EXTERNAL MEMORY
Ex. Each track contains 30 fixed-length sectors of 600 bytes each. Each sector holds 512 bytes of data plus control information useful to the disk controller. The ID field is a unique identifier or address used to locate a particular sector. The SYNCH byte is a special bit pattern that delimits the beginning of the field. The track number identifies a track on a surface. The head number identifies a head, because this disk has multiple surfaces. The ID and data fields each contain an error-detecting code.
Disk storage capacity in a CAV system is limited by the maximum recording density that can be achieved on the innermost track. To increase density, modern hard disk systems use a technique known as multiple zone recording, in which the surface is divided into a number of zones. Zones farther contain more bits than zones closer from the center. This allows for greater overall storage capacity at the expense of somewhat more complex circuitry.
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There must be some starting point on the track and a way of identifying the start and end of each sector. Thus, the disk is formatted with some extra data used only by the disk drive and not accessible to the user.
Track is a circular path on the surface of a disk. This gives rise to the organization of data on the platter. Each track is the same width as the head. There are thousands of tracks per surface.
Gaps Adjacent tracks are separated by gaps. This prevents, or at least minimizes, errors due to misalignment of the head.
Sectors Data are transferred to and from the disk in sectors. There are typically hundreds of sectors per track. In most contemporary systems, fixed-length sectors arc used, with 512 bytes being the nearly universal sector size.
Intratrack (Intersector) gaps To avoid imposing unreasonable precision requirements on the system, adjacent sectors are separated by intratrack (Intersector) gaps.
A bit near the center of a rotating disk travels past a fixed point slower than a bit on the outside. So we increasing the spacing between bits of information recorded in segments of the disk to compensate for the variation in speed. The information can be scanned at the same rate by rotating the disk at a fixed speed, known as the constant angular velocity (CAV).
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The advantage of using CAV is to move the head from its current location to a specific address, it only takes a short movement of the head to a specific track and a short wait for the proper sector to spin under the head. The disadvantage of CAV is that the amount of data that can be stored on the long outer tracks is the same as what can be stored on the short inner tracks.
DATA ORGANIZATION AND FORMATTING
The write mechanism is based on the fact that electricity flowing through a coil produces a magnetic field. Pulses are sent to the write head, and magnetic patterns are recorded on the surface below. The write head is in the shape of a rectangular doughnut with a gap along one side and a few turns of conducting wire along the opposite side. An electric current in the wire induces a magnetic field across the gap, which in turn magnetizes a small area of the recording medium.
The read mechanism is based on the fact that a magnetic field moving relative to a coil produces an electrical current in the coil. When the surface of the disk passes under the head, it generates a current of the same polarity as the one already recorded. The structure of the head for reading is same as for writing and therefore the same head can he used for both.
The read head consists of a partially shielded magnetoresistive (MR) sensor. By passing a current through the MR sensor, resistance changes are detected as voltage signals. The MR design allows higher-frequency operation, which equates to greater storage densities and operating speeds.
MAGNETIC READ AND WRITE MECHANISMS
Data are recorded from the disk via a conducting coil named the head, a read head and a write head. During a read or write operation, the head is stationary while the platter rotates beneath it.
BENEFITS OF GLASS SUBSTRATES
Improvement in the uniformity of the magnetic film surface to increase disk reliability
A significant reduction in overall surface defects to help reduce read-write errors
Ability to support lower fly heights
Better stiffness to reduce disk dynamics
Greater ability to withstand shock and damage
