Week 5

Question 1

What are the steps of the Fetch Decode Execute cycle?

Answer 1

Fetch - read an instruction from memory

Decode - identify the instruction

Execute - carry out the instruction

Question 2

What is a Pipelined Cycle?

Answer 2

An ideal scenario, the rate of instruction execution is multiplied by the number of stages

Question 3

What is Clock Cycle Time?

Answer 3

Time which each pipeline stage must complete its operation

Question 4

What is Clock Speed?

Answer 4

Frequency at which the CPU’s clock runs (1/clock cycle time)

Question 5

What is Execution Time (hint: … X … )?

Answer 5

Clock cycles for program x Clock cycle time

Question 6

What are the 2 Pipeline Hazards?

Answer 6

Control Hazards

Data Hazards

Question 7

What is a control hazard?

Answer 7

Occurs when a branch instruction changes the next instruction

Question 8

What are the 3 ways to deal with control hazards?

Answer 8

1. Stall the pipeline
2. Assume branch not taken
3. Branch prediction

Question 9

What is Stalling the pipeline?

Answer 9

Stop fetching and executing next instructions until the branch result becomes known

Question 10

What are the pros and cons of Stalling the pipeline?

Answer 10

Pros:

No need to do anything

Cons:

Wasting time and resources

Question 11

What is Assuming the branch not taken?

Answer 11

The processor assumes branch not taken

Squashing instructions if wrong

Question 12

What are the pros and cons of Assuming the branch not taken

Answer 12

Pros:

No need to guess the next instruction

Cons:

Wasting time and resources if branch taken

Question 13

What is Branch Prediction?

Answer 13

The processor may try branch prediction based on past branches

If prediction is incorrect, instructions are squashed

Question 14

What are the pros and cons of Branch Prediction?

Answer 14

Pros:

Can reduce wasted clock cycles

Cons:

Branch prediction overhead

Question 15

What is a Data Hazard?

Answer 15

Occurs when an instruction requires a result from a previous instruction before that result has been computed/written

Question 16

What are the types of Data Hazards?

Answer 16

Read after write (RAW) - True Dependency

Write after read (WAR) - Anti-dependency

Write after write (WAW) - Output dependency

Question 17

What is Read After write (RAW) - True Dependency?

Answer 17

Instruction writes, then another reads same location

Hazard if read happens before write completes

Question 18

What is Write After Read (WAR) - Anti-dependancy?

Answer 18

Instruction reads, followed by another writing to same location

Hazard if write completes before the read

Question 19

What is Write After Write (WAW) - Output Dependancy?

Answer 19

Two instructions write to same location

Hazard if writes occur in reverse intended order

Question 20

What are the side effects of Deeper Pipelines?

Answer 20

More pipeline stages means

A larger penalty for incorrect branch predictions

A bigger risk of data hazards

Question 21

What is Caching?

Answer 21

A processor fetches instructions and data from cache memory if possible

Question 22

What is Pipelining?

Answer 22

The execution of an instruction is divided into multiple stages

Question 23

What is a Branch Prediction?

Answer 23

A processor predicts branches based on past history, and squashes instructions if wrong

Question 24

What is Speculative Execution?

Answer 24

An optimization technique in which a processor (CPU) performs a series of tasks before it is prompted to, in order to have the information ready if it is required at any point

Question 25

What is Spectre and Meltdown?

Answer 25

Spectre and Meltdown are security vulnerabilities that affect modern computer processors. Discovered in early 2018, they exploit critical vulnerabilities in modern processors to potentially access sensitive data

Question 26

What is Spectre?

Answer 26

Vulnerability that affects modern computer processors

Unfortunate combo of:

Branch prediction

Speculative execution

Cache exploit of temporal locality

Question 27

What are the similarity and differences of Spectre and Meltdown?

Answer 27

Similarities:

Exploit built-in vulnerabilities, not bugs

Require attacker to be using the system

Differences:

Spectre reads memory from other processes

Meltdown can also read kernel memory from user space