Control Data Cyber 170 State Hardware Reference Manual page 31

Functional Descriptions
T h i s c h a p t e r p r o v i d e s f u n c t i o n a l d e s c r i p t i o n s of t h e c e n t r a l p r o c e s s o r (CP),
c e n t r a l memory (CM), and i n p u t l o u t p u t u n i t ( I O U )
as
shown i n t h e b l o c k diagrams
i n c h a p t e r 1. F u n c t i o n a l d e s c r i p t i o n s f o r t h e system d i s p l a y s t a t i o n a r e i n
t h e CDC 721 hardware r e f e r e n c e manual; d e s c r i p t i o n s of t h e water-cooling system
a r e i n t h e c o o l i n g system manual; b o t h manuals a r e l i s t e d i n t h e p r e f a c e .
Central Processor
The
CP
c o n s i s t s of t h e i n s t r u c t i o n s e c t i o n , r e g i s t e r s , t h e e x e c u t i o n s e c t i o n ,
cache memory, t h e a d d r e s s i n g s e c t i o n , and c e n t r a l memory c o n t r o l .
Instruction Section
The i n s t r u c t i o n s e c t i o n c o n s i s t s a f l o g i c f o r i n s t r u c t i o n c o n t r o l .
lnstruction Lookahead
The i n s t r u c t i o n lookahead hardware
(
ILH) pref e t c h e s a maximum of 12
i n s t r u c t i o n s t o make t h e n e x t i n s t r u c t i o n immediately a v a i l a b l e when t h e
execution of t h e p r e v i o u s i n s t r u c t i o n i s completed.
T h i s i s accomplished by
r e a d i n g i n s t r u c t i o n s from cache/CM i n t o a
series
o f b u f f e r r a n k s .
The ILH responds t o both n e g a t i v e and p o s i t i v e r e s o l u t i o n of a c o n d i t i o n a l
branch by purging t h e b u f f e r r a n k s and r e i n i t i a l i z i n g t h e i n s t r u c t i o n f e t c h
u n i t .
When ILH d e t e c t s a c o n d i t i o n a l branch,
i t
assumes t h a t t h e branch c o n d i t i o n
w i l l be met. ILH computes t h e branch t a r g e t a d d r e s s and r e a d s i n s t r u c t i o n s
from cache/CM s t a r t i n g a t t h e t a r g e t a d d r e s s . I f t h e branch i s t a k e n , t h e
b u f f e r ranks c o n t a i n t h e n e x t e x e c u t a b l e i n s t r u c t i o n words. I f t h e branch
i s
not t a k e n , t h e hardware purges t h e b u f f e r ranks and resumes p r e f e t c h i n g a t t h e
i n s t r u c t i o n word f o l l o w i n g t h e u n s a t i s f i e d branch i n s t r u c t i o n .
Maintenance Access Control
The maintenance a c c e s s c o n t r o l performs i n i t i a l i z a t i o n and maintenance
o p e r a t i o n s i n t h e CP.