Article ID: 44412
Article Last Modified on 11/21/2006
APPLIES TO
- Microsoft QuickBasic 4.0
- Microsoft QuickBASIC 4.0b
- Microsoft QuickBasic 4.5 for MS-DOS
- Microsoft BASIC Compiler 6.0
- Microsoft BASIC Compiler 6.0b
This article was previously published under Q44412
SUMMARY
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.
MORE INFORMATION
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)
Additional query words: QuickBas BasicCom
Keywords: KB44412
