SGS ATES Z80, Instruction Set

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The m i n u e nd si g n has c han ge d fr o m a p o sitive t o a
nega t ive, gi vin g a n in co rr e ct diff e r e nce, Ov e rfl o w is
t herefo r e se t ,
A n o t her me t ho d fo r pr e dicting a n o v e rfl o w is t o o bs e r ve
t he car r y i n t o and o ut o f th e sign bit, If th e r e is a
ca r ry in a n d n o carry out, o r if th e r e is no carry i n
and a carry o ut, t hen o v e rflow has o c c urr e d,
This flag is a l so us e d wit h l ogi cal ope r a ti o ns a nd
ro t a t e i n str u c ti o n s t o indi ca t e th e p a rit y o f th e
re s u lt , The nu m b e r of '1 ' bit s in a b y t e are co unt e d ,
I f t he t ota l is o dd, 'ODD ' parit y (P=O) is fla g ged, I f
the tota l is e v e n , 'EVEN' parit y is fla g ged (P=1) ,
During search i n stru c ti o n s ( C PI ,C PI R,C PD, CP DR ) a nd b l ock
transfe r ins tru c t ion s ( LDI , LDI R , LDD , LDDR) th e P/V fl ag
monitors the s t a t e of th e b y t e co unt r eg i s t e r (B C) ,
When decreme n t in g, t he b y t e c ou nt e r r e sults i n a zero
value, t he f l ag i s r e s e t t o 0, o th e rwis e th e fla g i s a
L ogic 1,
During L D A,I a nd LD A , R instructi o ns, th e P/V fla g will
be set with t he c o nt e nts o f t he int e rrupt e nabl e
flip - flop (IFF2) fo r st o r age o r t e stin g ,
W h en inpu tti ng a b y t e fr o m a n I / O d e vic e , IN r,(C) , t he
f l ag wi l l b e a d j ust e d to indi ca t e t he parit y o f t he
da t a,
T h e Hal f C a rr y Flag ( H ) will b e set o r r e s e t d e p e ndin g
o n t he ca rr y a nd b o rr o w st a tus b e tw ee n bits 3 and 4 of
an 8 -b i t ari t hme t ic o per a ti o n, This flag is u s e d b y the
dec i ma l adju st a ccu m ulat o r instructi o n (D A A) t o co rr ect
the result o f a p acke d B C D a dd o r subtract o perati o n,
H A DD
SUBTRACT
1 T he r e i s a c arry fr om
There is a
B it 3
t o Bit
4
borrow fr o m
bit 4
° Ther e is n o carry Ther e is n o
fr o m Bit 3 t o Bit 4 b o rrow fr o m
Bit 4