Control Data Cyber 170 State Hardware Reference Manual page 55

InputlOutput Unit
R Register
The 22-bit R r e g i s t e r , i n conjunction with t h e A r e g i s t e r , forms an a b s o l u t e CM
a d d r e s s f o r CM r e a d / w r i t e i n s t r u c t i o n s . When b i t 17 of t h e
A
r e g i s t e r i s s e t ,
t h e a b s o l u t e CM a d d r e s s
i s
formed by appending s i x 0 ' s t o t h e lower end of t h e
c o n t e n t s of t h e R r e g i s t e r and adding t o t h e r e s u l t b i t s 0 through
16
of t h e
c o n t e n t s of t h e A r e g i s t e r ( r e f e r t o f i g u r e 2-5).
A
Register
Figure
2-5. Formation of Absolute CM Address
The 18-bit A r e g i s t e r h o l d s one operand f o r a r i t h m e t i c , l o g i c , o r s e l e c t e d 110
o p e r a t i o n s .
The c o n t e n t of A may be a n a r i t h m e t i c o r l o g i c a l operand, CM
address o r p a r t of a CM a d d r e s s (depending on b i t 1 7 ) ,
I/o
f u n c t i o n ,
I/O
d a t a
word, o r a word count f o r block I / O i n s t r u c t i o n s .
Various i n s t r u c t i o n s o p e r a t e
on 6, 1 2 , 1 6 , o r 1 8 b i t s of t h e A r e g i s t e r .
When t h e A r e g i s t e r i s used a s t h e CM a d d r e s s , p a r i t y i s generated f o r
t r a n s m i s s i o n with t h e address t o memory c o n t r o l . At d e a d s t a r t , t h e A r e g i s t e r
i s
s e t t o
10000
( o c t a l ) .
When
b i t 17 of t h e A r e g i s t e r i s c l e a r , t h e A
r e g i s t e r i s used a s t h e
CM a d d r e s s ; however, when b i t
1 7 i s
s e t , t h e R r e g i s t e r
i s added t o t h e A r e g i s t e r ( a s d e s c r i b e d i n t h e
R r e g i s t e r d e s c r i p t i o n ) t o
o b t a i n t h e a b s o l u t e CM a d d r e s s f o r CM r e a d l w r i t e i n s t r u c t i o n s .
P Register
The 16-bit P r e g i s t e r i s t h e program a d d r e s s r e g i s t e r , except during t h e
execution
of
i n s t r u c t i o n s 61, 63,
7 1 ,
and
73.
For t h e s e i n s t r u c t i o n s , t h e P
r e g i s t e r c o n t a i n s t h e PPM a d d r e s s of t h e d a t a t r a n s f e r . A t d e a d s t a r t , t h e P
r e g i s t e r i s s e t t o
0.