When we store information in some kind of
circuit or device, we not only need some way to store and retrieve it, but
also to locate precisely where in the device that it is. Most, if not
all, memory devices can be thought of as a series of mail boxes, folders in
a file cabinet, or some other metaphor where information can be located in a
variety of places. When we refer to the actual information being stored in
the memory device, we usually refer to it as the data. The location
of this data within the storage device is typically called the address,
in a manner reminiscent of the postal service.
With some types of memory devices, the address in which certain data is
stored can be called up by means of parallel data lines in a digital circuit
(we'll discuss this in more detail later in this lesson). With other types
of devices, data is addressed in terms of an actual physical location on the
surface of some type of media (the tracks and sectors of
circular computer disks, for instance). However, some memory devices such as
magnetic tapes have a one-dimensional type of data addressing: if you want
to play your favorite song in the middle of a cassette tape album, you have
to fast-forward to that spot in the tape, arriving at the proper spot by
means of trial-and-error, judging the approximate area by means of a counter
that keeps track of tape position, and/or by the amount of time it takes to
get there from the beginning of the tape. The access of data from a storage
device falls roughly into two categories: random access and
sequential access. Random access means that you can quickly and
precisely address a specific data location within the device, and non-random
simply means that you cannot. A vinyl record platter is an example of a
random-access device: to skip to any song, you just position the stylus arm
at whatever location on the record that you want (compact audio disks so the
same thing, only they do it automatically for you). Cassette tape, on the
other hand, is sequential. You have to wait to go past the other songs in
sequence before you can access or address the song that you want to skip to.
The process of storing a piece of data to a memory device is called
writing, and the process of retrieving data is called reading.
Memory devices allowing both reading and writing are equipped with a way to
distinguish between the two tasks, so that no mistake is made by the user
(writing new information to a device when all you wanted to do is see what
was stored there). Some devices do not allow for the writing of new data,
and are purchased "pre-written" from the manufacturer. Such is the case for
vinyl records and compact audio disks, and this is typically referred to in
the digital world as read-only memory, or ROM. Cassette audio and
video tape, on the other hand, can be re-recorded (re-written) or purchased
blank and recorded fresh by the user. This is often called read-write
memory.
Another distinction to be made for any particular memory technology is
its volatility, or data storage permanence without power. Many electronic
memory devices store binary data by means of circuits that are either
latched in a "high" or "low" state, and this latching effect holds only as
long as electric power is maintained to those circuits. Such memory would be
properly referred to as volatile. Storage media such as magnetized
disk or tape is nonvolatile, because no source of power is needed to
maintain data storage. This is often confusing for new students of computer
technology, because the volatile electronic memory typically used for the
construction of computer devices is commonly and distinctly referred to as
RAM (Random Access Memory). While RAM memory is
typically randomly-accessed, so is virtually every other kind of memory
device in the computer! What "RAM" really refers to is the
volatility of the memory, and not its mode of access. Nonvolatile memory
integrated circuits in personal computers are commonly (and properly)
referred to as ROM (Read-Only Memory), but their
data contents are accessed randomly, just like the volatile memory circuits!
Finally, there needs to be a way to denote how much data can be stored by
any particular memory device. This, fortunately for us, is very simple and
straightforward: just count up the number of bits (or bytes, 1 byte = 8
bits) of total data storage space. Due to the high capacity of modern data
storage devices, metric prefixes are generally affixed to the unit of bytes
in order to represent storage space: 1.6 Gigabytes is equal to 1.6 billion
bytes, or 12.8 billion bits, of data storage capacity. The only caveat here
is to be aware of rounded numbers. Because the storage mechanisms of many
random-access memory devices are typically arranged so that the number of
"cells" in which bits of data can be stored appears in binary progression
(powers of 2), a "one kilobyte" memory device most likely contains 1024 (2
to the power of 10) locations for data bytes rather than exactly 1000. A "64
kbyte" memory device actually holds 65,536 bytes of data (2 to the 16th
power), and should probably be called a "66 Kbyte" device to be more
precise. When we round numbers in our base-10 system, we fall out of step
with the round equivalents in the base-2 system. |
