components of a computer system

• the purpose of a computer is to take data, process it then output it
• a computer system consists of hardware and software that work together to process data and complete tasks
• embedded systems are computers built into other devices like dishwashers, microwaves and tvs. easier to design, cheaper to produce and more efficient
• the cpu processes all the data and instructions and manages all the hardware, the processing power depends on different characteristics like clock speed, number of cores and cache size
• the cu is in the cpu. it controls the cpu, executes instructions by following the fetch decode execute cycle. also controls the flow of data in the cpu
• the alu does arithmetic calculations, performs logical operations, and performs shift operations. it contains the accumulator register
• cpu contains registers
• the cache is very fast memory inside the cpu, slower than registers but faster than ram. stores regularly used data so the cpu can access it quickly when it is needed. low capacity + expensive compared to ram and secondary storage.
• the different levels of cache are L1, L2 and L3. L1 is the quickest but lowest capacity, L2 is slower than L1, L3 is slower than L2 but can hold more
• von neumann architecture:

  • program counter - holds the memory address of the instruction for each cycle
  • accumulator - stores results of calculations in the alu
  • memory address register - holds any memory address about to be used by the cpu
  • memory data register - holds the actual data of the instruction, this may have been fetched from memory or be waiting to be written from memory
  • memory - holds program instructions and program data
• fetch:

  1. copy memory address from program counter to mar
  2. copy instruction stored in the mar to the mdr
  3. increase pc to point to the next instruction address
• decode:

  1. instruction in the mdr is decoded by the cu
  2. the cu prepares for the next step eg loading values into the mdr or mar
• execute:

  the instruction is performed
• clock speed

  • the number of instructions a single processor core can carry out per second
  • for most desktop computers this is around 3.5GHz
  • the higher the clock speed, the greater the number of instructions that can be carried out per second
  • some cpu can be overclocked to make them run at a higher clock speed. this is risky if not done properly as it can make the cpu overheat, causing crashes or permanent damage, high performance cooling systerms are needed
• ram

  • main memory in the computer, can be read and written to
  • volatile
  • where all data, files and programs are stored while they are being used
  • slower then cpu cache but much faster than secondary storage
• virtual memory

  • secondary storage used as extra ram
  • needed if too many applications are used at once or a memory intensive application is being used
  • if the cpu needs to read the data stored in virtual memory, it must move the data back to ram. this is slow as data transfer rates are much slower on secondary storage
  • can make a computer slow to respond and cause crashing
• rom

  • can only be read and not written
  • non-volatile
  • contains bios which are the instructions for a computer to start and boot up
  • the cpu read instructions from the rom for post, which tells the computer to perform self checks
• number of cores

  • each core in the cpu processes data independently
  • the more cores, the more instructions it can carry out at once so the faster it can process data
• cache size

  • cache is data storage in the cpu thats much faster than the ram
  • large cpu cache gives the cpu faster access to data it needs to process
• graphics processing units

  • specialised circuits for handling graphics and image processing
  • relieve processing load on the cpu so it can do other things and improves performance in graphic intensive applications
• effects of ram on the cpu

  • if the computer has too little ram it may run slowly due to use of virtual memory
  • easy to upgrade ram on a pc or laptop - replace ram sticks with higher capacity ones
• tiers of storage

  • primary storage - memory areas the cpu can access very quickly like cpu registers, cache, rom and ram. fastest read/write speeds and mostly volatile
  • secondary storage - where all data is stored when not in use. includes magnetic hard disk drives, solid state drives, CDs and SD cards. non-volatile and much slower compared to primary storage
• magnetic hard disks

  • hard disk drives are the traditional internal storage in PCs and laptops
  • made of a stack of magnetised metal disks
  • high capacity, reliable storage, long lasting
  • portable HDDs are popular for backing up and transporting large amounts of data
  • examples are hard disks, zip disks and floppy disks
• solid state

  • fast and reliable secondary storage
  • no moving parts
  • faster reading time than HDDs
  • portable SDDs can be used to back up and transport data
  • USB memory sticks are solid state
• optical

  • cheap and robust secondary storage. comes in the forms read only, write once, rewritable
  • examples are CDs (hold around 700MB of data), DVDs (4.7GB) and blu-rays (25GB)
  • cheap, portable and not damaged by water/shocks but easily scratched
  • low capacity per disc, very slow read/write speeds and poor reliability
• magnetic tape

  • much greater storage capacity then HDD
  • used by large organisations to store large amounts of data
  • comes in plastic cassetes which require a special tape/ drive to read /write
  • written or read sequencially so slow when finding specific data stored on it but fast read/write speed when it is in the correct place