IBM-PC Parallel Printer Port
Finding out how many ports are present and where

Normally, a PC can have up to 3 parallel printer ports - LPT1, LPT2 & LPT3. Their (16-bit) base addresses in the processor's I/O space are loaded into memory when the machine is booted, starting at address 408 Hex.

A well-written program requiring direct access to any of these ports should endeavour to find out which are present and where at run-time (rather than use constant declarations as in my earlier examples!)

Due to 80x86 processors being " little-endian", the addresses are stored as follows -

For example, the following values -

mean that LPT1 is at 378 Hex, LPT2 is at 278 Hex, and LPT3 is not present.

• Here are some example programs that report which LPT ports are present and where -
  Turbo Pascal • Borland C • MS-QBasic or you can use Debug

• An alternative method, which doesn't require direct memory access, would be to write a test byte to each of the three potential base addresses (278 / 378 / 3BC Hex) in the processor's I/O space. Any that can be read back successfully indicate that there must be a port at that address. But this won't tell you which corresponds to LPT1, 2 and 3 if more than one parallel port is present.

• You can use the Microsoft Diagnostics program to find out the base address of your PC's Parallel Printer Port(s).

• If you are running Windows95, you can view the PC's Printer Port Properties

last updated: 18-Apr-97 Ian Harries <ih@doc.ic.ac.uk>