Follow-Ups to Answer from Expert Bill A

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Lee wrote at 2009-04-03 18:50:04
Yes but a read is one operation and a write is another, so let suppose that 2 cores read at the same value, and then they increment it and store. That's at least 3 instructions.



CPU 1       CPU 2

Read 0001

         Read 0001

Inc  0002  

         Inc 0002

Write 0002

         Write 0002



An Atomic Increment would effectively block the read until the increment is complete.



CPU 1         CPU 2



Read 0001     Waiting For Atomic Lock

Inc  0002

Write 0002

         Read 0002

         Inc  0003

         Write 0003

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MichaelBinary wrote at 2010-12-07 17:01:53
That is not true.

If the int is lets say word(32 bit) aligned then it is an atomic update. However, if the int is 16 bit aligned the processor will write the data to memory in 2 16 bit chunks, allowing the possibility for another process to read the first 16 bits changed, but the old value for the second 16 bits.

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