Lammert Bies explains both their usage and the history behind.
28 – FS – File separator The file
separator FS is an interesting control
code, as it gives us insight in the
way that computer technology was
organized in the sixties. We are now
used to random access media like RAM
and magnetic disks, but when the ASCII
standard was defined, most data was
serial. I am not only talking about
serial communications, but also about
serial storage like punch cards, paper
tape and magnetic tapes. In such a
situation it is clearly efficient to
have a single control code to signal
the separation of two files. The FS
was defined for this purpose.
29 – GS – Group separator
Data storage was one
of the main reasons for some control
codes to get in the ASCII definition.
Databases are most of the time setup
with tables, containing records. All
records in one table have the same
type, but records of different tables
can be different. The group separator
GS is defined to separate tables in a
serial data storage system. Note that
the word table wasn't used at that
moment and the ASCII people called it
a group.
30 – RS – Record separator
Within a group (or table) the records
are separated with RS or record
separator.
31 – US – Unit separator
The smallest data items to be stored
in a database are called units in the
ASCII definition. We would call them
field now. The unit separator
separates these fields in a serial
data storage environment. Most current
database implementations require that
fields of most types have a fixed
length. Enough space in the record is
allocated to store the largest
possible member of each field, even if
this is not necessary in most cases.
This costs a large amount of space in
many situations. The US control code
allows all fields to have a variable
length. If data storage space is
limited—as in the sixties—this is a
good way to preserve valuable space.
On the other hand is serial storage
far less efficient than the table
driven RAM and disk implementations of
modern times. I can't imagine a
situation where modern SQL databases
are run with the data stored on paper
tape or magnetic reels...
A Unit separator could provide essentially the same purpose as a comma in a CSV file or a tab in a tab-delimited file.
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
