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Screen updates

Since you are looking at this text, I might assume that you have allready some programming experience. You most probably collected your experience programming raster screen 'machines'. Be it computers, arcades, consoles, handhelds or whatever. If you have allready programmed vector screen 'machines' you will find nothing new in this small section. If you on the other hand have no exp- erience programming vector displays, let me do a little introduction first, and be ready for a whole new experience in screen updates :-).
There are nice and not so nice aspects of vector screens, as you will see. You as a programmer have total control over the vector beam (or you let the BIOS do the job for you -> recomended!). If you want to draw a vector you have to place the beam at the position you want the vector to start. 'Switch the light on', move the beam and 'switch the light off'. Due to the persistence of the screen, the vector will than be displayed for some milliseconds (I don't exactly know for how long). Than the vector will fade away. Most of the time you will not be satisfied with a vector being displayed only for some milliseconds, so sooner or later you will start thinking about how to draw a stable image. Well, there is no secret to it, you just have to keep drawing... .Draw the vector you want to appear stable, about 50 times per second and you can happily view a stable vector image! While the BIOS has routines for vector drawing, even vector lists or string drawing, it does not update the screen automatically. You as a programmer are responsible for updating the screen regularily. If you lose to much time between these updates the display will surely flicker. As a rule of thumb the above mentioned value of 50Hz is sufficient for a stable image. The nice effect of this is, that you don't have to worry about garbage on your screen, if you move a 'sprite' you don't have to keep track of the background, since in the next round everything is 'clean' again.

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