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Darwin 0.2 Driver Kit

Here's what we decided (I think):

1. an "NRW DMA Device" is defined as a a device which resides in a slot 
   with whose NextBus slot Id bits 9 through 7 are '111'. 
   
2. There are three kinds of address space which a driver can map in:

   * Register space consists of one page of registers as defined in the NRW
     spec, sec. 5.2.8.2. A driver can map in a register page if and only if
     it is associated with an NRW DMA Device.
   
   * Slot Space consists of up to 16 MB of memory, starting at 0xfs000000.
     An  NRW DMA device driver can map in the first 0xf00000 (15 M) bytes 
     of its associated slot space. A non-NRW DMA device driver can map in all
     16 MB of its slot space.
     
   * Board space consists of up to 256 MB, starting at 0xs000000. Drivers 
     associated with slots 13 and 14 are allowed to map in all or part of 
     their board space.
     
Some examples: 

  * A driver for the internal SCSI device can map in one Register page 
    and up to 15 MB of slot space, although it won't ever need the slot 
    space. 
    
  * A driver for a non-NRW DMA device in slot 14 can map in 16 MB of 
    slot space and 256 MB of board space.
    
  * A driver for an NRW DMA device in slot 7 can map in one Register page
    and up to 15 MB of slot space. This slot space can be shared by all
    drivers associated with devices in slot 7. 
   
   

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