Chapter 01 Part III Hardware: Storage
Storage • Need some way to • Store large amounts of information • Store the information permanently (I.e., when power is turned off). • Examples: hard drives, CD-ROM, zip drives, etc.
Hard Drives • Hard disks were invented in the 1950s. • early disks up to 20 inches in diameter holding a few megabytes. • originally called "fixed disks" or "Winchesters" (a name of a popular IBM product). • Hard disk is similar to a cassette tape. • Both use the same magnetic recording techniques • Both can be easily erased and rewritten, and will "remember" the magnetic flux patterns stored onto the medium for many years. • Hard disks have a hard platter that holds the magnetic medium, • Tapes and floppies use a flexible plastic film
Hard Disk Parts Platter Read/Write Head Electronics control board
Hard Drives: how they work • Platter consists of a thin base material, • bonded to this base is a coating of ferric oxide powder (Fe2O3) . • Maghemite or gamma ferric oxide are common names for the substance. • About 3 millionths of an inch thick • This oxide is a ferromagnetic material, if exposed to a magnetic field it is permanently magnetized by the field. • Can read/write/erase at any time
Hard Drives: how they work • an electromagnet that applies a magnetic flux to the oxide on the platter. • The oxide permanently "remembers" the flux it sees. • The record head is a very small, circular electromagnet with a small gap in it, like this: • The electromagnet consists of an iron core wrapped with wire.
Hard Drives: how they work • an electromagnet that applies a magnetic flux to the oxide on the platter. • To write, CPU sends a signal to the circuit board on the hard drive. • The electronics controls the read/write head and the motor that spins the platters • The electronics also assemble the magnetic domains on the drive into bytes (reading) and turn bytes into magnetic domains (writing).
Hard Drives: how they work • A picture of the disk with the read/write head positioned over it. Platter Read/Write Head
Hard Drives: how they work • an electromagnet that applies a magnetic flux to the oxide on the platter. • To write (continued) • The electronics moves the read/write head to the correct track of the disk • When the disk spins so that the correct part of the disk is under the read/write head, the electronics sends a signal to the read/write head
Hard Drives: how they work • an electromagnet that applies a magnetic flux to the oxide on the platter. • To write (continued) • The signal goes through the coil of wire to create a magnetic field in the core. • At the gap, magnetic flux forms a fringe pattern to bridge the gap (shown in red), and this flux is what magnetizes the oxide on the platter. • To read, magnetic head detects the polarity of the bit under the head.
CD ROMs • A CD is a piece of plastic, about four one-hundredths (4/100) of an inch (1.2 mm) thick. • Most of a CD consists of an injection-molded piece of clear polycarbonate plastic. • During manufacturing, this plastic is impressed with microscopic bumps arranged as a single, continuous, extremely long spiral track of data. • Once the clear piece of polycarbonate is formed, a thin, reflective aluminum layer is sputtered onto the disc, covering the bumps. • Then a thin acrylic layer is sprayed over the aluminum to protect it. • The label is then printed onto the acrylic.
CD ROMs • A CD has a single spiral track of data, circling from the inside of the disc to the outside. • The fact that the spiral track starts at the center means that the CD can be smaller than 4.8 inches (12 cm) if desired, • there are now plastic baseball cards and business cards that you can put in a CD player. • CD business cards hold about 2 MB of data before the size and shape of the card cuts off the spiral. • the data track is approximately 0.5 microns wide, with 1.6 microns separating one track from the next.
CD ROMs • The elongated bumps that make up the track are each 0.5 microns wide, a minimum of 0.83 microns long and 125 nanometers high. • You will often read about "pits" on a CD instead of bumps. • They appear as pits on the aluminum side, but on the side the laser reads from, they are bumps. • If you could lift the data track off a CD and stretch it out into a straight line, it would be 0.5 microns wide and almost 3.5 miles (5 km) long.
CD ROM Parts • A drive motor spins the disc. This drive motor is precisely controlled to rotate between 200 and 500 rpm depending on which track is being read. • A laser and a lens system focus in on and read the bumps. • A tracking mechanism moves the laser assembly so that the laser's beam can follow the spiral track. The tracking system has to be able to move the laser at micron resolutions.
CD ROM: Reading • A laser is used to read • The laser beam passes through the polycarbonate layer, • reflects off the aluminum layer • Light hits a bump is scattered. • Light that hits a “land” is reflected back to the laser • The laser has a prism that redirects the reflected light to a light-detecting diode • The diode generates a small electrical voltage that creates a 1. • If the diode does not detect light, it does not generate voltage and a 0 is generated.
CD ROM: tracking • CDROM must keep the laser beam centered on the data track. (tracking system). • The tracking system, has to continually move the laser outward. • As the laser moves outward from the center of the disc, the bumps move past the laser faster - • this happens because the linear, or tangential, speed of the bumps is equal to the radius times the speed at which the disc is revolving (rpm). • Therefore, as the laser moves outward, the spindle motor must slow the speed of the CD. That way, the bumps travel past the laser at a constant speed, and the data comes off the disc at a constant rate.
CDROM: writing • CD-recordable discs, or CD-Rs, don't have any bumps or flat areas at all. Instead, they have a smooth reflective metal layer, which rests on top of a layer of photosensitive dye. • When the disc is blank, the dye is translucent: Light can shine through and reflect off the metal surface. • But when you heat the dye layer with concentrated light of a particular frequency and intensity, the dye turns opaque: It darkens to the point that light can't pass through. • By selectively darkening particular points along the CD track, and leaving other areas of dye translucent, you can create a digital pattern that a standard CD player can read. • The light from the player's laser beam will only bounce back to the sensor when the dye is left translucent, in the same way that it will only bounce back from the flat areas of a conventional CD.
CDROM: Writing • The CD burner has a moving laser assembly, just like an ordinary CD player. • In addition to the standard "read laser," it has a "write laser." • The write laser is more powerful than the read laser, It alters the surface instead of just bouncing light off it. • Read lasers are not intense enough to darken the dye material, so simply playing a CD-R in a CD drive will not destroy any encoded information.
Flash Drives • Flash memory is used in such devices as digital cameras and home video game consoles. • used more as a hard drive than as RAM. • Flash memory is a solid state storage device. • there are no moving parts -- everything is electronic instead of mechanical.
Flash Drives • Examples of Flash memory: • Your computer's BIOS chip • CompactFlash (most often found in digital cameras) • SmartMedia (most often found in digital cameras) • Memory Stick (most often found in digital cameras) • PCMCIA Type I and Type II memory cards (used as solid-state disks in laptops) • Memory cards for video game consoles • Flash drives
Flash Drives • Flash memory is a type of EEPROM chip. • It has a grid of columns and rows with a cell that has two transistors at each intersection. • The two transistors are separated from each other by a thin oxide layer. • One of the transistors is known as a floating gate, and the other one is the control gate. • The floating gate's only link to the row, or wordline, is through the control gate. As long as this link is in place, the cell has a value of 1. • To change the value to a 0 requires a curious process called Fowler-Nordheim tunneling.
Flash: Tunneling • Tunneling is used to alter the placement of electrons in the floating gate. • An electrical charge, usually 10 to 13 volts, is applied to the floating gate. • The charge comes from the column, or bitline, enters the floating gate and drains to a ground. • This charge causes the floating-gate transistor to act like an electron gun. • The excited electrons are pushed through and trapped on other side of the thin oxide layer, giving it a negative charge. These negatively charged electrons act as a barrier between the control gate and the floating gate. • A special device called a cell sensor monitors the level of the charge passing through the floating gate. If the flow through the gate is greater than 50 percent of the charge, it has a value of 1. • When the charge passing through drops below the 50-percent threshold, the value changes to 0. • A blank EEPROM has all of the gates fully open, giving each cell a value of 1.
Flash: Erasing • The electrons in the cells of a Flash-memory chip can be returned to normal ("1") by the application of an electric field, a higher-voltage charge. • Flash memory uses in-circuit wiring to apply the electric field either to the entire chip or to predetermined sections known as blocks. • This erases the targeted area of the chip, which can then be rewritten. • Flash memory works much faster than traditional EEPROMs because instead of erasing one byte at a time, it erases a block or the entire chip, and then rewrites it.
Example 1.3 • You have 64 Gbytes of information on your HD. You want to transfer to 1.4 Mbyte floppies. How many floppies do you need? Number of bytes on HD: 64 X 230 On each floppy: 1.4 X 220 64 X 230 1.4 X 220 = 45.7 X 210 Or about 45,700
Example 1.3 • If you have a CD/RW and each CD holds 600MB, how many 20MB files can you get on the CD? 600 X 220 20 X 220 = 30
Bar Code Readers • Photocells inside the bar code reader detect the light and dark regions and convert them to bits. • Light elements are 0’s, dark are 1’s UPC symbol from cereal box. Left 5 digits identify the Manufacturer (30000, Quaker). Right 5 identify product (06700, 100% Natural Cereal).
Part of the UPC symbol from the Quaker cereal box These characters appear on the left side of the label
UPC Code • Code is different for characters on the left half and on right half. • A dark bar is composed of from one to four adjacent dark regions. • Each decimal digit has two dark bars and two light spaces. • Characters on left half begin with a light space and end with a dark bar. • Characters on right half begin with a dark bar and end with a light space. • Each left character has an odd number of ones. • Each right character has an even number of ones.
UPC codes • Bit patterns for the decimal digits in the UPC code.