What is a Computer?

• This is an adaptation of chapter 1.2 of your book.
• A classic model for a computer is the Von Neumann Architecture.
  • (Taken from wikipedia).
  • Input devices
    • Keyboards, mice, network, ...
  • A CPU or Central Processing Unit
  • A Memory or storage location
  • Output devices (monitor, printer, speakers, ...)
  • This is, of course, idealized or abstracted.
• The heart of the system is a CPU
  • This is the "chip"
  • This executes, or runs, instructions.
  • These instructions are provided by the programmer.
  • They do whatever we want the computer to do.
• Memory is where we store data.
  • There are at least two types of memory.
  • RAM or Random Access Memory
  • Persistent or Long term storage.
• Persistent storage is the easiest.
  • Hard drives, flash drives, ,...
  • This holds information when no power is present.
  • This is generally larger, slower and less expensive.
  • For an analogy think books.
• RAM
  • This is short term storage.
  • Generally when the power is turned off this is erased.
  • Think scratch paper or a white board.
• Generally the CPU will access memory for computations.
• One feature of the Von Neumann architecture is that both program and data are stored in memory.
• The CPU will execute the fetch-decode-execute cycle when running a program

  • Fetch Decode Execute
       Get the next instruction to be executed from memory
       Do what that instruction says (execute the instruction)
          

  • We will discuss this a little right now.
  • But you will explore this in much more depth in Computer Architecture.
• Back to memory
  • The basic unit of memory is a bit or Binary DigiT.
  • This is somewhat akin to a penny.
  • A bit is a single 0 or 1.
  • We generally do not discuss bits, but collections of bits.
• Byte
  • A collection of 8 bits is called a byte
  • Just like a collection of pennies is called a dollar.
• Memory is a collection of bits that we address or access by an index value.

  • Address	Data
    0	xxxxxxxx
    1	xxxxxxxx
    2	xxxxxxxx
    ...	xxxxxxxx
    MAX Address	xxxxxxxx

  • Each x represents a 0 or a 1.
• We frequently don't deal with a single byte either.
  • A word is a hardware dependent measurement, but it is the natural unit f data for a given processor
    • your laptop probably has a 64 bit word. (8 bytes)
    • An older computer has a 32 bit word (4 bytes)
    • An old 8 bit Atari has an 8 bit word (1 bytes)
  • We use the SI Prefixes when referring to larger collections of bits/bytes.
    • 1000 (or $10^3$) bytes is 1 kilobyte
    • 1,000,000 (or $10^6$) bytes is 1 megabyte
    • 1,000,000,000 (or $10^9$) bytes is 1 gigabyte
  • Your author is not quite right in table 1.1
    • Due to litigation, a new unit has been introduced for powers of two.
    • These are the multi byte units
    • $1024$ (or $2^10$) bites are a Kibibyte
    • $1024^2$ (or $2^20$) bites are a Mebibyte
  • I really don't care about the difference in the two, but
    • Powers of 10 are the true SI system
    • Powers of 2 are close, but not quite.
  • By the way, these go up a long way.
    • I am not sure how much to trust this, but look at this blog entry
    • A Zetabtye is $10^{21}$ or 1,000,000,000,000,000,000,000 bytes.
    • That is not quite, but a yottabyte(s) (a joke)
      • yottabyte is the next unit, $10^{24}$.
      • That is a yot of zeros. (say it fast)