Magnetic material coated on both sides of an extremely flat smooth disk or platter. One or more platters permanently mounted on a rigid spindle. Read/write heads on both sides of each platter, but only one head active at a time.

Data layout

Data layed out in sectors (512 bytes commond) on separate parallel tracks. The set of tracks at the same location on each surface of the platters is called a cylinder. On early models, all tracks contained the same number of sectors. On newer models, tracks are grouped into zones, with number of sectors per track in any one zone being the same but varying between zones to allow more sectors on the outer tracks.

Addressing sectors

Legacy PC design provides for direct addressing of data on a disk by cylinder, head, and cylinder. The initial protocl provided 10 bits for the cylinder address, 8 bits for head address, and 6 bits for sector address. This provides for 1024 Cylinders, 256 Heads, and 63 Sectors. With a 512 byte sector, this provides 1024 * 256 * 63 * 512 = 8 Gig.

CHS - Sectors referenced by the default addressing protocol. Although the 8 Gig limit was available from the original pc design, in practice, the space allocated in the chasis of the pc provided for a drive with at most 8 platters (16 heads). With this limit, the maximum drive storage for PCs was 528 Meg.

Translation - Uses circuitry on the hard drive to remap cylinder, head, and sector addresses outside the legal ranges to appear legal. Example. A drive with 2048 Cylinders and 4 Heads could be addressed as 1024 Cylinders and 8 Heads.

LBA - Logical or Large Block Addressing - The sectors are numbered from 0 up to the number addressable by the 24 bits dedicated to the CHS address. This provides the 8 Gig access no matter how the CHS geometry is arranged. However, on most systems, additional software or newer bios support is required to use this form.

To access drives of greater than 8 Gig. of storage, bios extensions which override the orginal 24 address storage are required.

Disk Speed and sector access

Disk speed is set at a constant rotational or angular velocity. This means that the linear speed at the rim of the disk is faster than at the hub.

There are currently three speeds available for disks using IDE technology. 5400 RPM, 7200 RPM and 10,600 RPM.

When the sector count per cylinder remains the same for every cylinder on the disk then a sector at the rim is larger than at the hub. To compensate for this, a technique called zone formatting breaks the disk surface up into sets of cylinders or zones. All cylinders in a zone near the hub will have a certain number of sectors per cylinder. At a zone further from the hub, all cylinders will have a larger number of sectors.

This allows more data to be stored on the drive. But is also requires that LBA or drive hardware supported version of translation be implemented.

Data Access

Data access is a combination of random and sequential read. The read/write head mechanism can be randomly placed over the cylinder of interest and any one of the heads activated.

Once the read/write head is in place, the track of interest must be scanned sequentially for the desired sector and the sector sequentailly read.