The I/O configurations to the operating system (software) and the channel subsystem (hardware) must be defined. The Hardware Configuration Definition (HCD) component of z/OS® consolidates the hardware and software I/O configuration processes under a single interactive end-user interface.

The output of HCD is an I/O definition file (IODF), which contains I/O configuration data. An IODF is used to define multiple hardware and software configurations to the z/OS operating system.

When a new IODF is activated, HCD defines the I/O configuration to the channel subsystem or the operating system, or both. With the HCD activate function or the z/OS ACTIVATE operator command, changes can be made in the current configuration without having to initial program load (IPL) the software or power-on reset (POR) the hardware. Making changes while the system is running is known as dynamic configuration or dynamic reconfiguration.