1.1.2 Types of Processors

Question 1

What does RISC stand for?

Answer 1

• Reduced Instruction Set Computing

Question 2

What does CISC stand for?

Answer 2

• Complex Instruction Set computing

Question 3

What is an instruction set?

Answer 3

• An instruction set is written in machine code and are instructions recognisable by the CPU and executable
• Different CPU designs have different core instruction sets

Question 4

What are some of the characteristics of CISC?

Answer 4

• Used in laptops/desktops
• Has more complex hardware
• Multiple machine cyces per instruction
• Physically larger in size and requires more silicon (expensive)
• Greater energy consumption
• More intensive tasks will do better with CISC
• Cant support pipelining
• Support a wider range of addressing modes

Question 5

What are some of the characteristics of RISC?

Answer 5

• Used in smartphones and tablets
• Single machine cycle per instruction
• Smaller in size, less complex circuitry required, less silicon needed = cheaper
• Lower energy requirements (‘sleep mode when not processing)
• Small number of simple instructions and hardware
• Run at lower clock speed, simpler tasks faster
• Supports Pipelining

Question 6

Exam Question: As well as a CPU the console contains a GPU for 3D graphics. Explain why a GPU is more suitable than a CPU for this task.

Answer 6

• CPUs are general purpose processors (1) whereas GPUs are designed specifically for graphics (1).
• And so likely to have built in circuitry / instructions for common graphics operations (1).
• GPUs are able to perform an instruction on multiple pieces of data at one time (1) often we want to do this when processing graphics (e.g. transforming points in a polygon or shading pixels) (1)

Question 7

What is Pipelining?

Answer 7

• The output of 1 instruction is the input of the next
• While one instruction is being executed, another can be decoded and another fetched.
• Pipelining is aimed to reduce the amount of the CPU which is kept idle.

Question 8

What are Multi-core systems

Answer 8

• 1 method of Increasing Processing Speed is to increase the number of cores. There are 2 ways to organise cores:

1. Parallel Systems
2. Co-Processors

Question 9

In a Parallel System the computational problem should be able to:

Answer 9

• 2 or more processors work together to complete a task
• Be broken apart into discrete pieces of work and can be solved simultaneously
• Execute multiple program instructions at any time
• Be solved in less time with multiple computer resources

Question 10

Parallel Systems can be placed into 1 of 3 categories:

Answer 10

• Single instruction, multiple data (SIMD system)
• Multiple Instruction, single data (MISD system)
• Multiple Instructions, multiple data (MIMD system)

Question 11

What is a Single Instruction, Multiple Data

Answer 11

• Single Instruction - Multiple processors follow/ execute the same set of instructions at any given clock cycle
• Multiple Data - Each processing unit takes a different set of data.

SIMD process lots of different data concurrently using the same algorithm.

Question 12

What is Multiple Instruction, Single Data

Answer 12

• Multiple Instruction - Multiple processors. Each one operates on the data independently via separate instruction streams
• Single Data - A single data stream is fed into multiple processing units

perform different operations on the same data.

Question 13

What is Multiple Instruction, Multiple Data

Answer 13

• Multiple Instruction - Multiple processors. Each one could be executing a different instruction stream independently of the others
• Multiple data - Every processor may be working with a different data stream

Can process a number of different instructions simultaneously. Currently the most common type of parallel computers

Question 14

What is a Co- Processor

Answer 14

• Are additional processors which are designed for very specific tasks only (graphics processing or mathematical operations.
• The co-processor and central processor operate on different tasks at the same time which speeds up the overall execution
• However a co-processor operate on its own right. A CPU can

Question 15

What are the advantages of Multi-Core Systems

Answer 15

• More jobs can be done in a shorter time = executed simultaneously
• Tasks can be shared to reduce the load on individual processors + avoid bottlenecks

Question 16

What are the disadvantages of Multi-Systems

Answer 16

• Difficult to write programs, ensuring each task has the correct input data to work on.
• Results from different processors need to be combined adds to time taken to execute program
• Not all tasks can be split across multiple processors
• Additional complexity of implementing

Question 17

What are examples of co-processors

Answer 17

• Floating point units (FPU) : Built into the main CPU and run floating point mathematical operations.
• Digital Signal Processor (DSP) : Commonly used to to process sound effects and merge sound channels ao they can output in stereo and surround sound.
• Graphics Processing unit (GPU) : Complete big 3D calculations needed for games and effects.

Question 18

Exam Question: Describe 1 difference between RISC and CISC.

Answer 18

• More than one clock cycle for each instruction in CISC … …one clock cycle for each instruction in RISC
• CISC architecture has complex circuitry and is therefore more expensive to manufacture… …RISC architecture has simple circuitry minimising manufacture cost.
• Some instructions in CISC will rarely get used … … In RISC instructions are used regularly

Question 19

Exam Question: Explain two reasons why games designed for other companies’ consoles may not work on this machine

Answer 19

• Different consoles will have different processors
  
  • each with their own instruction sets and word sizes.
• Different consoles have different operating systems
  
  • and so games may be dependent on libraries in one operating system that don’t exist in another or may make different system calls
• Games might be on a different media
  
  • for example, might be on Blu-ray when console may only have a DVD player

Question 20

Exam Question: Describe 1 advantage to the company of using RISC over CISC.

Answer 20

Programs run faster… …due to simpler instructions

Costs less to design/produce

Cause it has fewer/ simpler instructions

Question 21

Exam Question: Compare a Complex Instruction Set Computer (CISC) architecture with a Reduced Instruction Set Computer (RISC) architecture.

Answer 21

• CISC:
  
  • Each instruction may take multiple cycles.
  • Single register set Instructions have variable format
  • Many instructions are available
• Many addressing modes are available
• Complicated processor design Integrated circuit is expensive
• RISC:
  
  • An instruction performs a simple task
  • Limited number of instructions available
  • Complex tasks can only be performed by combining multiple instructions
  • Simple processor design

Question 22

Discuss the different approaches that can be taken to provide increasingly larger amounts of computing power and the types of problem they are suited to

Answer 22

• Knowledge:
  
  • Processors can have multiple cores
  • Processors have increasingly large clock speeds and can be overclocked
  • GPUs can be applied to problems other than graphics processing
• Application
  
  • Having multiple cores can speed up smaller problems but this will not be enough for larger problems
  • GPUs tend to have large number of cores so can run on highly paralleliseable problems… …but only where the same instruction is being applied to multiple pieces of data
• Evaluation:
  
  • Increased clock speed is limited to smaller problems
  • Parallel processing isn’t suited to all problems - writing algorithms = hard
  • GPUs only good where the same calculation is repeated on multiple data sets

Question 23

Exam Question: Explain why using a RISC processor rather than a CISC processor is likely to result in increased battery life.

Answer 23

• RISC has a smaller instruction set (than CISC)
• Requires fewer transistors / less complex circuitry
• Means less power is required.