Cray CRAY-1 - Floating Point Arithmetic; Floating Point Data Format

FLOATING POINT ARITHMETIC
Floating point numbers are represented in a standard format throughout
the
CPU. This format is a packed representation of a binary coefficient
and an exponent or power of two. The coefficient is a 48-bit signed
fraction. The sign of the coefficient is separated from the rest of
the
coefficient as shown in figure 3-3. Since the coefficient is signed
magnitude,
it is not complemented for negative values.
o
I I
BINARY POINT
•
I~ 16
I
SIGN EXPONENT
COEF FIC
lENT
Figure 3-3. Floating point data format
63
I
The exponent portion of the floating point format is represented as a
biased integer in
bits 1 through 15. The bias that is added to the
exponents
is 400008. The positive range of exponents is 40000
8
through
5777780 The negative range of exponents is 377778 through 20000
8
,
Thus,
the unbiased range of exponents
is the following:
2-200008 through 2+177778
In terms of decimal values, the floating point format of the CRAY-l allows
the expression of
numbers accurate to about 15 decimal digits in the
I approximate decimal range of 10-
2466
through 10+
2466
•
A zero value or an underflow result is not biased and is represented as a
word of all zeros.
A negative zero is not generated by any functional unit.
2240004
3-21
E