Anatomy of a Computer

1. Anatomy of a Computer RAM, ROM, CPU, etc.

2. A computer is • a person or thing that computes • to compute is to determine by arithmetic means (The Randomhouse Dictionary) • so computing involves numbers • While typing papers, drawing pictures and surfing the Net don’t seem to involve numbers at first, numbers are lurking beneath the surface

3. Representing numbers • Some attribute of the computer is used to “represent” numbers (for example: a child’s fingers) • two kinds of representation are: • analogthe numbers represented take on a continuous set of values • digital thenumbers represented take on a discrete set of values

4. Pros and Cons • the analog representation is fuller/richer after all there are an infinite number of values available • the digital representation is safer from corruption by “noise;” there is a big difference between the various discrete values, and smaller, more subtle differences do not affect the representation

5. Digital signals 0 1 1 0 1 0 0 0

6. Our computers are • digital and electronic • (note that digital  electronic) • they are electronic because they use an electronic means (e.g. voltage or current) to represent numbers • Gives computers their speed and small size • they are digital because the numbers represented are discrete • Noise resistant

7. Binary representation • the easiest distinction to make is between • low and high voltage • off and on • then we can only represent two digits: 0 and 1 • but we can represent any (whole) number using 0’s and 1’s

8. Decimal vs. Binary • Decimal (base 10) • 124 = 100 + 20 + 4 • 124 = 1  102 + 2  101 + 4  100 • Binary (base 2) • 1111100 = 64 + 32 + 16 + 8 + 4 + 0 + 0 • 1111100 = 1  26 + 1  25 + 1  24 + 1  23 + 1  22 + 0  21 + 0  20

9. Bits and Bytes • A bit is a single binary digit (0 or 1). • The elementary unit of information • A byte is a group of eight bits. • A byte can be in 256 (28) distinct states (which we might choose to represent the numbers 0 through 255). • Note computer scientists like to start counting with zero.

10. Realizing a bit • We need two “states,” e.g. • high or low voltage (e.g. computer chips) • why you should protect computer from power surges • north or south pole of a magnet (e.g. floppy disks) • why you should keep floppies away from large magnets • light or dark (e.g. reading CD or DVD, also laser printers) • hole or no hole (e.g. punch card or CD)

11. More, more, more • Akilobyte is 1,024 (210) bytes • approx. one thousand • A megabyte is 1,048,576 (220) bytes • approx. one million • Agigabyte is 1,073,741,824 (230) bytes • approx. one billion • A terabyte is 1,099,511,627,776 (240) bytes • approx. one trillion

12. Storing it away • A standard 3.5 inch floppy disk holds 1.44 MB (megabytes) • An Iomega Zip disk holds approx. 100 MB or 250 MB • A CD (compact disk) holds approx. 650 MB • A DVD (digital versatile [video?] disc) holds several GB (gigabytes) • A typical hard drive holds several GB • Less portable, but faste

13. When bits are represented using voltage, the logical operators (gates) can be constructed from transistors The Pentium ® II has approximately 7.5 million transistors on it The transistors have lengths approximately 0.35 microns (millionths of a meter) Transistors

14. A chip off the old block • Millions of transistors are connected into what is called an integrated circuit or chip • Chips are made from silicon (a semiconductor, a material halfway between a conductor and an insulator) • “Silicon valley” is a nickname for the region south of San Francisco that contains an unusually high concentration of computer companies.

15. CPU • The most important chip in a computer is the microprocessor • The microprocessor houses the Central Processing Unit (CPU), the “brain” of the computer • Ex. Pentium IV is a microprocessor

16. Extending the brain analogy • Similar to dividing the brain into • Brain stem • Forebrain • We divide the CPU into • Control Unit • Arithmetic Logic Unit (ALU)

17. How good is your computer? • Computing means moving bits around, so an important question is how many bits can be handled at one time • Word size: how many bits are handled at a time (8, 16, 32, 64) by memory or the processor • Bus size: how many bits can move around at once • analogy: two-lane, four-lane or eight-lane highway • Bus speed: how fast they move

18. How fast is it? • Each of the computer’s manipulations (instructions) begins with a “tick” of the clock • So the faster the clock ticks, the faster the computer • Clock speed: a measure of how fast the computer is, given in MHz (megahertz - millions of cycles per second) • There are gigahertz machines now • Middle number written on LaSalle’s computers

19. Speed II • Sometimes one instruction can be started before the previous one was complete • Like having a batter on deck • So another measure of speed is useful • instructions per second, given in MIPS (millions of instructions per second) • (considered by some to be a misleading measure of speed)

20. The mother of all circuit boards • chips and other things are connected together on what is called a circuit board • the mother board, a.k.a. the system board, holds the main components of the computer • CPU • clock • connectors • expansion slots, ETC

21. If you can’t stand the heat • The chips, especially the microprocessor can get hot • heat sink: the strangely shaped metal or ceramic structure sitting on the processor that serves to draw away the heat • there’s also a little fan near the processor; that’s often what you hear whirring away on old computers

22. A link to the outside world • The process of putting information into or getting information out of a computer is called interfacing or input/output (IO) • ports are sockets, typically in the back of a computer, where one plugs in the cable connecting the computer to the IO devices • Newer computers have a port in the front for convenience

23. Two types • serial • data sent one bit at a time • for modems and some printers • cable can be very long • ex. MIDI, USB • parallel • data sent eight bits at a time • limit on length of cable • ex. SCSI

24. SCSI port • Small computer system interface • pronounced “scuzzy” • allows more than one device to be connected to a single port • daisy chain: getting the output for a second output device from the first (rather than directly from the computer), the output for a third can come from the second and so on

25. A connector in every port • Ports have connectors, as do cables • connectors come in two varieties • male: have pins sticking out • female: have holes to receive pins

26. Analog to Digital • Any measurement that can be converted to an electronic signal (voltage or current) can be directly fed into a computer • the original data is often continuous (analog) and must be converted into digital form • This signal can be fed in through a port so long as the appropriate software is installed

27. In the cards • Expansion Slot: A socket designed to hold the circuit board for the device, such as a sound or video card, that adds capability to the computer system • Adapter cards: additional circuitry and chips that extend your PC’s capabilities allowing you to customize it

28. Some types of cards • video or graphics card: enhancescomputer’s ability to convert output into video and send it to the monitor • Sound card: improves your computer’s sound capabilities, be it input (microphone) or output (speakers) • internal modem: allows computer to connect to networks via phone lines and such

29. Plug and play • refers to computer’s capability to figure out what to do when new expansion cards and devices are added • this way the user does not have to know how to “configure” the system

30. Memories • Saving information we have entered (e.g. onto floppies) is referred to as “storage;” it is long term and slow by computer standards (storage  memory) • Before we save the data, it is in the computer’s memory, i.e. in memory chips, which hold the information temporarily • Memory also holds the instructions a computer needs to operate (“stored program concept”)

31. Reading and Writing • The basic actions involving memory are • WRITING: putting information into memory • READING: getting information from memory • The rest of the time memory just holds onto information

32. ROM • Read Only Memory • This memory is loaded up by the manufacturer (some is programmable) • contains low-level instructions for the computer • Not lost when the computer is turned off • “nonvolatile” memory

33. RAM • Random Access Memory • The memory the user uses • The programs one loads and the data one enters are here • Lost when the computer is turned off • “volatile” memory • Why is it called random?

34. Random Vs. Sequential • A cassette tape is sequential access; you have to go through song one and two to get to song three • A CD is random access; you can jump directly to song three

35. Some Types of RAM • Dynamic RAM (DRAM): dynamic means changing, which for memory is not necessarily a good thing, so dynamic memory must be continually refreshed • Synchronous DRAM: blocks of data (instead of lines) read more efficiently • Static RAM (SRAM): doesn’t need constant refreshing, is faster but more expensive than dynamic

36. Cache • pronounced “cash” • The computer puts information it anticipates (guesses) you will use soon in a place which is accessed more quickly • A correct guess is called “a cache hit”, an incorrect guess “a cache miss” • Caching improves speed

37. Some kinds of cache • Memory cache: put information from DRAM into SRAM (high-speed memory) • L1 on the chip with the microprocessor • L2 on a different chip • Disk cache: put information from storage (floppy or hard drive) into memory • Browser cache: put information from internet into hard drive