chapter3-m2-ziavras

The writes end up being performed in the wrong order

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Unformatted text preview: before it is written by i. The writes end up being performed in the wrong order, leaving the value written by i rather than the value written by j in the destination. Corresponds to output dependence. WAW hazards are present only in pipelines that write in more than one pipe stage or allow an instr. to proceed even when a previous one is stalled S. Ziavras Data Hazards (3) 3. WAR (write after read): j tries to write a destination before it is read by i, so i incorrectly gets new value • Corresponds to antidependence Corresponds to antidependence • Cannot occur in most static issue pipelines (even deeper pipelines or floating-point pipelines) because all reads are early (in ID stage) & all writes are late (in WB stage) • Occurs either when there are some instrs. that write results early in the instr. pipeline & other instrs. that read a source late in the pipeline, or when instrs. are reordered 4. RAR (read after read): NOT A HAZARD S. Ziavras Control Dependences • Determine ordering of an...
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This document was uploaded on 02/09/2014.

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