Information Technology Reference
In-Depth Information
data and so are important RepInfo for interpreting data files that use IEEE 754 32
bit floating point values.
The IEEE 754 standard is good RepInfo for data files that contain IEEE 754
floating point values and it should be expected that Structure RepInfo describing
data should give the type of floating point values being used, i.e. via a reference to
the IEEE 754 standard or other documentation describing the bit structure of the
values if they are not IEEE 754. Not all data uses IEEE 754 floating point values.
For example data produced from VAX systems have a very different floating point
format. A list of floating point formats and their respective structure can be found in
the CCSDS green topic [
33
], though it is not a comprehensive list.
Floating point values can also, like integer values, be restricted. They can be
specified to have maximum or minimum value (or both), and fixed values.
7.3.1.5 Markers
In some instances it may be necessary to terminate a sequence of DVs in a data file
with a marker. This allows the number DVs to be variable. The marker could be a
DV of any of the PDT that has a size greater than zero and can be made unique (a
value that other DVs are guaranteed not to take), such PDT are usually Integer, Real
Floating Point, Character, or String. An important marker is the End of File (EOF)
marker. Although there is no specific value held in data representing the EOF, the
operating system usually provides some indication to software that the EOF has
been reached. This can be used by some data reading software to find the end of
a particular structure. For example, one may need to keep reading DVs from a file
until the EOF has been reached.
7.3.1.6 Enumerations
Enumerations are essentially a Lookup Table, or Hash Table. It consists, conceptu-
ally, of two columns of values where each column has values of a single PDT type.
The first column is referred to as the “keys” while the second column is referred to as
the “values”. When a data structure in the data file is indicated to contain values that
are to be “looked up” (enumeration type) the enumeration is used to find the correct
value by reading the DV from the file and then finding the corresponding value in
the enumeration. So here the DVs in the data file are “keys” and its corresponding
values in the enumeration are the “values”.
Enumerations can be used where data has only a fixed number of values, say
ten names of people in a family (Strings). The names can then be represented as 8
bit integer values (for example 1 to 10 in decimal notation). Here the 8 bit value
would be stored in the data, and when reading the data the enumeration would be
used to “look up” the name as a string. This results in a reduction of the number of
octets used in the data as a name as a string will be composed of a number of 8 bit
characters, but the stored data is only one 8 bit integer.
