When a GKS workstation is opened, the display surface is cleared. For some data reduction system architectures this is inappropriate as it prevents one applications program from adding to or interacting with a picture drawn by another. To circumvent this difficulty, Starlink has implemented an escape function which suppresses the screen clearing. This facility should only be used where absolutely necessary as it is unique to Starlink’s GKS and is only available on devices with device handlers written or modified by Starlink. It is only available for interactive devices where other techniques such as the use of metafiles to redraw pictures are too slow to be useful.
To enable the suppression of screen clearing on those devices that support it, the GKS routine GESC should be called with escape function identifier 1000 and an ASCII value of 1 as the first character of the data record. The feature is disabled by 0 in the first character.
An SGS function to access this facility is available and this should be used wherever possible because the details of the parameters may change in the future as function identifier values are registered with ISO.