Q45171: How to Detect Keypress in BASIC without Reading in Character

Article: Q45171
Product(s): See article
Version(s): 4.00 4.00b 4.50
Operating System(s): MS-DOS
Keyword(s): ENDUSER | B_BASICCOM | mspl13_basic
Last Modified: 13-DEC-1989

The program shown below demonstrates how to check for a keypress
without reading in the character, thus leaving the character in the
keyboard buffer.

This information applies to Microsoft QuickBASIC Versions 4.00, 4.00b,
and 4.50 and to Microsoft BASIC Compiler Versions 6.00, and 6.00b for
MS-DOS, and to Microsoft BASIC PDS Version 7.00 for MS-DOS.

The following program, INTKEY.BAS, uses an MS-DOS interrupt to check
if a key has been pressed. This can be used to check for a keypress
before doing an INPUT, LINE INPUT, or INKEY$ statement. This approach
is an alternative to invoking the INKEY$ function, which takes one
character at a time out of the keyboard buffer.

This example uses MS-DOS interrupt 33 (21 hex) with function call 11
(0B hex), "Check Input Status." For more information about MS-DOS
interrupts, please refer to "Advanced MS-DOS Programming" (Second
Edition) by Ray Duncan (published by Microsoft Press, 1988).

Example
-------

' $INCLUDE: 'qb.bi'
' For BC.EXE and QBX.EXE in BASIC 7.00 the include file is 'QBX.BI'

DIM inregs AS RegType, outregs AS RegType
inregs.ax = &HB00  ' Move a hex value of B into the AH register
outregs.ax = 0
PRINT "Press any key: "
DO
  CALL INTERRUPT(&H21, inregs, outregs)
LOOP UNTIL (outregs.ax AND &HFF) = &HFF
INPUT X$   ' The first character typed appears in the INPUT line.
' Or you can PRINT INKEY$ instead of using INPUT X$ to see the
' character waiting in the keyboard buffer.

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