1. Jazan University, Jazan KSA
Chapter 6
Lecture 13
faheem
Lecturer, CNET Dept.,
Jazan University, Jazan KSA
External Memory

2. Topics of this chapter Magnetic Disk
Magnetic Disk, Read and Write Mechanisms, Inductive Write MR Read, Data Organization and Formatting, Disk Data Layout, Disk Velocity, Disk Layout Methods Diagram, Finding Sectors, ST506 format (old!), Characteristics, Fixed/Movable Head Disk, Removable or Not, Multiple Platter, Cylinders.
RAID [Redundant Array of Independent Disks]
Removable, RAID 0,1,2,3,4,5, RAID comparison
Optical Memory.
CD-ROM, CD-Recordable (CD-R), CD-R/W, DVD

3. Magnetic Disk
A disk is a circular platter constructed of nonmagnetic material, called the substrate, coated with a magnetic material. Generally, the substrate is of an aluminum material. Now recently, glass substrates are introduced in new hard disk.
Recording and retrieval via conductive coil called a head, May be single read/write head or separate ones.
During read/write, head is stationary, platter rotates.
Book Page No 186

4. Read and Write Mechanisms
Current through coil produces magnetic field
Pulses sent to head
Magnetic pattern recorded on surface below
Read (traditional)
Magnetic field moving relative to coil produces current.
Coil is the same for read and write.
Book Page No 186

5. Inductive Write MR Read
Book Page No 187

6. Data Organization and Formatting
Concentric rings or tracks.
Gaps between tracks
Reduce gap to increase capacity
Same number of bits per track (variable packing density)
Constant angular velocity
Tracks divided into sectors.
Minimum block size is one sector.
May have more than one sector per block.
1. prevents, or at least minimizes, errors due to misalignment of the head or simply interference of magnetic fields.
2. Data are transferred to and from the disk in sectors
Book Page No 187

7. Disk Data Layout
Book Page No 188

8. Constant angular velocity & multiple zone recording.
The information that can be scanned at the same rate by rotating disk at fixed speed is called CAV(constant angular velocity).
Multiple zone recording: in which surface is divided into number of concentric zones. Within zone number of bits remain constant.

9. Disk Layout Methods Diagram
Book Page No 189

10. Fixed head and movable head
In fixed head there is one read write head per track.
In movable head there is only one read write head for all tracks.

11. Removable and non removable disk and cylinder
Non removable head is permanently mounted on disk.
Removable disk can be removed and replaced with another disk.
Cylinder is set of all tracks in the same relative position on the platterm.

12. Disk performance
Seek time: time it takes to position head at the track is called seek time
Rotational delay: time it takes for the beginning of sector to reach the head.
Transfer time: time required for transfer of data on disk.

13. Thank you!!!