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Q44412: Color and Monochrome Attributes in SCREEN 0 (Text) Explained

Article: Q44412
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | SR# S890508-33 B_BasicCom docerr | mspl13_basic
Last Modified: 20-DEC-1989

This information below refers to the SCREEN 0 description in the
following sources: the QB Advisor on-line Help system of QuickBASIC
Version 4.50; Page 373 of "Microsoft QuickBASIC 4.0: BASIC Language
Reference" for Versions 4.00 and 4.00b; Page 373 of "Microsoft BASIC
Compiler 6.0: BASIC Language Reference" for MS OS/2 and MS-DOS
Versions 6.00 and 6.00b; and Page 310 of the "Microsoft BASIC Version
7.0: Language Reference" for Microsoft BASIC PDS Version 7.00.

The documentation for SCREEN 0 states that you may assign "16 colors
to any of 16 attributes (with EGA)." However, this is misleading
because colors 0 through 15 are available on all color cards (CGA,
EGA, and VGA), and the word "attribute" is used inaccurately.

A more accurate statement is that for each character in SCREEN 0, you
can assign 16 colors for the foreground and any of 8 colors for the
background on a color adapter. In addition, you can make the character
blink (flash on and off).

On a monochrome adapter, only certain combinations of display
attributes are supported, as described further below.

Attribute Byte for Color Adapters
---------------------------------

The reason for having only 8 colors (0 through 7) available for the
background of each character is because of the arrangement of video
memory. In text mode, each character position on the display screen is
controlled by two contiguous bytes in video memory. The first byte
contains the ASCII code for the character, and the second byte is that
character's attribute byte, which controls colors, brightness, and
blinking.

In the foreground, there are 8 base colors, plus a 4th bit for
intensity, which gives 16 colors (2^4 = 16). The background has only
the 8 base colors. Another bit turns on blinking, which flashes
foreground and background together for a given character.

The following are the 8 bits of the attribute byte allocated per
character in text mode (SCREEN 0) for color display adapters:

 ---------------------------------------------------------------
|       |       |       |       |       |       |       |       |
| Blink |  Red  | Green | Blue  |Intense|  Red  | Green | Blue  |
|       |       |       |       |       |       |       |       |
|   7   |   6   |   5   |   4   |   3   |   2   |   1   |   0   |
 ---------------------------------------------------------------
          \______     ______/       \_________     _________/
                 \   /                        \   /
               Background                  Foreground

Attribute Byte for Monochrome Adapters
--------------------------------------

The following are the 8 bits of the attribute byte allocated per
character in text mode (SCREEN 0) for monochrome display adapters:

 ---------------------------------------------------------------
|       |       |       |       |       |       |       |       |
|   7   |   6   |   5   |   4   |   3   |   2   |   1   |   0   |
|       |       |       |       |       |       |       |       |
 ---------------------------------------------------------------
   \_/    \______     ______/      \_/     \______     ______/
    |            \   /              |             \   /
 Blinking      Background      Intensified      Foreground

Only the following combinations of attribute bits are recognized by a
monochrome adapter card. The appearance of some display attributes
depends on the setting of the enable-blink bit at I/O port 3B8 hex.
The attribute-bit combinations are as follows:

Value of Attribute Byte
  (Binary)      (Hex)    Description of Display Attributes
-----------------------  ---------------------------------
  00000000       00      Not displayed
  00000001       01      Underlined
  00000111       07      Normal (white on black)
  00001001       09      High intensity, underlined
  00001111       0F      High intensity
  01110000       70      White background, black foreground
                             (i.e., reverse video)
  10000111    *  87      If blinking enabled: blinking white on black
                         If blinking disabled: dim background, normal
                             foreground
  10001111    *  8F      If blinking enabled: blinking high intensity
                         If blinking disabled: dim background, high-
                             intensity foreground
  11110000       F0      If blinking enabled: blinking reverse video
                         If blinking disabled: high-intensity
                             background, black foreground

* Combinations marked with asterisk (*) are not displayed on all
  monochrome monitors.

For more information about video memory, please see the following
book, which is sold at bookstores or can be ordered by calling
Microsoft Press at (800) 638-3030 or (206) 882-8080:

   "Programmer's Guide to PC and PS/2 Video Systems," by Richard
   Wilton (published by Microsoft Press, 1987)

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