Memory Systems

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  • Memory
    • RAM
      • Random Access Memory
      • Computer stores data and instructions for the short term
      • Used while other instructions are being carried out
      • Volatile
        • Anything on it will disappear when the power is cut
    • ROM
      • Read Only Memory
      • Where instructions and data are stored don't need to be changed during normal use
      • Non-volatile
        • Data is still stored after the power is cut
    • Cache
      • Very fast memory
      • Usually stored within the processor
      • Stored while instructions are being carried out in the processor
      • Bigger caches are split into smaller sections to carry out instructions faster
        • Usually include layers
        • Goes from RAM to CPU with levels in between
    • Hard disc drives
      • Can store large amounts - up to 1TB
      • Disc spins and read-write heads move across them
      • Encoded as opposing magnetic polarities on the surface of the disc by electromagnets in the read-write heads
      • Stacks of non-removable discs coated in magnetic material
      • Moving components can break
      • Very reliable
      • Strong magnets could erase the data
      • Qualities
        • 500MB
        • Cheap
        • Internal devices are fixed, external hard discs are portable
        • Reasonably durable but can be damaged if dropped or knocked
        • Generally reliable if looked after
        • Long-term storage of instructions and data
    • Optical storage
      • Compact discs (CDs) can store up to 700MB
      • Digital versatile discs (DVDs) can store up to 4.7GB
      • Blu-ray discs can store up to 50GB
      • Single spiral track running 5km from the inside to the outside
      • Data stored by etching pits into the track
      • Light hits land = reflected back to a detector (1)
      • Light hits pit = shattered away (0)
      • Discs covered in a translucent dye
        • Light passes through and is reflected by disc surface
        • Powerful laser burns the dye and creates black spots
        • Reflective areas = 1, black spots = 0
      • Susceptible to scratches and shattering
    • Solid-state memory
      • Made from NAND flash memory
      • Stored as an electrical charge in the silicon
      • Non-volatile
        • Can electrically erase and reprogram it
      • All transistors are charged (set to 1)
      • Save operation begins = current is blocked and switched to 0

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