void sio_irq (void) __irq {
  volatile char dummy;
  volatile char IIR;
  struct buf_st *p;

  /*------------------------------------------------
Repeat while there is at least one interrupt source.
  ------------------------------------------------*/
  while (((IIR = U1IIR) & 0x01) == 0) {
     switch (IIR & 0x0E) {
       case 0x06: /* Receive Line Status */
         dummy = U1LSR;  /* Just clear the interrupt source */
         break;

       case 0x04: /* Receive Data Available */
       case 0x0C: /* Character Time-Out */
         p = &rbuf;

         if (((p->in - p->out) & ~(RBUF_SIZE-1)) == 0) {
           p->buf [p->in & (RBUF_SIZE-1)] = U1RBR;
           p->in++;
         }
         break;

       case 0x02: /* THRE Interrupt */
         p = &tbuf;

         if (p->in != p->out) {
           U1THR = p->buf [p->out & (TBUF_SIZE-1)];
           p->out++;
           tx_restart = 0;
         }
         else {
           tx_restart = 1;
         }
         break;

       case 0x00: /* Modem Interrupt */
         dummy = U1MSR;  /* Just clear the interrupt source */
         break;

       default:
         break;
     }
  }

  VICVectAddr = 0; /* Acknowledge Interrupt */
}