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PNP activation

    If PNP devices found
      PNP Bios support checks reserved non-volitile memory (cmos) for ESCD
        entry of device.

      ESCD - Extended System Configuration Data - contains configuration
        information for a previously discovered PNP device.

    If no new PNP devices found
      Original ESCD used.

    If new PNP devices found
      Configuration data reprocessed and PNP devices remapped to available
        interface resources (memory ranges, interrupts, DMA channels, etc).

    Devices reactivated using the re-assigned resources and communicated back 
      to PNP device. 

    (OS may reconfigure this one more time.)

    New ESCD stored in static memory.

    Begin building the BIOS interrupt vector table in 1st 1024 bytes of RAM
      memory.  256 4-byte pointers. Initially point to ROM but OS may later 
      redirect.

      4-byte pointer interpreted by cpu as segment:offset ( 20 bit address ).

      Intel 286 and up - protected mode  
        Once system has booted, the the IVT is replaced by IDT - interrupt
        descriptor table - which is referenced by a register in the cpu and
        can reside anywhere in the 1st Meg of memory. The entries in turn, 
        are then extended to 8 bytes allowing the actual bios code to exist
        anywhere in the 4GiB memory range. The address of Int calls (int 21) 
        are now treated as indexes in the IDT.

    Boot process now searches for an external boot device.
      Initially Floppy A.

    Current systems allow listing of alternative devices including hard drives,
      CD/DVD reader, network cards, and/or usb devices and provide for priority
      configuration of list.  List and order of search stored in CMOS memory.

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