28 lines
No EOL
905 B
Markdown
28 lines
No EOL
905 B
Markdown
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### Hardware primitives
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Atomic R/W registers provide quite a basic computational model.
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We can strenghten the model by adding specialized HW primitives, that essentially perform in an atomic way the combination of some atomic instructions.
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Usually, every operating system provides at least one specilized HW primitive.
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##### Most common ones:
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- **Test&set:** atomic read+write of a boolean register
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- **Swap:** atomic read+write of a general register (generalization of the above)
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- **Fetch&add:** atomic read+increase of an integer register
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- **Compare&swap:** tests the value of a general register, returns a boolean (result of the comparison, true if it is the same).
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#### Test&Set
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```
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Let X be a boolean register
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X.test&set() :=
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tmp <- X
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X <- 1
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return tmp
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(the function is implemented in an atomic way by the hardware by suspending the interruptions!)
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```
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###### How do we use it?
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```
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lock() |