Topic       : TOS - The Operating System
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Subject     : Programmieren/Atari
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View Ref-File8.2.5  The quarter-screen buffer                                   TOS

The quarter-screen buffer is required by the screen-manager to save 
the contents of the menu background when drop-down menus drop down. 
The 'QSB' (the usual abbreviation) is also used for the display of 
alert boxes. Normally its size should depend on the number of colour 
planes and the size of the system font, but not on the total size of 
the screen.

A good formula would be:
  500(characters) * space of one character * colour planes

In 'ST High' resolution this would give a value of exactly 8000 (i.e. 
a quarter of the screen memory). Unfortunately in many cases the AES 
is not so clever. The following table contains a summary of the algo- 
rithm used by some GEM versions:

 GEM version Method for setting the QSB

 1.0 and 1.2 Static, 8000 bytes
 1.4     Dynamic, a quarter of the screen memory
 3.0     Dynamic, half of the screen memory

Note: The GEM versions 1.0 and 1.2 (i.e. up to and including TOS 
Version 1.02) are not prepared by this for colour graphics cards - one 
of several reasons why even with the use of a special VDI driver under 
these GEM versions one can not make use of colour graphics cards.

See Also: GEM