The 8086 had
four segment registers:
     CS    - Code segment, used with IP register
     DS    - Data segment
     SS    - Stack segment, used with SP and BP registers
     ES    - Extra segment, used with DI for string instructions as
           destination (DS:SI as source)
  You could override instructions dealing with memory with any of the
 segment registers: 
 If you were writing an emulator, what would you have it do if an
 interrupt occurred while in a REP operation with a segment override?
 Just how accurately should an emulator match the behavior of the
 emulated processor?
 
 Halt and catch fire ... No that's 6800's.