Control Data Cyber 170 State Hardware Reference Manual page 187

CP
Programming
Overflow
Overflow of t h e floating-point range
i s
i n d i c a t e d by an exponent value of
p o s i t i v e 1777 (3777 o r 4000 i n packed form). This i s t h e l a r g e s t exponent
value t h a t can be represented i n t h e f l o a t i n g - p o i n t format. This exponent
v a l u e may r e s u l t from the calculation i n which t h i s exponent value, t o g e t h e r
with t h e computed c o e f f i c i e n t v a l u e , i s a c o r r e c t r e p r e s e n t a t i o n of t h e
r e s u l t . This s i t u a t i o n
i s c a l l e d a p a r t i a l overflow.
However, f u r t h e r
computation using t h i s r e s u l t generates an overflow.
A
complete overflow occurs whenever a r e s u l t r e q u i r e s a n exponent l a r g e r t h a n
p o s i t i v e 1777. I n t h i s case, a complete overflow value r e s u l t s . This r e s u l t
has a p o s i t i v e 1777 exponent and a zero c o e f f i c i e n t . The s i g n of the
c o e f f i c i e n t i s t h e same a s t h a t which generates i f t h e resu1.t had not
overflowed t h e floating-point range.
Underflow
Underflow of t h e floating-point range i s i n d i c a t e d by an exponent value of
negative 1777 (0000 o r 7777 i n packed form).
This i s t h e s m a l l e s t exponent
v a l u e t h a t can be represented i n t h e f l o a t i n g - p o i n t format. This exponent
v a l u e may r e s u l t from t h e c a l c u l a t i o n i n which t h i s exponent v a l u e , t o g e t h e r
with t h e computed c o e f f i c i e n t v a l u e , i s a c o r r e c t r e p r e s e n t a t i o n of t h e
r e s u l t . This s i t u a t i o n
is
c a l l e d
a p a r t i a l underflow.
Further computation
using t h i s r e s u l t m a y be detected a s an underflow.
A
complete underflow occurs whenever a r e s u l t r e q u i r e s an exponent smaller than
negative 1777. I n t h i s c a s e , a complete underflow value r e s u l t s . This r e s u l t
has a negative 1777 exponent and a zero c o e f f i c i e n t .
The complete underflow
i n d i c a t o r i s a word of a l l O ' s , and
i t
i s t h e same a s
a
zero word i n i n t e g e r
format.
Indefinite
An
i n d e f i n i t e r e s u l t i n d i c a t o r g e n e r a t e s whenever t h e c a l c u l a t i o n
i s
unresolvable.
A n example i s d i v i s i o n when the d i v i s o r i s 0 and the dividend is
a l s o 0. Another example i s m u l t i p l i c a t i o n of an overflow number times an
underflow number. The i n d e f i n i t e r e s u l t i n d i c a t o r i s a value t h a t cannot occur
i n normal f l o a t i n g - p o i n t c a l c u l a t i o n s . This i n d i c a t o r corresponds t o a -0
exponent and a 0 c o e f f i c i e n t (177770,
. . .
,0 i n packed form).
Any
i n d e f i n i t e i n d i c a t o r used a s an operand generates an i n d e f i n i t e r e s u l t no
matter what the other operand v a l u e is.
Although i n d e f i n i t e i n d i c a t o r s always
generate with a p o s i t i v e s i g n , they
may
occur a s operands with a n e g a t i v e s i g n .