MIPS pipeline: data hazards / data dependency

Data dependencies between instructions

Different instructions READ/WRITE the same register

Register contention

Read After Write (RAW)

Later instructions read from destination register written by an earlier instruction

Example - Incorrect execution

i1: add $1, $2, $3
i2: sub $4, $1, $5

If i2 reads register $1 before i1 can write back the result,

i2 will get a stale result (old result).

Example - illustration

Given the above instructions repeating usage of register $2, which instructions do we need to handle?

We observe the following:

  • result from sub instruction is produced after EX stage.

  • Data is needed by the “and/or” instructions at the beginning of EX stage

Solution:

  • Forward the result to instructions, before the READ/WRITE stage, as soon as it is available.

  • Replace the data read from register file.

Other dependencies

These do not cause pipeline hazards:

  • Write after Read (WAR)

  • WAW (Write after Write)

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