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-ROMs *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 links1 bookmark At the bottom of your page: your name, the date, & the URL of the page. Send me an telling what you learned from this that you hadn’t understood before. Due 12:00 noon.