How to Modify the Keyboard-Flags Byte from a C Program

MS-DOS                 | WINDOWS

The information in this article applies to:

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

SUMMARY

The following information, which may be used with Microsoft C and QuickC, describes how to access the keyboard-flags byte in the ROM BIOS data area from a C program. This procedure allows a keyboard key, such as INS (insert) or CAPS LOCK, to be set or cleared from within a program. Note that the methods described below can be used to access any particular memory location (not just the keyboard flags).

MORE INFORMATION

At 0000:0417H in the ROM BIOS data area there is a byte that contains information pertaining to the status of several keyboard keys. The following table lists each bit and its meaning when set:

   Bit No.   Decimal Value      Meaning If Set
   -------   -------------      --------------

     0            1             Right SHIFT key depressed
     1            2             Left SHIFT key depressed
     2            4             CTRL key depressed
     3            8             ALT key depressed
     4           16             SCROLL LOCK on
     5           32             NUM LOCK on
     6           64             CAPS LOCK on
     7          128             INS on

   char far * flags = (char far *)0x00000417L;

   if ( (*flags & 64) == 0 )
        printf("The CAPS LOCK key is not on.\n");

To turn off a particular flag, you must do a bitwise-and with the one's complement of the flag's value (that is, ~flag). For example, you can turn off the CAPS LOCK key as follows:

   *flags = (char)(*flags & ~64);

   *flags = (char)(*flags | 64);

The following sample program turns on the CAPS LOCK key and prompts for some keyboard entry to demonstrate that CAPS LOCK is really set.

Sample Code

/* Compile options needed: none
*/

#include <stdio.h>

void main(void);

void main()

   char far *flags = (char far *)0x00000417L;
   char string[10];

   *flags = (char)((*flags) | 64);

   printf("\nType something, it should be in CAPS: ");
   gets(string);

   *flags = (char)(*flags & ~64);