Q59131: How to Trap CTRL, ALT, and SHIFT Keys Alone in BASIC

Article: Q59131
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | SR# S900223-10 B_BasicCom | mspl13_basic
Last Modified: 28-FEB-1990

Normally, to trap CTRL (CONTROL), ALT, and SHIFT in combination with
other keys, you would set the bits for these keys in the first byte
(keyboard flag) of the two CHR$() bytes in the KEY statement.

However, to trap the CTRL, ALT, and SHIFT keys alone (not in
combination with other keys), the keyboard flag in the KEY statement
should be CHR$(0) for the standard keyboard or CHR$(&H80) for keys
specific to an extended keyboard.

This information applies to Microsoft QuickBASIC Versions 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) Version 7.00 for MS-DOS and MS OS/2.

The syntax of the KEY statement (where n=15 through 25 can be
user-defined keys) is as follows:

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

The following table lists the scan codes for the CTRL, ALT and SHIFT
keys:

   Key           Scan Code
   ---           ---------
   CTRL          &H1D
   ALT           &H38
   LEFT SHIFT    &H2A
   RIGHT SHIFT   &H36

NOTE: To trap the RIGHT CTRL or RIGHT ALT key (found only on an
extended 101 keyboard), the extended keyboard flag (&H80) must be used
in the keyboard flag byte.

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

The following code example traps the CTRL, ALT, and SHIFT keys
alone:

'NOTE:  This program requires NUM LOCK and CAPS LOCK to be off.
KEY 15, CHR$(&H0) + CHR$(&H1D)      'LEFT CTRL (on
ON KEY(15) GOSUB Ctrl               'Standard or Extended keyboard)
KEY(15) ON

KEY 16, CHR$(&H80) + CHR$(&H1D)     'RIGHT CTRL (on Extended keyboard)
ON KEY(16) GOSUB Ctrl
KEY(16) ON

KEY 17, CHR$(&H0) + CHR$(&H38)      'LEFT ALT (on
ON KEY(17) GOSUB alt                'Standard or Extended keyboard)
KEY(17) ON

KEY 18, CHR$(&H80) + CHR$(&H38)     'RIGHT ALT (on Extended keyboard)
ON KEY(18) GOSUB Alt
KEY(18) ON

KEY 19, CHR$(&H0) + CHR$(&H2A)      'LEFT SHIFT (on
ON KEY(19) GOSUB Shift              'Standard or Extended keyboard)
KEY(19) ON

KEY 20, CHR$(&H0) + CHR$(&H36)      'RIGHT SHIFT (on
ON KEY(20) GOSUB Shift              'Standard or Extended keyboard)
KEY(20) ON

WHILE INKEY$ <> CHR$(27): WEND      'Press ESC to end
END

Ctrl: PRINT "CTRL"
RETURN

Alt:  PRINT "ALT"
RETURN

Shift:  PRINT "SHIFT"
RETURN

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