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  The PC/AT and the PS/2s all have a real-time clock that keeps track of
  the current date and time. In the PC/AT, the real-time clock is part of
  the Motorola MC146818 chip that supports the PC/AT's nonvolatile CMOS RAM.
  In the PS/2s, the real-time clock is in custom silicon. In all these
  machines, the real-time clock runs off a battery so that the time and date
  are maintained even while the computer is turned off.