Trapping a Keystroke and Displaying Its Extended Scan Code

• Microsoft Macro Assembler for MS-DOS, versions 5.1 and 5.1a

SUMMARY

The following example causes a main C program to call an assembly routine to print the ASCII code and the extended code resulting from a keystroke.

For example, if you press the A key, the output is an ASCII 61 and an extended code of 1E indicating the position of the key. If F1 is pressed, there is no ASCII equivalent for it, and a 0 is returned for the ASCII code and hex 3B for the extended code.

MORE INFORMATION

A keystroke is sensed by the keyboard microprocessor that deposits an 8-bit scan code at Port A of the 8255 peripheral interface chip. (Bit 1=1 when the key contact is made; 0 when the key is released.)

Following the scan code, keyboard Interrupt 9H is invoked and a 2-byte character code is placed in the keyboard buffer. The status of the SHIFT and toggle keys is stored in memory and is checked before the character code is generated. This allows for uppercase or lowercase characters.

The upper byte of the character code contains an ASCII code if one is generated; the lower byte contains the extended code, depending on the position of the key pressed.

A few key combinations do not generate scan codes: <CTRL+BREAK>, <CTRL+ALT+DEL>, <PRINT SCREEN>, and <ALT+SYSRQ> on a 286 computer. These combinations cause special predefined results.

The following is the example:

Sample Code:

/* Compile options needed: none
*/

#include <stdio.h>

void main( )

   int *ascii_ptr, *scan_ptr, num, num1;
   num = 0;
   num1 = 0;
   ascii_ptr = #                 // initialize pointers to zero
   scan_ptr = &num1;

   key_read( ascii_ptr, scan_ptr );  // call assembly routine

// print the high byte - ASCII code, and the low byte - extended
// code of the character placed in the keyboard buffer

   printf( "The ASCII code is hex %x or decimal %d\n", *ascii_ptr,"
           " *ascii_ptr);
   printf( "The EXTENDED Code is hex %x "
           "or decimal %d\n", *scan_ptr, *scan_ptr);
}

; Assemble options needed: none

         .model small,c
         .data

         .code
PUBLIC   key_read

         PUSH  bp          ;save the base pointer
         MOV   bp, sp

         MOV   ah, 0CH
         MOV   al, 0
         INT   21h

         MOV   ah, 0H
         INT   16H

         MOV   bx, [bp+4]  ;ASCII returned
         MOV   [bx], al

         MOV   bx, [bp+6]  ;Extended code returned
         MOV   [bx], ah

         POP   bp
         RET

         END