computer science paper 1

• • The purpose of the Central Processing Unit (CPU) is to fetch, decode and execute instructions
• • The Fetch-Execute Cycle is the cycle that the central processing unit (CPU) runs through billions of times per second to make a computer work
• Fetch stage

  • During the fetch stage of the cycle, the next instruction or data must be fetched from the computer's memory (RAM)
  • The instruction or data is brought back to the CPU
• Decode stage

  • During the decode stage of the cycle, the CPU needs to work out what is required from the instruction 
  • This could be a range of tasks depending on what the instruction or data included
• Execute stage

  • During the execute stage of the cycle, the CPU will carry out the instruction that was fetched
  • Some examples that would take place at this stage are
  • Performing a calculation
  • Storing a result or data back in main memory (RAM)
  • Going to main memory to fetch data from a different location
• The important things to remember are:

  • An instruction or data is fetched from memory
  • The instruction is decoded 
  • The instruction is executed
  • The cycle repeats billions of times per second
• he CPU is made up of 4 key components

  • Arithmetic Logic Unit (ALU)
  • Control Unit (CU)
  • Cache
  • Registers
• Arithmetic logic unit (ALU)

  •  Performs arithmetic operations
  • Performs logical decisions
• Control unit (CU)

  • Coordinates how data moves around the CPU by sending a signal to control the movement of the data 
  • Decodes the instructions fetched from memory
• Cache

  • Very small, very fast memory located in the CPU which is used to provide quick access to frequently used instructions and data
  • The more cache there is, the more data can be stored, which speeds up the performance of the CPU
  • It prevents the CPU from having to repeatedly fetch frequently used instructions from RAM
• Registers

  • Extremely small, extremely fast memory located in the CPU
  • Each register has its very own specific purpose
• • The Von Neumann Architecture outlines how the computer memory, input / output devices and processor all work together 
  • The Von Neumann Architecture is a design of the CPU which was proposed by Mathematician John Von Neumann in the 1940s, which most general-purpose computers are built upon
• he Von-Neumann-architecture

  • It consists of 4 main registers
  • The Program Counter (PC)
  • The Memory Address Register (MAR)
  • The Memory Data Register (MDR)
  • The Accumulator (ACC)
• Program Counter (PC)

  • Holds the memory address of the next instructions to be executed 
  • Increments by 1 as the fetch-decode-execute cycle runs
• Memory Address Register (MAR)

  • Holds the memory address of where data or instructions are to be fetched from
• Memory Data Register (MDR)

  • Stores the data or instruction which has been fetched from memory
• Accumulator (ACC)

  • Stores the results of any calculations that have taken place in the Arithmetic Logic Unit (ALU)
• • There are 3 common characteristics of the CPU
  • Clock Speed
  • Cache Size
  • Number of Cores
• Clock speed

  • The clock speed is measured in Hertz (Hz)
  • The clock speed measures the number of fetch-decode-execute cycles that can take place in 1 second
  • The faster the clock speed, the more instructions can be fetched and executed per second
  • Modern computers have a clock speed in Gigahertz (GHz), meaning billion
  • A clock speed of 3.5GHz can perform up to 3.5 billion instructions per second
•  Cache size

  • Cache is very small, very fast memory on or close to the CPU 
  • Cache is used as temporary storage to provide quick access to a copy of frequently used instructions and data
  • The larger the cache size, the more frequently used instructions or data can be stored
  • This results in the CPU having to complete fewer fetch cycles from memory (RAM), speeding up the performance
  • Cache also has a significantly faster read/write speed than RAM, making it much quicker to retrieve instructions from there instead of from memory (RAM) 
• Number of cores

  • A core works like it is its own CPU
  • Multiple core processors mean they have multiple separate processing units that can fetch, decode and execute instructions at the same time
  • For example, a dual-core processor would have 2 processing units, each with their own
  • Control Unit (CU)
  • Arithmetic Logic Unit (ALU)
  • Accumulator (ACC)
  • Registers 
  • Multi-core processors can run more powerful programs with greater ease
  • Multiple cores increase the performance of the CPU by working with the clock speed
• What is an embedded system?

  • An embedded system is a computer system with a single function, inside a larger mechanical unit
  • Examples of embedded systems include
  • Heating thermostats
  • Hospital equipment
  • Washing machines
  • Dishwashers
  • Coffee machines
  • Satellite navigation systems
  • Factory Equipment
  • Traffic lights
• What are the properties of an embedded system?

  • They are small in size
  • They use less power than a general-purpose computer
  • They have a lower cost
• Primary storage


  Short term, working memory that is directly connected to the CPU
• Primary storage access times


  Considerably faster than secondary storage
• Faster access times


  Dramatically reduces time taken to complete operations such as the Fetch-Execute Cycle
• Primary storage


  Holds the data and instructions that the CPU needs to access whilst the computer is turned on
• Primary storage


  • Used as short term, working memory, in hardware that is directly connected to the CPU such as RAM, and components that reside inside the CPU such as Cache and Registers
• Performance of primary storage means a much higher cost which limits the amount that is used
• What is RAM?

  • RAM (Random Access Memory) is primary storage that is directly connected to the CPU and holds the data and instructions that are currently in use
  • RAM is volatile which means the contents of RAM are lost when the power is turned off
  • For the CPU to access the data and instructions they must be copied from secondary storage
  • RAM is very fast working memory, much faster than secondary storage
  • RAM is read/write which means data can be read from and written to
  • In comparison to ROM, it has a much larger capacity
• What is ROM?

  • ROM (Read Only Memory) is primary storage that holds the first instructions a computer needs to start up (Bootstrap)
  • ROM contains the BIOS (Basic Input Output System)
  • ROM is a small memory chip located on the computers motherboard
  • ROM is fast memory, much faster than secondary storage but slower than RAM
  • ROM is non-volatile which means the contents of ROM are not lost when the power is turned off
  • ROM is read only which means data can only be read from
  • In comparison to RAM, it has a much smaller capacity
• What is virtual memory?

  • Virtual memory is an extension of primary storage (RAM) located on secondary storage
  • In situations where RAM is close to being full, virtual memory can be used so that the computer remains operational
  • Programs and data not currently being executed are transferred to virtual memory
  • Programs and data are transferred back to RAM when they are needed
  • Virtual memory is much slower than RAM and it's use will impact negatively on system performance
  • To avoid the use of virtual memory, increase the size of the RAM
• The Need for Secondary Storage

  • Computer systems need both primary and secondary storage to operate
  • Both types of storage play a crucial role in the operation of a computer system
  • primary is volatile an has small storage
  • secondary is non-volatile and has a large capacity
• Why do you need secondary storage?

  • A computer needs secondary storage for long term storage of programs and data that are currently not in use 
  • Secondary storage is needed as ROM is read only and RAM is volatile
  • Secondary storage holds the programs and data whilst the computer is turned off (non-volatile)
  • Performance of secondary storage is slower than primary storage but capacity is much higher which makes it perfect for backup & archive of data files
• What are the characteristics of secondary storage?

  • Capacity What is the maximum amount of data that can be stored?
  • Speed How fast can data be read from and written to? (R/W)
  • Cost How much does it cost?
  • Portability How easy is it to move around? What is the physical size? Weight?
  • Durability How robust is the storage?
  • Reliability What is the probability that the device will consistently perform its function?
• • There are 3 types of secondary storage:
  • Magnetic
  • Solid State
  • Optical
• What is magnetic storage?

  • Magnetic storage is a type of non-volatile media that uses magnets (polarity) to store binary 0s and 1s
  • Advantages/disadvantages
  • Capacity High storage
  • Durability Moving parts can get damaged if dropped
  • Cost - Low per gigabyte
  • Portability Heavy & bulky making them less convenient for transport
  • Speed - Moderate read/write access 
  • Reliability - Prone to mechanical failure Noise Loud (spinning disks)
• What is solid state storage?

  • Solid state storage is a type of non-volatile media that uses electronic circuits to store binary 0s and 1s
  • AdvantagesDisadvantages
  • Capacity -Medium/high storage
  • Cost - Very high per gigabyte
  • Speed - Very fast read/write access
  • Reliability - Limited read/write cycles
  • Durability- No moving parts 
  • Portability -Small and no moving parts
  • Noise -Silent  
• What is optical storage?

  • Optical storage is a type of non-volatile media that uses lasers to burn the surface of a disk, creating pits and lands suitable for storing binary 0s and 1s
  • AdvantagesDisadvantagesCost - Very low per gigabyteCapacity Very lowDurability No moving partsSpeed - Very slow read/write accessPortability Small and no moving partsReliability - Prone to scratchesNoise Silent  
• What is optical storage?

  • Optical storage is a type of non-volatile media that uses lasers to burn the surface of a disk, creating pits and lands suitable for storing binary 0s and 1s
  • Advantages/Disadvantages
  • Cost - Very low per gigabyte
  • Capacity Very low
  • Durability No moving parts
  • Speed - Very slow read/write access
  • Portability Small and no moving parts
  • Reliability - Prone to scratches
  • Noise-Silent