Writing MS-DOS Device Drivers

#include  "char.h"

/*
 * r_write()
 *
 * r_write() puts a byte in the buffer. when is the buffer full?
 * when writing 1 more byte would set the read and write indices
 * equal to each other (which means the buffer is empty!!). does
 * nothing but return if it can't write the byte without
 * overflowing the buffer... if this was a real multi-tasking
 * system, we could sleep until somebody reads a byte, which
 * would allow us to do our write, but it isn't, so...
 */

void   r_write(c)
char   c;
   {
   if (((w_index + 1) & RLIMIT) == r_index)
       return;
   r_buf[ w_index++ ] = c;
   w_index &= RLIMIT;      /* wrap the index around */
   }

/*
 * r_puti()
 *
 * r_puti() converts a small (0 - 99) decimal number into two
 * ASCII digits and put them in the ring buffer
 */

void   r_puti(c)
char   c;
   {
   r_write((c / 10) + '0');
   r_write((c % 10) + '0');
   }