Section 1 - Components of a Computer System

Section 1 - Components of a Computer System

1. The CPU

   • The CPU is essentially the so called 'Brain' of the computer system
     • It processes all of the data and instructions in which make the system work
     • The processing power of a CPU is dependent on several characteristics
       • Clock Speed
         • This is the number of instructions a single processor core cam carry out per second (Hz)
           • For most desktop computers, this will be around 3.5GHz
             • This means 3.5 billion instructions per second
         • The higher the clock speed, the more instructions per second
           • Some CPUs are overclocked
             • This means that they are running at a higher clock speed than the factory-set rate
               • Can be risky if not done correctly
                 • CPU can overheat
                 • This will cause crashes or permanent system damage
                   • High performance cooling systems are usually needed
       • Number of Cores
         • Each core can process data independently from the rest
           • The more cores, the more instructions it can carry out at once, so the faster is can process a batch of data
             • Most PCs or Smartphones have at least 4 or more cores today
       • Cache Size
         • The cache (p2) is data storage inside the CPU that's much faster than RAM
           • Larger cache gives the CPU faster access to more data it needs to process
     • The CPU Architecture describes the main components of the CPU
       • How they interact with each other and other parts in the computer system
       • Von Neumann and Harvard
         • Two main types of architecture
         • Von Neumann's Design Revolutionised Computing
           • Th program counter holds the memory address of the instruction of each cycle
           • Accumulator stores intermediate results of calculations in the ALU
           • Memory holds program instructions and the program data
           • The memory data register (MDR) holds the actual data
             • This may have been fetched from the memory or waiting to be written to the memory
           • The memory address register (MAR) holds and memory address about to be used by the CPU
             • The address might point to data or a CPU instruction
   • The CPU has Three Main Parts
     • The Control Unit (CU)
       • The CU is in overall control of the CPU
         • Main job is to execute program instruction by following th fetch-decode-execute cycle
     • The Arithmetic Logic Unit (ALU)
       • The ALU does all of the calculations
         • Completes simple addition and subtraction
         • It also compares the size of numbers and can do multiplications and divisions
           • Uses repeated addition and subtraction
         • Performs logic operations such as AND, OR and NOT
           • Contains an accumulator
     • The Cache
       • The cache is very fast memory in the CPU
         • Slower than the registers
           • Faster than RAM
           • The CPU contains various registers
             • Temporarily hold tiny bits of data needed by the CPU
               • Super quick to read/write to
       • Stores regularly used data so the CPU can access it quickly
         • When the CPU requests data, it checks the cache first
           • If not there, it goes to the RAM
       • Very low capacity
         • Very expensive compared to RAM and secondary storage
       • There are different levels of cache memory
         • L1
           • L1 is the quickest and has the lowest capacity
         • L2
           • L2 is slower than L1, however it can hold more
         • L3
           • L3 is slower than L2, however it can hold more
   • The Fetch - Decode - Execute Cycle
     • Fetch
       • 1 - Copy memory address from the program counter to the MAR
       • 2 - Copy the instruction stored in the MAR address to the MDR
       • 3 - Increment the program counter to point to the address of the next instruction
         • This prepares the CPU for the next cycle
     • Decode
       • The instruction in the MDR is decoded by the CU
         • The CU may then prepare for the next step e.g. loading values into the MAR or MDR
     • Execute
       • The instruction is performed
         • This could be:
           • Load data from memory
           • Write data to memory
           • Do a calculation/logic operation
           • Change the address in the PC
           • Halt the program

2. System Performance

   • RAM
     • If a computer has too little RAM it may run slowly because of the use of virtual memory
     • The more RAM, the more apps, or more memory-intensive apps it can smoothly run, therefore making it faster
     • It's easy to upgrade the RAM on a PC or Laptop
       • Just replace the existing RAM sticks with higher capacity or higher speed ones
     • If a computer already has plenty of RAM to run everything the user wants, increasing RAM will more than likely make no difference
   • GPU
     • GPUs (Graphical processing units) are specialised circuits for handling graphics and image processing
       • They relieve the processing load on the CPU, freeing it to do other things
     • Computes already have basic GPUs integrated onto the motherboard or the CPU
       • For better graphics performance, a dedicated GPU is required
     • Using high-end graphics cards can greatly improve graphical performance in graphics-intensive apps
       • E.g. PC gaming and design software

3. Memory

   • RAM is used as the main memory in a computer, it can be read and written to
     • RAM is volatile
       • Volatile memory is temporary and it requires power to retain any of its data
         • Non-volatile memory is permanent memory; it keeps its contends when there is and isn't power
     • The main memory is where all data, files and programs are stored while they're being used
       • When software apps, documents and files are opened they're copied from secondary storage to RAM
         • They remain there until it's closed
     • Slower than CPU cache, but way faster than secondary storage
   • Virtual Memory is Secondary Storage used as extra RAM
     • When RAM is full the computer needs somewhere else to put app data
       • Moves data that hasn't been recently used to virtual memory on secondary storage
         • May be needed if many apps are open or a memory-intensive app is being used
         • The CPU needs to read data stored in virtual memory
           • Must move data back to RAM
         • Using VM can make a computer slow to respond when switching between apps when one is in VM or when using a memory-intensive app
   • ROM tells the CPU how to boot up
     • ROM is non-volatile memory and it can only be read from not written to
     • ROM comes on a small, factory-made chip built into the motherboard
     • Contains all of the instructions a computer needs to properly boot
       • These instructions are called the BIOS
         • Basic Input Output System
       • As soon as the computer is powered on the CPU reads the instructions from the ROM
         • This tells the CPU to perform self checks and set up the computer
     • Even though it's read only it's possible to update ('flash') the BIOS on a ROM chip

4. Secondary Storage

   • Three Tiers of Storage
     • Primary storage refers to the memory areas that the CPU can access quickly and easily
     • Secondary storage is non-volatile
       • This is where all data is stored when not in use
         • This includes Operating systems, applications and user files
     • Tertiary storage is non-volatile and is used for storing data more long term
       • Used mainly for archives or backups of large amounts of data
         • An example would be a magnetic tape library which used a robotic arm to access the tapes
   • Magnetic Hard Disks are High-Capacity, Reliable Storage
     • HDDs are the traditional internal storage
       • Made up of a stack of magnetised metal disks spinning at a rate between 5400 - 15000 rpm
         • Data is stored magnetically in small areas called sectors within circular tracks
           • Read/write heads on a moving arm are used to access sectors on the disks
     • Backup HDDs are popular for backing up and transporting large amounts of data
       • Generally long lasting and reliable (despite moving parts)
   • SSDs are Fast and Reliable secondary storage
     • Storage devices with no moving parts, most use a type of flash memory
       • Same purpose as HDDs for internal storage
         • Hybrid devices exist which use solid state storage for the OS and programs, and a hard disk for data
           • Faster boot with SSDs

5. System Software - Utilities

   • Section 1 Extension Topic - Software
   • Defragmentation Utilities put broken files back together
     • Files are stored in hard disks in available spaces, ideally entire files would be stored together
       • As files are moved, deleted and change size, many small gaps begin to appear on the disk
         • The OS will split newly saving files into smaller blocks to fill up the gaps
     • Over time disks become more and more fragmented
       • Slows reading and writing as the read/write head has to move back and forth across the disk
         • Defrag reorganises data on the hard drive to put fragmented files back together
           • Also collects and moves all the free space together to help prevent further fragmentation
   • A utility application performs a specific task in relation to the computer's operation/ performance

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