Source : Free On-Line Dictionary of Computing

XT bus architecture
     
         (After the {IBM PC XT}) An eight-bit
        {ISA} bus architecture used by {Intel 8086} and {Intel 8088}
        systems in the {IBM PC} and {IBM PC XT} in the 1980s.  It
        predates the 16-bit ISA architecture used on the {Intel 80286}
        based machines.
     
        The XT bus has four {DMA} channels, of which three are brought
        out to the {expansion slots}.  Of these three, two are
        normally allocated to machine functions:
     
         DMA channel  Expansion	 Standard function
         0	      No	 {dynamic RAM} refresh
         1	      Yes	 add-on cards
         2	      Yes	 {floppy disk} controller
         3	      Yes	 {hard disc} controller
     
        (1997-09-15)