COMPUTER SYSTEMS ORGANIZATION - Structured Computer Organization - page 87

Cache memories take a small multiple of CPU registers. Main memory accesses
are typically 10 nanoseconds. Now comes a big gap, as disk access times are at
least 10 times slower for solid-state disks and hundreds of times slower for mag-
netic disks. Tape and optical disk access can be measured in seconds if the media
have to be fetched and inserted into a drive.
Second, the storage capacity increases as we go downward. CPU registers are
good for perhaps 128 bytes, caches for tens of megabytes, main memories for a
few gigabytes, solid-state disks for hundreds of gigabytes, and magnetic disks for
terabytes. Tapes and optical disks are usually kept off-line, so their capacity is lim-
ited only by the owner's budget.
Third, the number of bits you get per dollar spent increases down the hier-
archy. Although the actual prices change rapidly, main memory is measured in
dollars/megabyte, solid-state disk in dollars/gigabyte, and magnetic disk and tape
storage in pennies/gigabyte.
We have already looked at registers, cache, and main memory. In the follow-
ing sections we will look at magnetic and solid-state disks; after that, we will study
optical ones. We will not study tapes because they are rarely used except for back-
up, and there is not a lot to say about them anyway.
2.3.2 Magnetic Disks
A magnetic disk consists of one or more aluminum platters with a magnetiz-
able coating. Originally these platters were as much as 50 cm in diameter, but at
present they are typically 3 to 9 cm, with disks for notebook computers already
under 3 cm and still shrinking. A disk head containing an induction coil floats just
over the surface, resting on a cushion of air. When a positive or negative current
passes through the head, it magnetizes the surface just beneath the head, aligning
the magnetic particles facing left or facing right, depending on the polarity of the
drive current. When the head passes over a magnetized area, a positive or negative
current is induced in the head, making it possible to read back the previously stor-
ed bits. Thus as the platter rotates under the head, a stream of bits can be written
and later read back. The geometry of a disk track is shown in Fig. 2-19.
The circular sequence of bits written as the disk makes a complete rotation is
called a track . Each track is divided up into some number of fixed-length sectors ,
typically containing 512 data bytes, preceded by a preamble that allows the head
to be synchronized before reading or writing. Following the data is an Error-Cor-
recting Code (ECC), either a Hamming code or, more commonly, a code that can
correct multiple errors called a Reed-Solomon code . Between consecutive sectors
is a small intersector gap . Some manufacturers quote their disks' capacities in
unformatted state (as if each track contained only data), but a more honest meas-
urement is the formatted capacity, which does not count the preambles, ECCs, and
gaps as data. The formatted capacity is typically about 15 percent lower than the
unformatted capacity.