2/19 how disk storage works (part ii) roll call. video: disk storage. step-by-step lecture: reading...
TRANSCRIPT
2/19 How disk storage works (part II)
• Roll call.• Video: disk
storage.• Step-by-step
lecture: reading from a disk
• Step-by-step lecture: how a hard drive works.
Part 1: Reading from a disk
• How does a program get a file back from a disk?
1. A request is made.
• A program asks the OS to get a file.
• EX: MS Word asks for the file “boringmesilly.doc”
2. The OS looks it up in the VFAT.
• The operating system looks in the VFAT for a reference to the file.
• It looks in the current folder only.
LETMEGO.DOC 3
3. The OS gets the location.
• The operating system gets the address of the first cluster of the file.
• It also gets the addresses of any other clusters that the file is in.
ITSFRIDAY.DOC 3
4. The OS gets the BIOS involved.
• The operating system gives the address details to the BIOS.
• The BIOS tells the disk controller to move to get the data.
HAVEMERCY.DOC
5. The BIOS gives it to the RAM.
• The BIOS accepts the data from the disk drive and passes it on to the RAM.
• Now it can be used by a program.
PLEASE.DOC 3
Part 2: How a hard drive works
• Materials & methods
• Comparisons with other storage media
1. The protective case
• A sealed metal housing protects the read/write heads and platters from foreign particles.
2. The logic board
• On the bottom of the drive is a logic board.
• It receives commands from the disk controller (which is run by the OS).
2A. What the logic board does
• The logic board converts digital commands into voltages that guide the head actuator to move the heads across the platters.
Head actuator
2B. Keeping the CAV
• The logic board also monitors the spindle to ensure a Constant Angular Velocity.
• It also tells the heads when to read and when to write.
Spindle
3. About the spindle
• The spindle is connected to an electric motor.
• It turns up to 8 platters at 3,600 to 10,000 rpm.
Spindle
4. Moving the heads
• The head actuator pushes and pulls the read/write head arms across the surfaces of the platters.
4A. Gang of heads
• The read/write heads move in unison -- they are fixed to one another.
5. Platters
• The number of platters and the type of magnetic material determine the capacity for data storage.
platters
5A. Platter materials
• Substrate: aluminum alloy, glass, glass composites, magnesium alloys.
• Magnetic coating: magnetic alloys, ~3 microin. thick.
• Protective film: keep out scratches.
platters
6. Readin’ ‘n’ (W)Ritin’
• The data is written to (or read from) the disk similar to a floppy disk drive: magnetic.
7. To start reading or writing a file
• The read/write heads go to the VFAT first, to get the address of the file.
• The VFAT is ALWAYS in sector 0 of the disk.
8. Find that file!
• The VFAT tells the OS of the chained record of clusters of the file.
8A. Write it down.
• The OS writes the VFAT of any changes that were made to the data on the disk.
9. Capacity: floppies, hard drives, & CD-ROMsTracks /
sideSectors /
trackBytes /sector
Floppy disk(1.44 Mb)
80 18 512
Hard drive(3.8 Gb)
6,810 ~ 63 - 232 512
CD-ROM(650 Mb)
20,000* n/a n/a
*CD-ROMs have one continuous track.
Ind. Assignment #2: Webpage
• Pick a subject of interest to you.• Make a webpage about it.• Post it somewhere• Minimums: 1 page 4 images• 4 links 1 bookmark• At the bottom of your page: your name, the date, & the
URL of the page.• Send me an email telling what you learned from this
that you hadn’t understood before.• Due 3/1/99 @ 12:00 noon.