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CPU RESGISTERS
The Z-80 CPU �on t a i n s 208 b i t s o f -R /W memory tha t are acc e s sab l e to the
p rog ramme r . F igu re
4-3
i l l us t ra tes how th i s memory is con f i gured int o
e i gh t een 8-b i t re g t e r s an d fou r 1 6-b i t reg i s t e r s . A l l Z-80 reg i s t e r s are
impl emen t e d us ing s t a t i c
RAM.
The reg i s t e r s inc l ude two se t s of s i x
gene r a l purpose reg i s t e r s that ma y b e us e d i n d i v idua l l y as 8-b i t re g i s t e r s
o r in pa i r s as 1 6-bit reg i s t er s . The r e a r e a l s o two se t s of accumu l a t o r
a n d f la g reg i s t e rs .
SPECIAL PURPOS E REGISTERS
( 1 ) P rogramme Coun t e r ( PC ) .
The p rogramme cou n t e r ho l d s the 1 6-b i t a dd re s s of the current
i n s truc t i on be ing f e t che d f rom memory . The PC is au toma t ica l ly
increme n t e d a f te r i t s con t e n t s have been trans f e r red to the add r e s s
l i nes .
Wh en a programme j ump occu rs t he new va l ue i s a u toma t i c a l ly
p l a c e d in the PC , ove r r i d i ng
the incrementer.
( 2 ) S t ack Point e r ( SP ) .
The s t ack po i n t e r ho lds the 1 6-b i t a dd re s s of the current top o f a
s t ack l oca ted anywhe re in ex t e rnal sy s t em RAM memory . The ex t e r n a l
s t ack memo r y i s or igan i ze d as a la s t- in f i r s t -ou t ( L I FO ) f i l e . D a t a
c an b e pushe d o n t o t he s t ack f rom spec i f ic C P U reg i s t ers or popped o f f
o f t h e s t ack in t o spec i f i c CPU reg i s t e r s th rough the e
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ecu t i on o f PUSH
and POP i n s t r u c t ions . The d a ta popped f rom the s t ack i s a lway s the
l a s t data pushed on to i t .
The stack a l l ows s imp l e imp l eme n t a t ion of
mu l t i p l e l e ve l i n t e rrup t s , un l im i te d subrou t i ne ne s t ing and
s imp l i f i cat i on of many type s of data man ipu l a t i on .
MA IN REG SET
ACCUMULATOR FLAGS
F
B c
D E
H L
4 . 3
ALTERNATE REG SET
ACCUMULATOR FLAGS
A ' F'
B'
C '
D'
E '
H ' L '
GENERAL
P URPOSE
REG ISTER