Problems With Data Dependencies
Executing instructions in parallel (i.e. simultaneously) raises problems with data dependencies, meaning that some instructions may be dependent on the results of others, and hence care must be taken to execute in the correct order.
Take for example these sequence of instructions:
r1 = r2 + r3
r7 = r1 + r4
We have a RAW (Read After Write) data dependency here, meaning that we must wait for instruction 1 to finish before executing instruction 2, as we require the correct value of r1 (register 1). Hence these instruction cannot be executed simultaneously.
Read more about this topic: Shelving Buffer
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