Spectravideo MKI, Service – Technical Manual

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INTERRUPT
MEMORY
VECTOR REFRESH
I
R
INDEX REGISTER IX
INDEX REGISTER IY
STACK POINTER S P
P ROGRANHE COUNTER PC
Z-80
CPU REGI STER CONFIGURATION
F ig.
4-3
( 3 ) Two Ind e x R eg i s t e rs ( IX and I Y ) .
The two independent index reg i s t e r s ho l d a 1 6-b i t ba s e addre s s t ha t is
u se d in indexe d a dd r e s s ing modes . In t h i s mod e , an index r e g i s t e r i s
us e d a s a ba s e t o po int to a re g i on i n memory f rom wh ich da ta i s to be
s t o re d or r e t r i e ve d.
An a dd i t i ona l byte i s i nc l uded in i ndex e d
in s t r uc t i on s t o spec i f y a d i sp l aceme nt i s spec i f i e d as a two ' s comp le­
me n t s i g ne d in t e ge r. Th i s mod e of a dd re s s ing grea t ly s imp l i f ie s many
t yp e s of prog ramme s , e spec i a l l y whe r e tab l e s of data are use d .
( 4)
I n t e rrupt Page Add r e s s Reg i s t e r ( I ) .
The Z-80 CPU can be ope r a t e d in a mode whe r e an ind i re c t ca l l to any
memory l oc a t i on c an be ach i eve d in r e s ponse to an i n t e r r up t . The I
Reg is ter i s use d for th i s pu rpose to s to re the h igh order 8-b i t s of
t he ind i rect a dd re ss wh i l e t he i n t e r r up t ing dev i ce p rovides the l ower
8-b i t s of the a dd re s s . This fea t u re a l l ows int e r rupt rou t ines to be
dyn am i c a l ly l oc a te d anywhe re in memory w i th a b s o l u te m inima l access
t ime to the rou t i ne .
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