Magnetic disk Physical characteristics

Physical characteristics are how the computer sees the magnetic disk.

[cartoon - what's on a disk?]

Physical Access methods

A disk drive is a device to hold and rotate a disk at high speed.

Accessing a sector

The heads are moved to the cylinder that the directory is stored on.
The directory is read when the disk rotates to its sector. This tells where the file of interest is stored.
Then the heads are moved to the desired cylinder until the correct sector comes around and is read.

Magnetic disk Terms

To access a sector of Disk a Read Write head is moved to the appropriate track. The time taken to perform this movement is called the track time. When the disk has rotated to bring the data under the seek head, data transfer takes place. The total time is called access time.

The two most common measures of a Disk speed are;

Average access time

The average time the drive head takes to reach and read a random spot on the disk

Data Transfer Rate

Measure of how fast data moves from the drive to your PC's memory.

Examples

Technical feature comparison

Factors affecting access times

	Latency	The time required to Spin the disk up to speed from stationery. Move the read head from stationary.
	Seek time	Time taken to find the right data
	Track-to-track movement	Speed to go between tracks

Physical Data Organisation

How the COMPUTER stores the data.

Sectors and tracks

Data is written to the disk surface on concentric circles (tracks)
unlike a record where a continuous track stating from the outside is used.

Each track is divided into SECTORS. Data is written to each sector but not necessarily in sequential sector/track order.

When files are deleted free sectors become available. The computer via a File Allocation Table writes new files into the free space.

Disks may therefore get highly fragmented files, and on large disks performance may suffer. Hence there are fix-it type programs that will reorganise the data into "contiguous" sectors. (for example,. Norton's Utilities)

A standard 360Kbyte floppy is made up as follows;
Soft sectored (only one index hole)
Tracks= 40 per side
Sectors= 9 per track
Bytes= 512 per sector
Total = 368,640 bytes (characters)

Note:. Hard sectored disks have holes in the front of each sector.

Exercise 1: For a Data Disk;

How would you read data?
How would you insert data?

cylinder

To increase the speed of data transfer, both top and bottom surfaces are written on simultaneously, tracing out a cylinder.

Finally, to allow for the management of data, each track is subdivided into a sector.

Answers

Answers 1: Physical Data Storage - Disk File Storage

For disk operations;

To read any data requires;

Use file location table (the File allocation table - FAT in DOS) to find the files position, then move the read/write heads (as you would on a record /Compact disk player), and read the data.

To insert data,

As for read - locate the file required. depending on the file type, the record may be accessed directly, and updated.