How to Detect Keypress in Basic without Reading in Character (45171)

SUMMARY

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.

MORE INFORMATION

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.