Chapter 13: System Resources^301
device to a different I/O address. Remember that it is unusual for a PC to have even two
serial ports or parallel ports. The I/O addresses for the second serial port or the second
parallel port are set aside and are usually available for use, if needed.Logical Devices
In computing, physical and logical have opposite meanings, although they can be used to
describe the same input/output or storage devices. A physical device is the actual hard-
ware and its support circuitry, such as a serial or a parallel port. A logical device is a serial
or parallel port and disk drive, devices that are assigned a name that can be used in lieu of
its actual (and physical) address. For example, the first serial port on a PC may have a
32-bit address that would be very awkward for general reference use. A logical name like
COM1 is much more practical for referencing the PC’s first serial port. Likewise, LPT1 is
thelogicalnameofthefirstparallelportandA:andC:arethelogicaldevicenamesforthe
floppy disk drive and the hard disk drive, respectively.
The POST (Power-On Self-Test) process assigns logical device names during the sys-
tem boot sequence. The BIOS locates each physical device using a predefined order of
I/O addresses of each device and assigns it an appropriate logical name. The serial ports
become COM ports; the parallel ports are assigned the logical name of LPT; and the disk
drives are identified as A:, B:, C:, etc. Table 13-4 lists the logical device names assigned to
the COM and LPT ports.Figure 13-13. The Properties window of a specific device showing its I/O address assignments