1.2
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What does volatile mean?
• Data in memory is lost when power is turned off
What are the differences between RAM and ROM?
RAM: - Volatile - Stores currently running instructions and data temporarily - Data can be changed - Big compared to ROM: - Non-volatile - Read only (data cannot be changed) - Stores boot-up instructions (bootstrap) - Small compared to
Give some examples of primary storage
• RAM • ROM • Registers • Cache
What is primary storage?
• Temporarily holds data and instructions which CPU needs access to while computer is running • Can access data faster than secondary • Volatile
What is virtual memory?
• Part of hard disk is used as an extension when RAM is nearly full • Allows more applications to be open at once • Transferred out to VM from RAM if not currently being executed • Transferred back to RAM from VM when needed
What is secondary storage?
• Needed since ROM is read only and RAM is volatile • Long term/permanent storage of programs and data when power is turned off • Backup of data files • Archive of data files • Read and write
State the three types of secondary storage
• Optical (CDs/DVDs/Blu-Ray) • Magnetic (Hard disk drive) • Solid State (SSD, Memory sticks, Flash memory cards)
What is optical storage?
• Used for music, films and archive files • Low storage capacity compared to • Slow access • Thin, lightweight, portable • Prone to scratches • Cheap
What is magnetic storage?
• Used to store operating system and application • High storage capacity • Quick access • Moving parts which eventually fail • Low portability • Medium cost • Reliable but not durable • Perform better if defragmented
What is solid state storage?
• Used for digital cameras and smartphones • Medium storage capacity • Very quick access • No moving parts, reliable and durable • Low power • No noise • No need to defragment • Limited R/W cycles • Relatively expensive
What are the units of storage from smallest to largest?
• Bit (b): binary digit (0 or 1) • Nibble: 4 bits/half a byte • Byte (B): 8 bits, can represent one ASCII character • Kilobyte (KB): 1 thousand • Megabyte (MB): 1 million • Gigabyte (GB): 1 billion • Terabyte (TB): 1 million million • Petabyte (PB): 1 thousand million million
Mnemonic for data units
Burger King Makes Great Toast, Perfect
What are the characteristics that should be considered when choosing secondary storage?
• Capacity • Speed • Portability • Durability • Reliability • Cost
Why do computers use binary?
• Ideal for representing two states (on or off), simple for computers to process data efficiently • Operate with switches that have only two states • RAM: charge or no charge • CD/DVD: pit or land
With two states, electronic components are:
• Easier to manufacture • Cheaper • More reliable
How do you calculate text files?
Bits per Character x Number of Chararacters
How do you calculate sound files?
Sample Rate x Duration (s) x Bit Depth
How do you calculate image files?
Colour Depth x Image Height (px) x Image Width (px) + 10% for metadata
What is a character set?
• Defined list of characters recognised by the computer • Each character is represented by a unique binary number
ASCII, Extended ASCII, UNICODE
• ASCII: 2^7 (7 bits) • Extended ASCII: 2^8 (8 bits) • UNICODE: 2^24 (24 bits)
What can images stored in binary be?
• Bitmaps: constructed from coloured squares (pixels) • Vectors: created using mathematical equations to draw shapes
What are pixels stored as?
Binary
How can the number of colours be calculated?
• 2^n • n is the number of bits per pixel
What is colour depth?
• Number of bits used to indicate the colour of a pixel