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Q59953: 9th Pixel of 9x16 VGA Fonts Mimics 8th Pixel for ASCII 192-223

Article: Q59953
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | SR# S900206-119 B_BasicCom | mspl13_basic
Last Modified: 26-MAR-1990

The width of CGA, EGA, MCGA, and VGA text characters is 8 pixels. In
the case of VGA however, 9 pixels are actually used for displaying the
characters. The 9th pixel is appended to the right end of each pixel
row. If the character being displayed has an ASCII code ranging from
192 to 223 and the 8th pixel in a given pixel row is on, the 9th pixel
in that row will be on also. If the 8th pixel in the row is off or the
ASCII code for the character is not in the range 192 to 223, the 9th
pixel will not be turned on.

This hardware information may be of interest to users of Microsoft
QuickBASIC Versions 4.00, 4.00b, and 4.50 for MS-DOS, Microsoft BASIC
Compiler Versions 6.00 and 6.00b for MS-DOS and MS OS/2, and Microsoft
BASIC Professional Development System (PDS) Version 7.00 for MS-DOS
and MS OS/2.

The following diagram shows the pixel make-up of a sample VGA
character and how it would be displayed on the screen if it had an
ASCII code within the range 192 to 223, and if its ASCII code was also
outside that range:

   * = lit pixel
   O = unlit pixel

   Pixel make-up        ASCII 192             ASCII 255
   -------------        ---------             ---------

   OO****OO             OO****OOO             OO****OOO
   O******O             O******OO             O******OO
   ********             *********             ********O
   ********             *********             ********O
   ********             *********             ********O
   ********             *********             ********O
   ********             *********             ********O
   ********             *********             ********O
   ********             *********             ********O
   ********             *********             ********O
   ********             *********             ********O
   ********             *********             ********O
   ********             *********             ********O
   ********             *********             ********O
   O******O             O******OO             O******OO
   OO****OO             OO****OOO             OO****OOO

Note how the 9th pixel mimics the status of the 8th pixel preceding it
when the ASCII code is 192. When the ASCII code is 255 (outside the
range 192 to 223), the 9th pixel is always off, regardless of the
status of the 8th pixel. This behavior is limited to VGA text mode and
is not found in CGA, EGA, or MCGA.

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