Compaq 4000N - Manual (Page 107)

Technical Reference Guide
Compaq Deskpro 4000N and 4000S Personal Computers
First Edition - September 1997
5-17
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 5-12.
Table 5–12. Serial Interface Control Registers
Table 5-12.
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.