How to prevent duplicate chars when I press keys on the keyboard

Question

I am trying to learn how to prevent the keyboard sending multiple chars to the screen and to scanf under DOS. I am using Turbo-C with inline assembly.

If the characters entered on the keyboard are:

mmmmmmmmyyyyy nnnnnaaaaammmmmmeeeeee iiiiiissss HHHHaaaaiiiimmmm

The characters seen on the console and processed by scanf would be:

my name is Haim

The basic output comes from the code in C which I am not allowed to touch. I must implement eliminate_multiple_press and uneliminate_multiple_presswithout touching the code in between.

The Turbo-C code I've written so far is:

#include <stdio.h>
#include <dos.h>
#include <string.h>

volatile char key;
volatile int i=0;
void interrupt (*Int9save) (void);

void interrupt kill_multiple_press()
{
 asm{
     MOV AL, 0
     MOV AH,1
     INT 16h
     PUSHF
     CALL DWORD PTR Int9save
     MOV AX,0
 }

 asm{
  JZ notSet
  MOV key, AL
  MOV AH, 04H
  INT 16H

 }
 notSet:
 //I am not sure what to do from here...............
  I also know that it should be related to the zero flag, but what I          
  wrote so far didn`t effect on multiple characters.
}

void eliminate_multiple_press()
{
 Int9save=getvect(9);
 setvect(9,kill_multiple_press);
}

void uneliminate_multiple_press()
{
  setvect(9,Int9save);
}

void main()
{
  char str[10000]="";
  clrscr();
  eliminate_multiple_press();
  printf("Enter your string: ");
  scanf("%s",&str);
  printf("\n%s",str);
  uneliminate_multiple_press();
 }

Information I have been given that relate to a solution are the keyboard BIOS routines that can be found at this link:

The problems I'm having are probably related to not understanding what to do at the label notSet. The solution seems to be related to using a buffer and the register AX (especially AL), but I really have no Idea how to make scanf to get the result I need. Does anyone have any ideas how I can complete this code to achieve the desired effect?

Answer 1

There are multiple layers of buffers that may be used by the BIOS, DOS, and the C library (including scanf). When your machine starts up the interrupt vector table is modified to point IRQ1/INT 9h (the external keyboard interrupt) to a BIOS routine to handle characters as they are typed. At the lowest level there is usually a 32 byte⁶ circular buffer that is maintained in the BIOS Data Area (BDA) to keep track of the characters. You can use the Int 16h BIOS calls¹ to interact with this low level keyboard buffer. If you remove characters from the BIOS keyboard buffer at interrupt time then DOS and the C library scanf⁵ routine will never see them.

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Method to Eliminate Duplicate Characters at the BIOS/Interrupt Level

It appears that the exercise is to eliminate all duplicate² characters entered into scanf³ by intercepting keystrokes via Interrupt 9 (IRQ1) and throwing duplicates away. One idea for a new keyboard interrupt handler to eliminate the duplicates before DOS (and eventually scanf) ever see them:

• Keep track of the previous character pressed in a variable
• Call the original (saved) Interrupt 9 so that the BIOS updates the keyboard buffer and the keyboard flags as DOS expects them to appear.
• Query the keyboard to see if a character is available with Int 16h/AH=1h.The Zero Flag (ZF) will be set if no characters are available and clear if there is one available. This keyboard BIOS call peeks into the beginning of the keyboard buffer without actually removing the next character available.
• If a character is available then compare it with the previous character.
  • If they are different then update the previous character with the current character and exit
  • If they are the same then use Int 16h/AH=0h to remove the duplicate character from the keyboard buffer and exit

A Turbo-C 3.0x version of the code⁴:

#include <stdio.h>
#include <dos.h>
#include <string.h>
#include <conio.h>

volatile char key = 0;
void interrupt (*Int9save)(void);

void interrupt kill_multiple_press(void)
{
    asm {
     PUSHF
     CALL DWORD PTR Int9save       /* Fake an interrupt call to original handler */

     MOV AH, 1                     /* Peek at next key in buffer without removing it */
     INT 16h                     
     JZ noKey                      /* If no keystroke then we are finished */
                                   /*     If ZF=1 then no key */

     CMP AL, [key]                 /* Compare key to previous key */
     JNE updChar                   /*     If characters are not same, update */
                                   /*     last character and finish */

     /* Last character and current character are same (duplicate)
      * Read keystroke from keyboard buffer and throw it away (ignore it)
      * When it is thrown away DOS and eventually `scanf` will never see it */
     XOR AH, AH                    /* AH = 0, Read keystroke BIOS Call */

     INT 16h                       /* Read keystroke that has been identified as a */
                                   /*     duplicate in keyboard buffer and throw away */
     JMP noKey                     /* We are finished */
    }
updChar:
    asm {
     MOV [key], AL                 /* Update last character pressed */
    }
noKey:                             /* We are finished */
}

void eliminate_multiple_press()
{
    Int9save = getvect(9);
    setvect(9, kill_multiple_press);
}

void uneliminate_multiple_press()
{
    setvect(9, Int9save);
}

void main()
{
    char str[1000];
    clrscr();
    eliminate_multiple_press();
    printf("Enter your string: ");
    /* Get a string terminated by a newline. Max 999 chars + newline */
    scanf("%999[^\n]s", &str);
    printf("\n%s", str);
    uneliminate_multiple_press();
}

---

Notes

• ¹Within the keyboard interrupt handler you want to avoid any keyboard BIOS call that will block waiting for keyboard input. If using Int 16h/AH=0 make sure there is a character available first with Int 16h/AH=1 otherwise Int 16h/AH=0 will block while waiting for another character to arrive.
• ²Removing duplicate characters is not the same as disabling the keyboard repeat rate.
• ³Because the duplicates are removed before DOS routines see them (and functions like scanf that rely on DOS), they will never be seen by scanf.
• ⁴Some modifications may have to be made to be compatible with versions of Turbo-C other than 3.0x.
• ⁵This method only works because scanf will be indirectly making BIOS calls keeping the keyboard buffer clear. This code does't work in all generic cases especially where keystrokes may be buffered by the BIOS. To get around that the keyboard interrupt routine would have to remove all the duplicates in the keyboard buffer not just at the head as this code does.
• ⁶Each keystroke takes up 2 bytes of space in the BIOS keyboard buffer (in the BDA). 2 of the 32 bytes are lost because they are used to detect if the keyboard buffer is full or empty. This means the maximum number of keystrokes the BIOS can buffer is 15.