PRB: getch() Does Not Behave as Expected with ALT+Q and ALT+R

MS-DOS                      | WINDOWS

The information in this article applies to:

• The C Run-time (CRT), included with:

      - Microsoft C for MS-DOS, versions 5.1, 6.0, 6.0a, and 6.0ax
      - Microsoft C/C++ for MS-DOS, version 7.0
      - Microsoft Visual C++ for Windows, versiona 1.0 and 1.5
  

SYMPTOMS

The program accepts all keyboard inputs, but does not behave as expected if the input is ALT+Q or ALT+R.

CAUSE

This is not a problem with the Microsoft C Optimizing Compiler run-time function getch(). The problem is caused because MS-DOS uses these two key combinations for other control purposes.

MORE INFORMATION

In this case, the second scancode of ALT+Q is 16 and the second scancode of ALT+R is 19 (both decimal). MS-DOS uses scancode 16 to mean "echo subsequent output to the printer" and scancode 19 to mean "pause output." These are equivalent to CTRL+P and CTRL+S. When a printer is not connected to the computer, the PRINT SCREEN key must wait for time-out. For the SCREEN PAUSE key, another key must be pressed to activate the screen again.