Logical input devices can be operated in three modes: Request, Sample, and Event ('GREQU', 'GSAMPL', 'GEVENT'). In Request mode the application enables a device and then waits for the user to trigger input, perhaps by pushing a key. This is similar to issuing a FORTRAN READ. In Sample mode the application can measure the current value of the device, for example the locator position, without waiting for a trigger. Finally, in Event mode, the application can enable a set of several logical devices simultaneously. Output measures from devices which have been triggered will be placed in an Event Queue, from whence they can be extracted by the application.
Note that in GKS input level 'b' only Request mode input is available, and that it is not possible to request GKS input and issue a FORTRAN I/O operation at the same time. (However, although not required by the standard, the GKSGRAL level 'b' implementation provides Sample input for some devices.)