📄 ibm-pc parallel printer port - finding out how many ports are present and where.htm
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<CENTER>IBM-PC Parallel Printer Port<BR>Finding out how many ports are present
and where </CENTER></H2>
<P>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.
<P>A <I>well-written</I> 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!)
<P>Due to 80x86 processors being <!--"<a href="http://www.cis.ohio-state.edu/htbin/ien/ien137.html">-->"<A
href="http://sunsite.doc.ic.ac.uk/rfc/ien/ien-137.txt.3"> little-endian</A>",
the addresses are stored as follows -
<P>
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<TABLE bgColor=#0000cc border=0>
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<TR>
<TD>
<TABLE bgColor=#ddddee border=1 cellPadding=5>
<TBODY>
<TR align=middle>
<TD>408</TD>
<TD>409</TD>
<TD>40a</TD>
<TD>40b</TD>
<TD>40c</TD>
<TD>40d</TD></TR>
<TR align=middle>
<TD colSpan=2><B>LPT1</B></TD>
<TD colSpan=2><B>LPT2</B></TD>
<TD colSpan=2><B>LPT3</B></TD></TR>
<TR align=middle>
<TD>low<BR>byte</TD>
<TD>high<BR>byte</TD>
<TD>low<BR>byte</TD>
<TD>high<BR>byte</TD>
<TD>low<BR>byte</TD>
<TD>high<BR>byte</TD></TR></TBODY></TABLE></TD></TR></TBODY></TABLE></CENTER>
<P>For example, the following values -
<P>
<CENTER>
<TABLE bgColor=#000000 border=0>
<TBODY>
<TR>
<TD>
<TABLE bgColor=#eeeeee border=1 cellPadding=5>
<TBODY>
<TR align=middle>
<TD>408</TD>
<TD>409</TD>
<TD>40a</TD>
<TD>40b</TD>
<TD>40c</TD>
<TD>40d</TD></TR>
<TR align=middle>
<TD>78</TD>
<TD>03</TD>
<TD>78</TD>
<TD>02</TD>
<TD>00</TD>
<TD>00</TD></TR></TBODY></TABLE></TD></TR></TBODY></TABLE></CENTER>
<P>mean that LPT1 is at 378 Hex, LPT2 is at 278 Hex, and LPT3 is not present.
<P>
<HR>
<UL>
<LI>Here are some example programs that report which LPT ports are present and
where -
<P>
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