INT 13H fns return communications port status in AH and AL.

LineStatusRec  -- returned in AH by all fns
 7 6 5 4 3 2 1 0 
                 
                  bit mask
                  0:  01H data ready status
                  1:  02H overrun error
                  2:  03H parity error
                  3:  08H framing error
                  4:  10H break detect
                  5:  20H trans holding reg empty
                  6:  40H trans shift reg empty
                  7:  80H time-out (or used to flag any error)

ModemStatusRec  -- returned in AL by fns 00H, 01H, and 03H.
 7 6 5 4 3 2 1 0 
                 
                  bit mask
                  0:  01H delta clear to send
                  1:  02H delta data set ready
                  2:  03H trailing edge ring detector
                  3:  08H delta recv line signal detect
                  4:  10H clear to send
                  5:  20H data set ready
                  6:  40H ring indicator
                  7:  80H received line detect signal

- -

BIOS COM status codes