Q64439: Trapping a Keystroke and Displaying Its Extended Scan Code
Article: Q64439
Product(s): Microsoft Macro Assembler
Version(s): 5.1,5.1a
Operating System(s):
Keyword(s):
Last Modified: 06-MAY-2001
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The information in this article applies to:
- Microsoft Macro Assembler (MASM), versions 5.1, 5.1a
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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>
extern key_read( int*, int* );
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
key_read PROC
PUSH bp ;save the base pointer
MOV bp, sp
; Invoke Int 21h Function Ch to clear the keyboard buffer before
; accepting a keystroke.
MOV ah, 0CH
MOV al, 0
INT 21h
; Invoke Int 16h Function 0h to place the character code in the AX
; register.
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
key_read ENDP
END
Additional query words: 5.10 5.10a
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Keywords :
Technology : kbMASMsearch kbAudDeveloper kbMASM510 kbMASM510a
Version : :5.1,5.1a
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