HP Vectra Technical Reference Manual page 226

SECTION 7.
SERIAL AND PARALLEL 1/0
This section covers the ROM BIOS support for the system serial and parallel I/O ports. The ROM
BIOS supports up to four parallel ports and up to four serial ports. However, DOS only provides
logical devices for three parallel printer ports and two serial ports.
7.1 Overview
The ROM BIOS provides two STD-BIOS drivers that control the serial (INT 14H) and parallel (INT
17H) ports. The functions in these drivers provide a means of setting communication parameters
and transmitting data. These drivers have expanded functionality that provide the programmer
with the additional ability to set higher baud rates and to transfer strings of data. In addition to
these drivers, the print screen driver (INT OSH) will be discussed in this section.
7.1.1 Serial And Parallel Port Addresses
The STD-BIOS data area contains two tables used by the serial and parallel port drivers. The Serial
Base Port Address Table contains the base port addresses for the serial ports. The Parallel Base
Port Address Table contains the base addresses of the parallel ports. The ROM BIOS checks
during SYSGEN for the presence of serial and parallel adapter cards at the addresses listed in
table 7.1. When a valid port is found, the base address of that port is placed in the next
available entry of the appropriate table. Application programs may add additional parallel ports
or serial ports to the port tables. An application program can also replace the values in the table
with new ones to support non-standard port addresses. Each table contains space for four
entries.
Serial and Parrallel I/O 217