Writing MS-DOS Device Drivers

#include     "char.h"

/*-------- Prototypes ---------*/

              /* handle init call */
extern void char_init (void);
              /* look up key code for reassignment */
extern char *k_seek (void);
              /* read the keyboard */
extern void rawread (void);
              /* see if char is available at the keyboard */
extern int rawstat (void);
              /* write byte into ring buffer */
extern void r_write (char);
              /* write character to screen */
extern void w_putc (void);

/*
 * rd_getc()
 *
 * rd_getc() reads a character from the keyboard, hanging until
 * there is one. If the character has been reassigned, copy the
 * reassignment buffer into the ring buffer. Otherwise, write
 * the character itself (with leading nul byte for extended
 * keys) into the ring buffer
 */

void   rd_getc()
   {
   if (r_index == w_index)
      {
      rawread();
      if (k_seek())
         {
         for (k = 0; k <*len; k++)
             r_write(*ptr++);
         }
      else
         {
         if (keycheck & 0177400)
             r_write(0);
         r_write(((char) keycheck) & 0000377);
         }
      }
   }

/*
 * interrupt()
 *
 * interrupt() takes care of the commands as they come in from
 * the request header. Of all the commands, only the device
 * initialization call is a separate function; this reduces
 * stack overhead. char_init() is a separate function, alone in
 * its own module, so that it can report its own address as the
 * end of the driver.
 */

void  interrupt()
   {
   count = rh->count;
   transfer = rh->transfer;
   switch (rh->command)
      {
      case   0:    /* initialization */
         char_init();
         break;
      case   4:    /* read */
         while (count)
            {
            rd_getc();
            *transfer++ = r_buf[ r_index++ ] & 000377;
            r_index &= RLIMIT;
            count--;
            }
         rh->status = DONE;
         break;
      case   5:      /* non-destr uctive read */
         if (r_index == w_index)
            {
            if (!rawstat())
                {
                rh->status = BUSY | DONE;
                break;
                }
            rd_getc();
            }
         rh->status = DONE;
         rh->data = r_buf[ r_index ];
         break;
      case   7:    /* flush input buffers */
         r_index = w_index = 0;
         while (rawstat())
            rawread();
         rh->status = DONE;
         break;
      case   8:      /* write */
      case   9:      /* write with verify */
         while (count)
            {
            outchar = *transfer++;
            w_putc();
            count--;
            }
      case   1:     /* media check */
      case   2:     /* build parameter block */
      case   6:     /* input status */
      case   10:    /* output status */
      case   11:    /* flush output buffers */
         rh->status = DONE;
         break;
      case   3:      /* ioctl read */
      default:
         rh->status = UNKNOWN_COMMAND | ERROR | DONE;
         break;
      }
   }