Information Technology Reference
In-Depth Information
whereever they may be. The location of the first sector is recorded
in the appropriate directory, as we have seen. Then, for each sector,
the computer records the location of the sector that contains the
next file data.
With this bookkeeping, retrieval of a file begins by consulting a
directory to determine the track and sector for the initial file data.
Once that material is obtained, the computer determines the loca
tion of the sector where the file data continue. Once material from
that second sector is read, the computer again identifies where it
should go next. This continues until the end of the file is found.
Although file data may be found physically anywhere on a disk, re
trieval proceeds sequentially—one sector at a time.
For a CD, storage can proceed in much the same way as a disk,
except that, for traditional CD storage, once a CD is prepared with
data, then the data are fixed. (The jargon is that a CD is burned
with its data.) As with a disk, a CD may contain multiple files, in
dexed by multiple directories or folders, and the location of the
main directory is determined by convention. However, because files
normally are just added to CDs and not deleted, CDs do not have
the problem that files are located on sectors that can be scattered
around. Instead, files can be stored in sequential portions of a CD.
On a personal level, you may have experienced the differences
in organization of files on tapes and CDs when listing to audio.
Music on audiotapes is stored sequentially. Thus, you have to
rewind and fastforward to find a song; you are never quite sure
where a song begins, and you are using sequential access to find it.
In contrast, a CD maintains a directory of songs. Thus, you can hit
rewind once, and the CD player automatically knows where the
previous song begins by consulting the directory. Similar comments
apply to playing videotapes in contrast to DVDs.
Virtual Memory and Its Relationship to Files and Main Memory:
When we discussed main memory and CPU registers in Chapter 1,
we noted that the CPU does the actual processing, so that data must
be moved to the CPU in order for work to be accomplished.
However, the CPU contains very little storage space, and main mem
ory provides significantly more storage. CPU registers are fast, but
main memory provides bulk storage for applications during process
ing. Also, to improve performance, computers utilize cache memory
between the CPU registers and main memory. Cache memory is
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