//to request a byte to be transmitted:

CirBuf[xstoffs] = byte;
xstoffs++;
xstoffs &= 0x3f;
if (!xmitBusy)
{ 
sbuf = byte;
xftoffs++
xftoffs &= 0x3f;
xmit Busy = TRUE;
}

//TI interrupt
if ( xstoffs == xftoffs) xmitBusy = FALSE;
else
SBUF = CirBuf[xftoffs] ;
xftoffs++
xftoffs &= 0x3f;