Spectravideo MKI, Service – Technical Manual

Page: 38 / 134

( 5 )
Memory
Re f r e sh Reg
i
st er
(
R
).
The Z-80 CPU conta i n s a memory re ftesh counter to ena b l e dynamic memories
to be used w i th the s ame e a s e as stat ic memori es. Seven b it s of this
8-b i t regis ter are automa t ic a l ly inc remented after each ins t ruct ion fetch.
The e i gltth b i t w ill remai n as p rogramme d as the result of an LD R,
A
i n s t ruc tlon. The data in the re f resh counter is sent out on the lower
po r t i on of t he addre s s bus a l ong w i th a refresh control signal while
the
CPU
is dec&d ing and execu t i ng the fet che d instruction. Th i s mode o f
re f r e sh i s to�� ly t ran sparent t o the p rogrammer and does not slow down
th e
CPU
ope rat ion. The p rogrammer can l oad the
R
r
e g
i
s t e
r
for t e s t ing
purpos e s, b u t t h i s reg i ste r is normally not used by the programmer.
Du r i ng re f re sh , the co ntent s of th e
I
re g i s t er are placed on the upper 8
b i t s of the addre s s b us .
ACCUMULATOR
AND
FLAG REGISTERS
The
CPU
inc l udes two independent 8-b i t accumu l a tors and ass oc i a t e d 8-bit f lag
regis t e rs . The ac cumu l a t or ho l d s t he resu l ts of 8-b it ar i thme t ic or l og i c a l
ope r a t i ons wh i l e the f l ag reg i s t e r ind i c a t e s spe c i f ic condi t ions for 8 o r
1 6 -b i t ope r a t i ons , s uch a s ind i c a t ing whe ther or not t he re s u l t of an o p e r a t ion
i s e qu a l to zero . The p rogramme r se l ects the accumu lator and flag pa i r that he
w i sh e s to work wi th a s ing l e exchange i n s t r uc t i on so that he may e a s i ly work
w i t h e i t h e r pai r .
GENERAL PURPOSE REGISTERS
The re are two ma tche d s e t s o f general purpose reg isters , each s e t con t a i n i ng
s ix 8 -b i t reg i s t e rs that
may
be used indiv idua l ly as 8-b i t reg i s t ers or a s
1 6-b it reg i s t e r
pairs
by th
e programmer .
One
se t is
ca l l e d BC, DE
and
HL
wh i l e t he comp l e me n t a r y s e t is ca l led BC ' , DE ' and HL' . At any one t ime the
p rog ramme r can se l ec t e i t her se t of reg is ters to work w i th through a sin g l e
exchange command for t he ent ire set . In sys tems where f as t int err upt response
is re qu i re d , one se t of gener a l purpose register s and an accumula 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 . Only a s imp l e
excha nge commandss nee d b e executed t o go be tween the rou t ine s . Th i s grea t l y
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. The se genera l purpose regi s t ers are used for a w ide
range of app l icat ions by the programmer . The y a l so simp l i fy programm ing ,
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 .
4 .5