I/O deviceparameters either perform actions that cause the device to do something (for example, CLEARSCREEN), or specify characteristics that modify the way the device subsequently behaves (for example, RECORDSIZE, ALLOCATION). When an I/O command has multiple action deviceparameters, GT.M performs the actions in the order of the deviceparameters within the command argument. When a command has characteristic deviceparameters, the last occurrence of a repeated or conflicting deviceparameter determines the characteristic.

Deviceparameters often relate to a specific device type. GT.M ignores any deviceparameters that do not apply to the type of the device specified by the command argument. Specified device characteristics are in force for the duration of the GT.M image, or until modified by an OPEN, USE, or CLOSE command.

When reopening a device that it previously closed, a GT.M process restores all characteristics not specified on the OPEN to the values the device had when it was last CLOSEd. GT.M treats sequential disk files differently and uses defaults for unspecified sequential disk file characteristics on every OPEN (that is, GT.M does not retain sequential disk file characteristics on a CLOSE).

The ZSHOW command with an argument of "D" displays the current characteristics for all devices OPENed by the process. ZSHOW can direct its output into a GT.M variable. For more information on ZSHOW, refer "Commands".