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Computer Architecture Interview Questions

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1. address lines to refer to the address of a block

2. data lines for data transfer

3. IC chips 4 processing data 

The MESI protocol is also known as Illinois protocol due to its development at the University of Illinois at Urbana-Champaign and MESI is a widely used cache coherency and memory coherence protocol.
MESI is the most common protocol which supports write-back cache. Its use in personal computers became widespread with the introduction of Intel's Pentium processor to "support the more efficient write-back cache in addition to the write-through cache previously used by the Intel 486 processor" 

There are situations, called hazards, that prevent the next instruction in the instruction stream from executing during its designated clock cycle. Hazards reduce the performance from the ideal speedup gained by pipelining.

There are three classes of Hazards:

  1. Structural Hazards: It arise from resource conflicts when the hardware cannot support all possible combinations of instructions simultaniously in ovelapped execution.
  2. Data Hazards: It arise when an instruction depends on the results of previous instruction in a way that is exposed by the ovelapping of instructions in the pipeline.
  3. Control Hazards: It arise from the pipelining of branches and other instructions that change the PC.

How to Avoid Hazards:

  1. Structural Hazard: This arise when some functional unit is not fully pipelined. Then the sequence of instructions using that unpipelined unit cannot proceed at the rate of one one per clock cycle. Another common way that it may appear is when some resources are not duplicated enough to allow all combination of instructionsin the pipeline to execute. So by fully pipelining the stages and duplicating resouces will avoid structural pipeline.
  2. Data Hazards: A major effect of pipelining is to change the relative timing of instructions by overlapping their execution. This overlap introduce the data and control hazards. Data hazards occur when the pipeline changes the order of read/write accesses to operands so that the order differs from the order seen by sequentially executing instructions on an unpipelined processor. It can be mimimized by simple hardware technique called forwarding or by adding stalls.
  3. Control Hazards: They are also know as Branch Hazards. The simplest scheme to handle branches hazard is to freeze or flush the pipeline, holding or deleting any instructions after the branch until the branch destination is known. In this case branch penalty is fixed and cannot be reduced by software. The other scheme is predicted-not-taken or predicted-untaken and delayed branch. 

A technique used in advanced microprocessors where the microprocessor begins executing a second instruction before the first has been completed. That is, several instructions are in the pipeline simultaneously, each at a different processing stage.

64k/32 = 2000 blocks
2 way set assoc- 2000/2 = 1000 lines-> 10 bits for index
32B block-> 5 bits for block offset
32-10-5= 17 bits for tag 

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