Accessing the Serial Port registers, Dlab

Accessing the Serial Port registers

Figure 13 lists the address map for the 16450 and 16550 UARTs. Each register can be accessed by reading from or writing to the proper I/O address. This I/O address is determined by adding an offset to the base address set for the particular serial port. The base address is set using DIP switches on the ES-100D (see section III).

Notice that two locations access different registers depending on whether an I/O read or I/O write is attempted. Address [base+0] accesses the receive buffer on an I/O read, or the transmit buffer on an I/O write. Address [base+2] accesses the Interrupt Identification register on an I/O read or the FIFO control register (16550 only) on an I/O write. Also, notice that if address [base+0] or [base+1] is used with the DLAB bit from the Line Control Register set to '1', the baud rate divisor latches are accessed.

NOTE: All figures displaying bitmapped registers are formatted such that bit 7 is the high-order bit.

UART Addressing		Register Description
DLAB	I/O Address

0	Base + 0	Receive buffer (read)
		Transmit holding register (write)
0	Base + 1	Interrupt enable

X	Base + 2	Interrupt identification (read) (16450 and 16550)
		FIFO control (write) (16550 only)
X	Base + 3	Line control

X	Base + 4	MODEM control

X	Base + 5	Line status

X	Base + 6	MODEM status
X	Base + 7	Scratchpad

1	Base + 0	Baud rate divisor latch (LSB) *

1	Base + 1	Baud rate divisor latch (MSB) *

(X = don't care)

* DLAB in Line Control Register must be set to access baud rate divisor latch.

Figure 13 --- Serial port register address map for 16450/16550 UART

Quatech ES-100D User's Manual