Q59133: How to Trap PRINT SCREEN Key on an Extended (101-Key) Keyboard

Article: Q59133
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | SR# S900222-147 B_BasicCom | mspl13_basic
Last Modified: 2-MAR-1990

It is possible to detect (trap) the PRINT SCREEN (or SHIFT+PRINT
SCREEN, PRT SCR, etc.) key with 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.

Although BASIC can detect the key press with the ON KEY GOSUB
statement, the PRINT SCREEN action is controlled through the ROM BIOS
interrupt table and cannot be disabled directly with BASIC. To disable
the print screen utility, the interrupt vector must be remapped with a
C or assembly routine.

Here is the syntax of the KEY statement, where n=15 through 25 can be
user-defined key combinations to trap:

   KEY n, CHR$(keyboardflag) + CHR$(scancode)

The following chart shows the keyboard flags and scan codes for PRINT
SCREEN and SHIFT+PRINT SCREEN for standard and extended keyboards:

                          Standard   Extended
                          --------   --------
   Keyboard flag for:

      No    SHIFT         &H00       &H80
      LEFT  SHIFT         &H02       &H82
      RIGHT SHIFT         &H01       &H81

   Scan Code for:

      PRINT SCREEN        &H37       &H2A

NOTE: The keyboard flags are affected by CAPS LOCK and NUM LOCK. The
above values assume that CAPS LOCK and NUM LOCK are off. Add &H40 to
the keyboard flag for CAPS LOCK, and add &H20 for NUM LOCK.

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

The following code example detects each of the PRINT SCREEN and
SHIFT+PRINT SCREEN combinations listed above:

'NOTE: This program detects PRINT SCREEN and SHIFT+PRINT SCREEN key
'      combinations on standard and extended keyboards with CAPS LOCK
'      and NUM LOCK off.
'
'ALSO NOTE: This program does NOT disable the PRINT SCREEN action
'           (that is, the screen is still printed). See the explanation
'           in the text of the above article.
KEY 15, CHR$(&H00) + CHR$(37)        'Standard no SHIFTs
KEY 16, CHR$(&H02) + CHR$(37)        'Standard LEFT SHIFT
KEY 17, CHR$(&H01) + CHR$(37)        'Standard RIGHT SHIFT
KEY 18, CHR$(&H80) + CHR$(2A)        'Extended no SHIFTs
KEY 19, CHR$(&H82) + CHR$(2A)        'Extended LEFT SHIFT
KEY 20, CHR$(&H81) + CHR$(2A)        'Extended RIGHT SHIFT
FOR i%=15 TO 20
  ON KEY(i%) GOSUB Handle:
  KEY(i%) ON
NEXT
WHILE INKEY$<>CHR$(27) : WEND        'Press ESC to end
END

Handle: PRINT "Trapped PRINT SCREEN keystroke"
        PRINT "Screen will still print unless"
        PRINT "Interrupt vector changed with"
        PRINT "C or Assembly routine."
        RETURN

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