flash memory), an optical fiber, a portable compact disc read-only memory (CDROM), an optical storage device, a transmission media such as those supporting the Internet or an intranet, or a magnetic storage device. Note that the computer-usable or computer-readable medium could even be paper or another suitable medium upon which the program is printed, as the program can be electronically captured, via, for instance, optical scanning of the paper or other medium, then compiled, interpreted, or otherwise processed in a suitable manner, if necessary, and then stored in a computer memory. In the context of this document, a computer-usable or computer-readable medium may be any medium that can contain, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device. The computer-usable medium may include a propagated data signal with the computer-usable program code embodied therewith, either in baseband or as part of a carrier wave. The computer usable program code may be transmitted using any appropriate medium, including but not limited to wireless, wireline, optical fiber cable, RF, etc. 
     Computer program code for carrying out operations of the present invention may be written in any combination of one or more programming languages, including an object oriented programming language such as Java, Smalltalk, C++ or the like and conventional procedural programming languages, such as the “C” programming language or similar programming languages. The computer program code logic  1304  of  FIG. 13  represents an embodiment of program code. The program code may execute entirely on the user&#39;s computer, partly on the user&#39;s computer, as a stand-alone software package, partly on the user&#39;s computer and partly on a remote computer or entirely on the remote computer or server. In the latter scenario, the remote computer may be connected to the user&#39;s computer through any type of network, including a local area network (LAN) or a wide area network (WAN), or the connection may be made to an external computer (for example, through the Internet using an Internet Service Provider). 
     The present invention is described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems) and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. 
     These computer program instructions may also be stored in a computer-readable medium that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable medium produce an article of manufacture including instruction means which implement the function/act specified in the flowchart and/or block diagram block or blocks. 
     The computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide processes for implementing the functions/acts specified in the flowchart and/or block diagram block or blocks. 
     The flowchart and block diagrams in the Figures illustrate the architecture, functionality, and operation of possible implementations of systems, methods and computer program products according to various embodiments of the present invention. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one ore more other features, integers, steps, operations, element components, and/or groups thereof. 
     The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated. 
     The flow diagrams depicted herein are just one example. There may be many variations to this diagram or the steps (or operations) described therein without departing from the spirit of the invention. For instance, the steps may be performed in a differing order or steps may be added, deleted or modified. All of these variations are considered a part of the claimed invention. 
     While the preferred embodiment to the invention had been described, it will be understood that those skilled in the art, both now and in the future, may make various improvements and enhancements which fall within the scope of the claims which follow. These claims should be construed to maintain the proper protection for the invention first described.