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The PNP one can go... it was only marginally useful back in the day,

Browse source 
and now it reads like a bad wikipedia article. - imp.

Approved by:	bcr (mentor)
This commit is contained in:
Eitan Adler 2012-12-10 15:31:01 +00:00
parent 2acf5ee995
commit 824fa6f4f8
Notes: svn2git 2020-12-08 03:00:23 +00:00 
svn path=/head/; revision=40323
1 changed files with 0 additions and 87 deletions

87
en_US.ISO8859-1/books/faq/book.xml
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@ -9363,93 +9363,6 @@ hint.sio.7.irq="12"</programlisting>
</answer>
</qandaentry>
<qandaentry>
<question id="pnp-initialize">
<para>How are Plug&nbsp;N&nbsp;Play ISA cards detected and
initialized?</para>
</question>
<answer>
<para>By: Frank Durda IV
<email>uhclem@nemesis.lonestar.org</email></para>
<para>In a nutshell, there a few I/O ports that all of the PnP
boards respond to when the host asks if anyone is out there.
So when the PnP probe routine starts, it asks if there are
any PnP boards present, and all the PnP boards respond with
their model # to a I/O read of the same port, so the probe
routine gets a wired-OR <quote>yes</quote> to that question.
At least one bit will be on in that reply. Then the probe
code is able to cause boards with board model IDs (assigned
by &microsoft;/&intel;) lower than <literal>X</literal> to
go <quote>off-line</quote>. It then looks to see if any
boards are still responding to the query. If the answer was
<literal>0</literal>, then there are no boards with IDs
above <literal>X</literal>. Probe will then ask for boards
below <literal>X</literal>. Finally, probe requests boards
greater than
<literal>X&nbsp;-&nbsp;(limit&nbsp;/&nbsp;4)</literal> to go
off-line. It then repeats this query. By repeating this
semi-binary search of IDs-in-range enough times, the probing
code will eventually identify all PnP boards present in a
given machine with a number of iterations that is much lower
than what 2<superscript>64</superscript> would take.</para>
<para>The IDs are two 32-bit fields (hence
2<superscript>64</superscript>) + 8-bit checksum. The first
32&nbsp;bits are a vendor identifier. They never come out
and say it, but it appears to be assumed that different
types of boards from the same vendor could have different
32-bit vendor IDs. The idea of needing 32&nbsp;bits just
for unique manufacturers is a bit excessive.</para>
<para>The lower 32&nbsp;bits are a serial #, or something else
that makes this one board unique. The vendor must never
produce a second board that has the same lower 32&nbsp;bits
unless the upper 32&nbsp;bits are also different. So you
can have multiple boards of the same type in the machine and
the full 64&nbsp;bits will still be unique.</para>
<para>The 32-bit groups can never be all zero. This
allows the wired-OR to show non-zero bits during the initial
binary search.</para>
<para>Once the system has identified all the board IDs
present, it will reactivate each board, one at a time (via the
same I/O ports), and find out what resources the given board
needs, what interrupt choices are available, etc. A scan is
made over all the boards to collect this information.</para>
<para>This info is then combined with info from any ECU files
on the hard disk or wired into the MLB BIOS. The ECU and
BIOS PnP support for hardware on the MLB is usually
synthetic, and the peripherals do not really do genuine PnP.
However by examining the BIOS info plus the ECU info, the
probe routines can cause the devices that are PnP to avoid
those devices the probe code cannot relocate.</para>
<para>Then the PnP devices are visited once more and given
their I/O, DMA, IRQ and Memory-map address assignments. The
devices will then appear at those locations and remain there
until the next reboot, although there is nothing that says
you cannot move them around whenever you want.</para>
<para>There is a lot of oversimplification above, but you
should get the general idea.</para>
<para>&microsoft; took over some of the primary printer status
ports to do PnP, on the logic that no boards decoded those
addresses for the opposing I/O cycles. I found a genuine
IBM printer board that did decode writes of the status port
during the early PnP proposal review period, but &microsoft;
said <quote>tough</quote>. So they do a write to the
printer status port for setting addresses, plus that use
that address + <literal>0x800</literal>, and a third I/O
port for reading that can be located anywhere between
<literal>0x200</literal> and <literal>0x3ff</literal>.</para>
</answer>
</qandaentry>
<qandaentry>
<question id="alternate-directory-layout">
<para>What about alternative layout policies for