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.