Data dependency
Data dependency ( data dependency in) describes computer science , the situation that data which are used by an instruction, also be used by a previous instruction.
A distinction is made between the following types of data dependencies:
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Real data dependency ( true dependence , read-after-write ) exists when an instruction has read access to the result of a preceding instruction. Example:
- a = b + c
- d = a + 1
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Counter-dependence ( anti-dependence , write-after-read ) exists when an instruction (over-) writes a value that is read from the previous instruction. Example:
- a = b + c
- b = d + 1
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Output dependence ( output dependence , write-after-write ) is when both instructions write to the same variable. Example:
- a = b + c
- a = d + e
Data dependencies are not a problem when, as in conventional programs, one instruction after the other is executed. However, in pipelined processors that execute several instructions in parallel, they can lead to data conflicts .
Individual evidence
- ↑ a b c John L. Hennessy ; David A. Patterson : Computer Architecture: a quantitative approach (3rd ed.) . Morgan Kaufmann, 2003, ISBN 1-55860-724-2 .