Magnetic Storage Principles All magnetic storage devices, such as floppy disk drives and hard disk drives, read and write data by using electromagnetism Random storage vs. serial storage. IBM Disks consist of magnetic material coated on mylar, glass, or aluminum.

More Principles Figure 9.4 Current flowing through a conductor produces a magnetic field. During recording, the head changes electrical impulses to magnetic fields. “Moving a conductor through a magnetic field” produces a current. During reading, the head changes magnetic fields to electrical impulses.

Data Encoding Schemes Are techniques for encoding data for magnetic storage Examples are: FM - Frequency Modulation (no longer in use) MFM - Modified Frequency Modulation (used in floppy disks) RLL - Run Length Limited (used in hard drives)

More on encoding The reading and writing processes require perfect synchronization Because it is twice as efficient as FM encoding, MFM encoding also has been called Double Density recording Instead of encoding a single bit, RLL normally encodes a group of data bits at a time

Floppy Disk Storage Invention in 1967 Types of drives include 1.44 Mb 3.5 inch drive 2.88Mb 3.5 Inch Drive 720Kb 3.5 Inch Drive 1.2Mb 5.25 Inch Drive 360Kb 5.25 Inch Drive

More about floppy drives R/W heads are moved by a motor called a head actuator. Floppy disk has 2 sides; thus, only need 2 heads: top and bottom. Top and bottom heads cannot move independently. Heads must touch the disk! Disk spins at 300 or 360 rpm. Tunnel erasure. Alignment.

Even More about floppy drives Logic board lives on the drive; logic boards do occasionally fail Floppy controller lives on the motherboard (little change in technology over time). Floppy needs an IRQ, DMA, and I/O port. Faceplate is called a bezel. Need a power connector and a disk drive cable.

Operation of Floppy Drives Formatting the disk writes the information that the o/s needs to maintain a “file table of contents.” Low-level and high-level are done at the same time. The outermost track is reserved for the o/s. Cleaning Typically not needed very often Simple head-cleaning kits available from computer- or office-supply stores Use a cleaning swab with a liquid such as pure alcohol or trichloroethane (not recommended) Physical construction notes: pp 626-628.

Care for disks and drives Page 629 Worst danger (to disks) is from magnetic fields, especially those from computer monitors. Metal detectors are bad but x-ray machines are not a problem (to disks). Troubleshooting tips: pp 631-633. Smoking around the drive is bad. Don’t bother re-aligning a drive. Drives are cheap.