Control Data Cyber 170 State Hardware Reference Manual page 103

CP Floating-point Arithmetic Instructions
Round f l o a t i n g d i f f e r e n c e of
(xj) and (Xi) t o X i
R X i Xj
-
X k
This i n s t r u c t i o n r e a d s 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 floating-point d i f f e r e n c e , 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
X j and Xk.
These operands
a r e i n floating-point format and a r e not n e c e s s a r i l y normalized.
The r e s u l t of
s u b t r a c t i n g t h e q u a n t i t y i n Xk from t h e q u a n t i t y i n Xj i s d e l i v e r e d t o
X i
i n
floating-point format and
is
not n e c e s s a r i l y normalized.
The round floating-point d i f f e r e n c e
i s
a s i n g l e - p r e c i s i o n , f l o a t i n g - p o i n t
difference with a round b i t ( o r b i t s ) i n s e r t e d before t h e s u b t r a c t o p e r a t i o n
t a k e s place.
A
round b i t i s always i n s e r t e d i n t h e c o e f f i c i e n t with t h e l a r g e r
exponent.
I f the two exponents are e q u a l , t h e round b i t i s added t o t h e
c o e f f i c i e n t f o r Xk. The round b i t
i s
equal t o
the
complement of the s i g n b i t
and i s i n s e r t e d immediately t o t h e r i g h t of the lowest-order b i t i n t h e
c o e f f i c i e n t . This has t h e e f f e c t of i n c r e a s i n g the magnitude of t h e
c o e f f i c i e n t by one-half of t h e l e a s t - s i g n i f i c a n t b i t . A second round b i t
i s
i n s e r t e d i n a corresponding manner t o t h e o t h e r c o e f f i c i e n t i f both operands
a r e normalized o r have l i k e s i g n s . The second round b i t i s i n s e r t e d before t h e
c o e f f i c i e n t i s s h i f t e d
by
t h e d i f f e r e n c e o f . t h e exponents. I n f i n l t e
(3777xxx...x o r 4000xxx
...
x) o r i n d e f i n i t e (1777xxx...x o r
6000xxx
...
x)
operands cause corresponding e x i t conditions t o s e t i n t h e 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.