General Purpose Registers - Spectravideo SVI 318 Service – Technical Manual

Memory Re f r e sh Reg
( 5 )
The Z-80 CPU conta i n s a me mory re f tesh counte r to ena b l e dynamic memories
to
be used w i
8-b i t reg i s t e r are a u toma t i c a l l y inc remente d after each i n s t r u c t ion f e tch.
The e i gltth b i t w ill rema i n as p rogramme d as the result of an LD R,
i n s t r uc tlon.
p o r t i on of t he addre s s b u s a l ong w i th a refresh control signal while
CPU
is dec&d i ng and e xe c u t i ng the fet c h e d instruction.
re f r e sh i s to�� l y t ran s p a r e n t t o the p ro grammer and does not slow down
CPU
th e op e r a t i o n.
purp o s e s, b u t t h i s reg i ste r i s no rmally n o t u s e d b y t h e p r o g r ammer.
Du r i ng re f re sh , the co ntent s
b i t s of t he add re s s b us .
AND
ACCUMULATOR FLAG REGISTERS
CPU
The inc l udes two independent 8-b i t ac cumu l a t o r s and ass oc i a t e d 8-bit f l ag
regis t e rs . The ac cumu l a t or ho l d s t he r e su l t s of 8-b i t ar i t hme t ic or l og i c a l
ope r a t i o ns wh i l e the f l a g reg i s t e r i n d i c a t e s s p e c i f i c cond i t i o n s for
1 6 -b i t ope r a t i ons , s uc h a s i nd i c a t ing whe ther or not t he re s u l t of an
i s e qu a l to z e ro .
w i sh e s to work w i th
w i t h e i t h e r pa i r .

GENERAL PURPOSE REGISTERS

The re a r e two ma tche d s e t s o
s i x 8 -b i t reg i s t e rs t h a t
1 6 -b it reg i s t e r pairs
wh i l e t he comp l e me
p rog ramme r can se l ec t e i t her se t
e x change command for t he ent ire s e t .
is re qu i r e d , one se t of gener a l purpose reg i s t er s and an accumul a tor / f l ag
reg i s t e r may be reserve d for hand l ing th is very fa st rou t ine .
excha nge commandss nee d b e execu t e d t o go be tween the rou t ine s .
re duces interrupt servi ce time by e l imina t ing the requirement for saving and
re tr ieving reg is ter contents in the externa l stack dur ing interrupt or
subrou t i ne proce ss ing .
range of app l icat ions by the programmer .
espec i a l ly in ROM based sy s t ems whe re l i t t l e external read/wr i te memory i s
ava i l ab l e .
R
(
).
i st e r
th the s ame e a s e as s t a t i c memori es.
The data i n the re f resh counter is sent out on the lower
The p ro grammer can l oa d the
of
The p ro g ramme r se l ects
a s ing l e exchange i n s t r uc t i on so that he may e a s i l y
f gener a l purpose reg i s t ers , each
be used indiv i dua l ly as 8-b i t reg i s t ers or a s
may
b y t h e programmer .
y s e t is ca l l e d
n t a r
o f
The se gener a l purpose reg i s t ers are used for a w ide
R
I
th e re g i s t er are plac e d on the upper 8
the accumu l a t o r and flag pa i r t hat he
One se t is ca l l e d BC, DE
BC ' , D E ' and HL' .
reg is t ers to work w i th through a sin
In sys tems where f as t int err upt response
The y a l so simp l i fy programm ing ,
4 .5
S e ven b i t s of t his
A
Th i s m o de o f
r
i
e g s t e for t e s t i n g
r
8 o r
o p e r
work
s e t con t a i n i ng
HL
and
At any one t ime the
g l
Only a s imp l e
Th i s grea t l y
the
a t ion
e