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How are PlugNPlay ISA cards detected and initialized?

April 26, 2017cards detected initialized ISA PlugnPlay

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How are PlugNPlay ISA cards detected and initialized?

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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, 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 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. Probe will then ask for boards below X. Finally, probe requests boards greater than X-(limit/4) 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