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Q58567: Any EGA/VGA Video RAM Above 256K Not Usable in Compiled BASIC

Article: Q58567
Product(s): See article
Version(s): 2.00 2.01 3.00 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | SR# S900201-17 B_BasicCom | mspl13_basic
Last Modified: 13-FEB-1990

Any RAM above 256K on an EGA or VGA video card cannot be used to store
screen pages during the execution of a compiled BASIC program.

This information applies to Microsoft QuickBASIC Versions 2.00, 2.01,
3.00, 4.00, 4.00b, and 4.50 for MS-DOS, to Microsoft BASIC Compiler
Versions 6.00 and 6.00b for MS-DOS, and to Microsoft BASIC
Professional Development System (PDS) Version 7.00 for MS-DOS.

These compilers are compatible with video cards that conform to the
IBM standard. Accessing video RAM above 256K is not part of this
standard.

For example, when reading the documentation on the SCREEN statement in
either the manuals or online Help included with the above products, it
may appear that more than 2 pages of screen memory are available to a
512K VGA card operating in screen mode 9. Footnote 2 on the bottom of
Page 314 of the "Microsoft BASIC 7.0: Language Reference" manual
states the following:

   Pages = Screen memory divided by page size. Eight page maximum,
           one page minimum.

The page size of screen 9 for VGA is 128K. Applying the above formula,
we get the following:

   Pages = 512K / 128K = 4 pages

However, 4 pages is incorrect because all the memory above 256K on
the VGA card is not usable. Therefore, the maximum number of pages is
as follows:

   Pages = 256K / 128K = 2 pages

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