BIOS COM status codes

 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

See Also: INT 14H: Serial Port I/O
          Asynchronous Adapter Ports
          BIOS Data Area
          ROM-BIOS Functions
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