Q29943: SCREEN Function Gives COLOR Attribute; Foreground, Background

Article: Q29943
Product(s): See article
Version(s): 1.00 1.02 2.00 2.01 3.00 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BasicCom B_GWBasicI | mspl13_basic
Last Modified: 15-JAN-1991

The SCREEN function can return the color attribute of a character on
the screen. If the SCREEN function is invoked at a location where a
character is printed, the color attribute returned is a combination of
the foreground and background colors.

If the character is not blinking (that is, the high-order, eighth bit
is off), the foreground color can be obtained by taking the color
attribute MOD 16, and the background color can be obtained by dividing
the color attribute by 16. If the character is blinking, just turn off
the eighth bit in the attribute byte before this calculation. For
example:

   X% = SCREEN (1, 1, 1)
   X% = X% AND &H7F   ' Turns off bit 8.

The following formula shows the relationship between the attribute and
the foreground and background colors, taking into account all eight
bits of the attribute:

   ATTRIBUTE = 128*(FOREGROUND\16) + BACKGROUND*16 + (FOREGROUND MOD 16)

This information applies to Microsoft QuickBASIC versions 1.00, 1.01,
1.02, 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 MS OS/2, and to
Microsoft BASIC Professional Development System (PDS) versions 7.00
and 7.10 for MS-DOS and MS OS/2.

The following is a bitmap of the attribute returned by the SCREEN
function:

        BIT ->    7     6     5     4     3     2     1     0
               _________________________________________________
               |  0  |  1  |  0  |  0  |  0  |  0  |  0  |  1  |
               -------------------------------------------------
                  ^     ^     ^     ^     ^     ^     ^     ^
                  |     |     |     |     |     |     |     |
        Blinking --     -------------     |     -------------
                       |      Intensity --     |
     Background Color --                       -- Foreground Color

Code Example
------------

CLS
COLOR 4, 1
PRINT "hello";
X% = SCREEN (1, 1, 1)
PRINT "Attribute: "; X%
X% = X% AND &H7F          ' Makes sure blinking bit (bit 8) is off.
PRINT "Foreground: "; X% MOD 16
PRINT "Background: "; X% \ 16

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