IBM 5150 PC Technical Reference (6025005, August, 1981) (PDF)

Modes of Operation
The different modes of operation are selected by programming the
8250 Asynchronous Communications Element. This is done by
selecting the I/O address (3F8 to 3FF) and writing data out to the card.
Address bit AO, Al and A2 select the different registers which define
r"'\ the modes of operation. Also, the Divisor Latch Access Bit (Bit 7) of
the line control register is used to select certain registers.
I/O Decode for Communications Adapter
Table 21. I/O Decodes (3F8 to 3FF)
I/O
DECODE REGISTER SELECTED DLABSTATE
3FB TX BUFFER DLAB=O (WRITE)
3FB RX BUFFER DLAB=O (READ)
3FB DIVISOR LATCH LSB DLAB=l
3F9 DIVISOR LATCH MSB DLAB=l
3F9 INTERRUPT ENABLE REGISTER DLAB=O
3FA
3FB
3FC
3FD
3FE
INTERRUPT IDENTIFICATION
REGISTERS
LINE CONTROL REGISTER
MODEM CONTROL REGISTER
LINE STATUS REGISTER
MODEM STATUS REGISTER
ADDRESS BITS
A9 A8 A7 A6 A5 A4 A3 A2 A1 AD DLAB REGISTER
3F8 to 3FF 1 1 1 1 1 1 1 X X X
0 0 0 0 Receive Buffer
(read), Transmit
Holding Reg.
(write)
0 0 1 0 Interrupt Enable
0 1 a x Interrupt Identification
0 1 1 X Li ne Control
1 a 0 X Modem Control
1 0 1 X Line Status
1 1 a X Modem Status
1 1 1 X None
0 a 0 1 Divisor Latch (LSB)
0 0 1 1 Divisor Latch (MSB)
A2, Al and AO bits are "Don't Cares" and are used to select the
different register of the communications chip.
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