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.