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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.
- Fall '09