Special Characters Aborting and Cancelling

Several of the normal printable characters have a special significance for UIPACK. These include the ``@" character, the ``?" character and several others. In addition to these printable characters, several non-printable (or CONTROL) characters have special significance, or may be granted special significance by an Application Program. The following CONTROL characters have significance in this fashion:-

Ctrl-C

This may be treated as a request for cancellation. It has no effect on either synchronous or asynchronous input, but the detection of this may be enabled by an Application Program. When Ctrl-C is detected then either it may activate a user-specified AST routine, or it may be polled for at intervals from within the body of an Application Program. As an example of such useage, the standard UIPACK Process Server (described in a later Section), uses the detection of Ctrl-C to disconnect the physical keyboard from a Subprocess and connect it to a menu of available options.

Ctrl-W

This only has significance in ViewText or ViewCursor modes of operation and causes the screen image to be refreshed. Thus any corruption of the image may be removed.

Ctrl-Z

This is used as an indicator that Input has been aborted.