Data Format and Conversion Information for Heritage Data at the
National Space Science Data Center
Space Flight Center
Greenbelt, MD 20771, USA
NSSDC holds many hundreds of digital data sets from
the early years of the space program.
Some of these
data sets have their numeric data coded in vendor-dependent
representations and have insufficient future-use potential to justify
an expensive transformation of their data to more modern word representation
standards (or even to ASCII). However, they have sufficient future-use
potential to warrant a documentation of these vendor-dependent representations
so that a sufficiently motivated future researcher will be able
to transform the data to a form usable in his/her day. Others
may find these reference pages of use, also.
Numeric data may have character-oriented or binary representations. In
the former, the number "27" would be represented by
two pairs of (typically) 6 or 8 bits, one set representing
the character "2" and the other set the character "7". One
example of this kind of representation is known as "ASCII" characters.
ASCII is the abbreviation for the "American Standard Code for Information
Interchange". Specific information about character representations
is documented here for various formats (see
Alphanumeric Characters , below).
Binary Data uses sets of N bits (N is typically 16, 18, 32, 36, 48, or
60 bits - or more!) to represent numbers of a wide range
of sizes. Binary representations may be pure
point (also called "real") numbers.
NOTE: Some of the tables in the links below are wide; if you print
pages with these tables, you might have to use landscape
mode to prevent table truncation.
Unisys Fieldata Characters
Floating point numbers are numbers which do have
decimal points and digits (zero or non-zero) to the right of
the decimal point. These numbers are frequently, but
not always, approximations (albeit highly precise approximations)
as represented internally in the computer: -3.333, 0.0, 1.0, 3.14,
5E+308 (that's 5 followed by 308 zeroes).
A floating point number, N, is represented
in the computer by
N = (sign) × (fraction) × (
Sign is + or -; fraction is usually 0<= fraction
<1; base is usually 2; and exponent and
bias are integers.
The exponent has a bias (sometimes called
excess) of varying values. If the bias is 64,
the value of 0 to 127 in a seven-bit exponent field is interpreted
as -64 to +63.
The fraction is sometimes referred to
as mantissa, magnitude, or significand
; the exponent is sometimes referred to as the
The fraction is usually, but not always,
normalized such that the leading bit is a
one. Since it is always a one in such systems, it is usually
dropped. Hence, there are 33 bits of information in such a
32-bit word! This is called "hidden point normalization".
Bit representations for fractions represent powers of 1/2. Thus,
a fraction with bits of 1011 is:
+ [0*(0.5)**2] + [1*(0.5)**3]
+ [ 1*(0.5)**4] = 11/16, or 0.6875.
Numbers Stored in Specific Manufacturer Formats
Note: The most significant bit as shown in the tables referred
to in the links below is the leftmost bit. These tables sometimes
refer to that bit as bit "1", sometimes as another number.
We have tried to be consistent with the original manufacturer's
Data General Eclipse
PDP-10 Note: The PDP-10 is the same computer
as the DEC-10.
Unisys (and Univac) 36-bit
Xerox/Scientific Data Systems Sigma 9
Xerox/Scientific Data Systems
A comment on how the data may be "packed".
Some of these data were originally written on computers with 6-bit
bytes (and 36 bit "words"); most others were written using 8-bit bytes (and
32-bit "words"). Additionally, the data may have been written onto
7- or 9-track tapes. One track on each tape is a parity track; the
remaining 6 or 8 tracks stored the data, which matches 6- or 8-bit bytes.
Occasionally, 6-bit data may have been written onto 9-track tapes,
or even 8-bit data onto 7-track tapes. If these data have been subsequently
written onto other media (such as DLT's or even CD's), whatever format the
data were written in may have been retained.
The 6-bit data written on 9-track tapes may have been written with two
extra bits of zero (called "fill") so that one 6-bit byte resides in one
8-bit byte of data (called a "frame" on the tape). On the other hand,
it may have been writen such that the 8-bit byte contains 6 bits of one data
byte, and two bits of another; with the next frame having 4 bits of the next
byte, and 4 bits of another byte. And so on.
Likewise, any 8-bit data written onto 7-track tapes would use more than
one frame to store each byte. These data were almost always written
without usuing any zero fill. Thus, for a string of 8-bit bytes, the
first frame would have the first six bits of data; the next frame, the following
two bits from the first byte, and four bits of the next byte. And so
Nathan L. James, email@example.com, +1-301-286-9789
Code 690.1, NASA Goddard Space Flight
Greenbelt, MD 20771, USA
Original Author: George Fleming
NASA Goddard Space
Greenbelt, MD 20771, USA
NASA Official: Dr. Edwin Grayzeck, Head, NSSDC
Version 1.0x December 17, 2002