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Chapter 5 Input/Output Interfaces
5-16 Compaq Deskpro EN Series of Personal Computers
Desktop and Minitower Form Factors
Third Edition – September 1998
5.4.2.2 Serial Interface Control
The BIOS function INT 14 provides basic control of the serial interface. The serial interface can
be controlled by software through the registers listed in Table 5-11.
Table 5–11
. Serial Interface Control Registers
Table 5-11.
Serial Interface Control Registers
Address Register R/W
Base Receive Buffer / Transmit Holding [1] R/W
Base, Base + 1 Baud Rate Divisor Latch [2] R/W
Base + 1 Interrupt Enable R/W
Base + 2 Interrupt ID RO
Base + 3 Line Control R/W
Base + 4 Modem Control R/W
Base + 5 Line Status RO
Base + 6 Modem Status RO
Base + 7 Scratch Pad R/W
NOTES:
Base Address:
COM1 = 3F8h
COM2 = 2F8h
[1] This register holds receive data when read from and transmit data when written to.
[2] When bit <7> of the Line Control register is set (1), writing to 3F8h and 3F9h
programs the divisor rate for the baud rate generator.
Receive Buffer / Transmit Holding Register, I/O Port 3F8h/2F8h
When read by the CPU, this byte contains receive data. When written to by the CPU, the byte
contains data to be transmitted.