Understanding Primary & Secondary Storage: RAM, Cache, ROM, HDDs, and Virtual Memory, Study notes of Computer science

Download Understanding Primary & Secondary Storage: RAM, Cache, ROM, HDDs, and Virtual Memory and more Study notes Computer science in PDF only on Docsity! Primary Storage  Primary storage is directly accessible by the CPU, it includes RAM, Cache, and ROM  The purpose of Random Access Memory (RAM) is to store data, instructions and software (including parts of the OS) currently in use o RAM is also called main memory o RAM has faster read/write speeds than secondary storage o RAM is volatile which means it is temporary memory and all data will be lost when the computer is switched off o The larger the main memory, the more data the computer access quickly, which can improve the computer's performance o A standard desktop computer has between 4GB and 16GB of RAM  The parts of the operating system currently in use will be stored in RAM  The purpose of cache is to store frequently/recently used instructions and data o Cache has faster read/write speeds than RAM o There are different levels of cache  Level 1 cache is situated within the CPU  Level 2 and 3 cache are situated on the motherboard o If you increase the amount of cache you can store more frequently used data and increase the performance of your computer  If you have too much cache it will decrease the data access speeds because the data requested will take longer to find o Cache is volatile and all data will be lost when the computer is switched off A diagram showing the parts of a computer system which are classed as primary storage  The role of Read-only memory (ROM) is to store the computer’s boot- up sequence o The boot-up sequence is a set of instructions the computer executes when then computer is initially switched on o This sequence will run a series of checks to ensure all of the system hardware is working o It will then load the operating system which will move from secondary storage to RAM  ROM is non-volatile which means data will be stored when the computer is switched off  ROM is read-only which means data cannot be written to ROM The difference between ROM and RAM RAM ROM Types of secondary storage  There are three types of secondary storage - magnetic, solid state and optical Magnetic  Magnetic hard disk drives have been the main type of internal secondary storage in personal computers for many years however solid state drives are increasingly popular due to their durability, low power consumption and high read/write speeds  A magnetic hard disk is made up of several metal discs coated with a magnetic material o These are called platters o Iron particles on each platter are magnetised to represent a 0 or 1  Each platter is divided by concentric circles creating several tracks and wedge shaped sectors. Where they intersect is a track sector  The hard drive spins the metal disk(s) at a high speed (typically around 5400-7200 RPM) using a motor  A read/write arm, controlled by an actuator, moves the head over the surface of the disc to the location of the data  The data is read/written using electromagnets Solid State  Solid State secondary storage is made of transistors that are arranged in a grid layout  It uses NAND and NOR gates in electrical circuits to persistently control the flow of electrons  Current flowing is 1, no current is 0 -  which is how electrical flow can represent binary values  A NAND gate will take in two inputs and produce an output (1/electrical current) as long as both inputs are not 1 Optical  Optical devices include CDs, DVDs and Blu-rays o Blu-rays have the largest capacity o CDs have the lowest capacity o CD-R are read-only (you cannot save data on to them) o CD-RW can be written to and read from o DVD-RW can be written to and read from  All optical devices work by shining a laser at the disk and processing the reflection  An arm moves the laser across the surface of the disk  In CD-Rs a laser burns the data, permanently on to the disk, by creating pits and lands  The laser is also used to read the data from the pits and lands  When the laser light hits the point where the pit changes into a land or vice versa the light scatters and is not reflected back as well. This is captured by a sensor and can be interpreted as a change in the binary value Virtual Memory  The purpose of RAM is to store current data and program instructions in use  The amount of physical RAM available is limited. The average modern desktop computer has between 4GB and 16GB of RAM. Some programs are larger than this  Virtual memory is part of secondary storage which acts as RAM when RAM is full  It prevents the computer system crashing when RAM is full  Data that needs to be accessed by the CPU directly is switched by the operating system from virtual memory into RAM and an inactive page from RAM replaces it in virtual memory. This is called page switching A diagram showing page switching between RAM and virtual memory