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LOCK             Lock the Bus                        Flags: not altered

LOCK

    LOCK is a one-byte prefix that can precede any instruction. LOCK
    causes the processor to assert its bus lock signal while the
    instruction that follows is executed. If the system is configured such
    that the LOCK signal is used, it prevents any external device or event
    from accessing the bus, including interrupts and DMA transfers.

  --------------------------------------------------------------------------
   Operands                  Clocks   Transfers  Bytes   Example
   no operands                 2          -        1     LOCK XCHG FLAG,AL
  --------------------------------------------------------------------------

       Note:          This instruction was provided to support multiple
                      processor systems with shared resources. In such a
                      system, access to those resources is generally
                      controlled via a software-hardware combination using
                      semaphores.

                      This instruction should only be used to prevent
                      other bus masters from interrupting a data movement
                      operation.  This prefix should only be used with
                      XCHG, MOV, and MOVS.