GETSCANLINE
This will report on the line that is currently being drawn on the VGA monitor in the range of 0 to 525. This is irrespective of the current MODE.
Using this to time updates to the screen can avoid timing effects caused by updates while the screen is being updated.
The first visible line will return a value of 0. Any line number above 479 is in the frame blanking period.
GPS functions
The GPS functions are used to return data from a serial communications channel opened as GPS.
The function GPS(VALID) should be checked before any of these functions are used to ensure that the returned value is valid.