Control Data Cyber 170 State Hardware Reference Manual page 108

CP Floating-point Arithmetic Instructions
4 5 i j k Bound f l o a t i n g d i v i d e (Xj) by
( ~ k ) t o X i
This i n s t r u c t i o n reads operands from two
X
r e g i s t e r s , o p e r a t e s on them t o form
a rounded f l o a t i n g - p o i n t q u o t i e n t , and d e l i v e r s t h i s r e s u l t t o a t h i r d X
r e g i s t e r . The operands f o r t h i s i n s t r u c t i o n a r e i n Xj and Xk. These operands
a r e i n floating-point format. The r e s u l t of d i v i d i n g t h e content of Xj by t h e
content of Xk i s delivered t o X i .
I f both operands a t e normalized, t h e
q u o t l e n t
i s
a l s o normalized. The remainder from t h e d i v i s i o n process i s
discarded
.
The two operands a r e unpacked from floating-point format i n t h i s operation.
The exponents a r e subtracted with a c o r r e c t i o n f a c t o r t o determine the exponent
f o r t h e r e s u l t . The c o e f f i c i e n t from Xj i s positioned i n a dividend r e g i s t e r .
The X j q u a n t i t y
is
modified by i n s e r t i n g a 2525
...
25 round p a t t e r n below the
lowest-order b i t of t h e dividend c o e f f i c i e n t . The c o e f f i c i e n t from Xk i s
t r i a l - s u b t r a c t e d repeatedly from the dividend.
The q u o t i e n t b i t s a r e assembled
i n a q u o t i e n t r e g i s t e r .
When
48
b i t s of t h e q u o t i e n t a r e assembled, they a r e
packed with t h e r e s u l t exponent i n t o floating-point format and d e l i v e r e d t o X i .
If the dividend i s not normalized, the q u o t i e n t cannot be normalized. However,
t h e q u o t i e n t i s c o r r e c t even though t h e r e may be l e a d i n g z e r o s i n the
c o e f f i c i e n t . I f t h e d i v i s o r i s not normalized, the q u o t i e n t may be i n c o r r e c t .
I f t h e c o e f f i c i e n t f o r the content of Xj
i s
l a r g e r than t h e c o e f f i c i e n t f o r t h e
content of Xk by a f a c t o r of two or more, a divide f a u l t occurs.
A d i v i d e
f a u l t causes an i n d e f i n i t e r e s u l t t o be returned t o X i .
This i n s t r u c t i o n i s used i n single-precision, 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 where
rounding of operands i s d e s i r e d t o reduce t r u n c a t i o n e r r o r s .
I f i n f i n i t e (3777mrx...x o r 4000xxx
...
x ) o r i n d e f i n i t e (1777xxx
...
x or
6000xxx...x) operands
are
used, corresponding e x i t c o n d i t i o n s a r e s e t i n the CP
f o r e x i t mode a c t i o n .
For f u r t h e r information, r e f e r t o Floating-point Arithmetic under CP
Programming i n chapter 5.