Providing indirect data addressing for a control block at a channel subsystem of an I/O processing system

An apparatus for providing indirect data addressing for a control block at a host computer system configured for communication with a control unit, the host computer system performing:

• obtaining a transport command word (TCW) for an I/O operation, the TCW specifying a location address and indicating whether the TCW directly or indirectly addresses a message for transmitting one or more commands to the control unit;
• extracting the specified location address from the TCW;
• obtaining the message from the specified location address based on the TCW indicating direct addressing, the message including the one or more I/O commands;
• gathering the one or more I/O commands from a plurality of storage locations specified by a list of addresses identified by the specified location address to form the message based on the TCW indicating indirect addressing;
• and
• forwarding the message to the control unit for execution.

A method for providing indirect data addressing for a control block at a host computer system, the host computer system including a channel subsystem configured for communication with a control unit, the method comprising:

• obtaining a transport command word (TCW) for an I/O operation by the channel subsystem, the TCW specifying a location address and indicating whether the TCW directly or indirectly addresses a message for transmitting one or more commands to the control unit;
• extracting the specified location address from the TCW;
• obtaining the message from the specified location address based on the TCW indicating direct addressing, the message including the one or more I/O commands;
• gathering the one or more I/O commands from a plurality of storage locations specified by a list of addresses identified by the specified location address to form the message based on the TCW indicating indirect addressing;
• and
• forwarding the message to the control unit for execution.