Chapter 9: Hard Disks and Floppy Disks^191
disk. The result is the number of bits (expressed as megabits and gigabits) per
square inch on the disk. An area density of around 1.5Gb per square inch is
common on newer disk drives.Interleaving
Although most newer drives no longer need it,interleavingwas applied to older disk
drives, such as the ST506/412, to offset the impact of latency and to all slow disk control-
lers to keep up with data transfer rates without missing sectors.
Interleavingallowstheread/writeheadtousetherotationofthedisktoitsadvantage.
A disk drive with an interleave ratio of 3:1 (which stands for 3 to 1, but really means 3 mi-
nus 1) writes one sector and then skips two before writing the next. Likewise, an inter-
leave of 2:1 means that it writes to every other sector (2 minus 1 equals 1). An interleave
of 1:1 is the same has having no interleaving at all.
As the disk rotates under the read/write head, the disk controller may need the
amount of time it takes to ski pover one or two sectors to get ready for its next read or
write action. By interleaving “empty” sectors into the process, the disk controller is better
synchronized to the speed of the disk’s rotation. However, the “empty” sectors skipped
over in the interleave action do not remain empty and will be used by other read/write
actions. Applying interleaving by changing the controller card to one that supported it
enabled some older drives to double or triple their transfer rates.Formatting the Disk
Hard disk drives must be formatted before data can be stored on them. Two formatting
levels are performed on a disk media to prepare it for use: low-level and high-level
formatting. On most new PCs, the hard disk media is low-level formatted at the factory
and pre-formatted diskettes are commonly available, so formatting is used largely to
prepare a hard disk for the operating system or to erase it for reuse. Here is more detail on
the two formatting types: Low-level formatting A low-level format permanently erases the disk and
is not reversible because it performs a destructive scan of the disk to find any
defects in the recording media. The location of any defect found is recorded
as unusable to avoid data problems.
High-level formatting High-level formatting is done after low-level formatting
and after the disk has been partitioned (see next section). The high-level format
prepares the disk’s partitions by creating a root directory and the File Allocation
Table (FAT). The FAT is used to record the location and relationships of files and
directories on the disk.A low-level format should not be done on an IDE or SCSI hard disk. This is per-
formed during manufacturing and should not ever be needed again. Because the