By: Frank Durda IV
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, he 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
``yes'' 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 X to go
``off-line''. It then looks to see if any boards are still
responding to the query. If the answer was ``0'', then
there are no boards with IDs above X. Now probe asks if there
are any boards below ``X''. If so, probe knows there are boards
with a model numbers below X. Probe then asks for boards greater
than X-(limit/4) to go off-line. If repeats the 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 2^64 would take.
The IDs are two 32-bit fields (hence 2ˆ64) + 8 bit checksum. The first 32 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 bits just for unique manufacturers is a bit excessive.
The lower 32 bits are a serial #, ethernet address, something that makes this one board unique. The vendor must never produce a second board that has the same lower 32 bits unless the upper 32 bits are also different. So you can have multiple boards of the same type in the machine and the full 64 bits will still be unique.
The 32 bit groups can never be all zero. This allows the wired-OR to show non-zero bits during the initial binary search.
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.
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 don't 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.
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 can't move them around whenever you want.
There is a lot of oversimplification above, but you should get the general idea.
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 MS said ``tough''. So they do a
write to the printer status port for setting addresses, plus that
use that address + 0x800, and a third I/O port for reading
that can be located anywhere between 0x200 and 0x3ff.