Processor Synchronization

• Shared memory updates must be synchronized among processors

  • Use memory locations as ‘locks’ to synchronize operations

• One way to do this is the test-and-set instruction

  • First test the value of a memory location
  • If ZERO, then the lock is free, set the memory location to ‘1’ indicating that the lock has been grabbed
  • The Read followed by Write must be ATOMIC - can’t allow another processor to access the lock between the read/write operations. TWO bus transactions.

• While processor is waiting (spinning) on the lock, it is idle.

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