<s>
The	O
Zilog	B-Device
Z800	I-Device
was	O
a	O
16-bit	B-Device
microprocessor	B-Architecture
designed	O
by	O
Zilog	O
and	O
meant	O
to	O
be	O
released	O
in	O
1985	O
.	O
</s>
<s>
It	O
was	O
instruction	O
compatible	O
with	O
their	O
existing	O
Z80	B-General_Concept
,	O
and	O
differed	O
primarily	O
in	O
having	O
on-chip	B-General_Concept
cache	I-General_Concept
and	O
a	O
memory	B-General_Concept
management	I-General_Concept
unit	I-General_Concept
(	O
MMU	O
)	O
to	O
provide	O
a	O
16MB	O
address	O
range	O
.	O
</s>
<s>
Zilog	O
essentially	O
ignored	O
the	O
Z800	O
in	O
favor	O
of	O
their	O
32-bit	O
Z80000	B-Device
and	O
the	O
Z800	O
never	O
entered	O
mass	O
production	O
.	O
</s>
<s>
After	O
more	O
than	O
five	O
years	O
had	O
elapsed	O
since	O
it	O
was	O
originally	O
introduced	O
,	O
the	O
effort	O
was	O
redubbed	O
the	O
Z280	B-Device
in	O
1986	O
.	O
</s>
<s>
An	O
actual	O
product	O
,	O
the	O
Z280	B-Device
would	O
ship	O
in	O
1987	O
with	O
almost	O
the	O
same	O
design	O
as	O
the	O
Z800	O
,	O
but	O
this	O
time	O
implemented	O
in	O
CMOS	B-Device
.	O
</s>
<s>
The	O
Z800	O
contrasts	O
with	O
Zilog	O
's	O
first	O
16-bit	B-Device
effort	O
,	O
the	O
Zilog	B-Device
Z8000	I-Device
,	O
in	O
that	O
the	O
Z800	O
was	O
intended	O
to	O
be	O
Z80	B-General_Concept
compatible	O
,	O
while	O
the	O
Z8000	B-Device
was	O
only	O
Z80-like	O
and	O
did	O
not	O
offer	O
any	O
direct	O
compatibility	O
.	O
</s>
<s>
Zilog	O
sought	O
to	O
rectify	O
the	O
lack	O
of	O
Z80	B-General_Concept
compatibility	O
exhibited	O
by	O
the	O
Z8000	B-Device
when	O
introducing	O
the	O
Z800	O
,	O
seeking	O
to	O
offer	O
Z80	B-General_Concept
binary	O
compatibility	O
with	O
an	O
eightfold	O
performance	O
increase	O
over	O
the	O
Z80	B-General_Concept
,	O
mirroring	O
plans	O
by	O
National	O
Semiconductor	O
to	O
incorporate	O
emulation	O
of	O
the	O
Intel	B-General_Concept
8080	I-General_Concept
in	O
certain	O
products	O
in	O
its	O
own	O
32000	B-Device
series	I-Device
of	O
microprocessors	B-Architecture
.	O
</s>
<s>
There	O
was	O
no	O
expansion	O
of	O
the	O
register	B-General_Concept
set	O
but	O
the	O
registers	O
and	O
instructions	O
were	O
significantly	O
orthogonalized	O
in	O
order	O
to	O
make	O
them	O
more	O
general-purpose	O
and	O
powerful	O
.	O
</s>
<s>
Many	O
new	O
8-bit	O
and	O
16-bit	B-Device
operations	O
were	O
added	O
,	O
and	O
the	O
HL	O
,	O
IX	O
,	O
and	O
IY	O
registers	O
were	O
upgraded	O
from	O
their	O
rather	O
limited	O
possibilities	O
as	O
accumulators	O
in	O
the	O
Z80	B-General_Concept
to	O
more	O
versatile	O
accumulators	O
.	O
</s>
<s>
In	O
addition	O
to	O
the	O
register	B-General_Concept
operands	O
possible	O
in	O
the	O
Z80	B-General_Concept
,	O
they	O
could	O
be	O
used	O
with	O
immediate	O
data	O
,	O
direct	O
address	O
,	O
register	B-General_Concept
indirect	O
,	O
or	O
indexed	O
operands	O
,	O
even	O
program	O
counter-relative	O
.	O
</s>
<s>
Eight-bit	O
operations	O
had	O
even	O
more	O
possibilities	O
,	O
including	O
stack	O
pointer-relative	O
addressing	O
and	O
a	O
choice	O
of	O
8-bit	O
or	O
16-bits	B-Device
immediate	O
offsets	O
.	O
</s>
<s>
Like	O
the	O
Z80	B-General_Concept
before	O
it	O
,	O
the	O
Z800	O
retained	O
the	O
internal	O
DRAM	O
controller	O
and	O
clock	O
,	O
but	O
added	O
256	O
bytes	O
of	O
RAM	O
that	O
could	O
be	O
used	O
either	O
as	O
"	O
scratchpad	O
"	O
RAM	O
,	O
or	O
as	O
a	O
cache	B-General_Concept
.	O
</s>
<s>
When	O
used	O
in	O
cache	B-General_Concept
mode	O
the	O
programmer	O
could	O
configure	O
it	O
as	O
a	O
data	O
or	O
instruction	O
cache	B-General_Concept
,	O
or	O
both	O
,	O
and	O
the	O
internal	O
memory	O
controller	O
then	O
used	O
it	O
to	O
reduce	O
access	O
to	O
(	O
slower	O
)	O
external	O
memory	O
.	O
</s>
<s>
Another	O
change	O
was	O
the	O
addition	O
of	O
an	O
optional	O
16-bit	B-Device
data	O
bus	O
,	O
which	O
doubled	O
the	O
rate	O
at	O
which	O
it	O
could	O
access	O
memory	O
if	O
set	O
up	O
properly	O
.	O
</s>
<s>
The	O
Z800	O
was	O
,	O
in	O
most	O
ways	O
,	O
a	O
minicomputer-inspired	O
"	O
super	O
Z80	B-General_Concept
"	O
that	O
would	O
run	O
existing	O
,	O
and	O
larger	O
,	O
programs	O
at	O
considerably	O
higher	O
speeds	O
.	O
</s>
<s>
Calculation	O
of	O
exact	O
execution	O
times	O
was	O
also	O
very	O
much	O
harder	O
to	O
do	O
than	O
for	O
the	O
Z80	B-General_Concept
.	O
</s>
<s>
Moreover	O
,	O
the	O
plain	O
Z80	B-General_Concept
were	O
good	O
enough	O
for	O
most	O
applications	O
at	O
the	O
time	O
so	O
the	O
extra	O
computing	O
power	O
was	O
,	O
in	O
many	O
cases	O
,	O
not	O
worth	O
the	O
added	O
complexity	O
.	O
</s>
<s>
Hitachi	O
developed	O
the	O
HD64180	B-General_Concept
,	O
as	O
a	O
less	O
ambitious	O
Z80	B-General_Concept
derivative	O
.	O
</s>
<s>
It	O
had	O
great	O
success	O
,	O
probably	O
because	O
it	O
is	O
almost	O
as	O
simple	O
to	O
program	O
and	O
interface	O
to	O
as	O
the	O
original	O
Z80	B-General_Concept
.	O
</s>
<s>
Apart	O
from	O
the	O
successful	O
Zilog	B-Device
Z180	I-Device
(	O
developed	O
largely	O
by	O
Hitachi	O
)	O
and	O
Zilog	O
Z182	O
other	O
attempts	O
were	O
made	O
to	O
extend	O
the	O
Z80	B-General_Concept
architecture	O
,	O
the	O
32-bit	O
Z380	B-Device
(	O
introduced	O
1994	O
)	O
was	O
a	O
commercial	O
disappointment	O
except	O
for	O
some	O
specific	O
telecom	O
applications	O
.	O
</s>
<s>
On	O
the	O
other	O
hand	O
,	O
the	O
fast	O
24-bit	O
eZ80	B-Device
(	O
introduced	O
2001	O
)	O
has	O
been	O
both	O
commercially	O
successful	O
and	O
won	O
engineering	O
awards	O
.	O
</s>
<s>
Unlike	O
the	O
Z800	O
,	O
Z280	B-Device
,	O
and	O
Z380	B-Device
,	O
the	O
eZ80	B-Device
does	O
not	O
introduce	O
many	O
new	O
instructions	O
or	O
addressing	O
modes	O
,	O
in	O
comparison	O
to	O
the	O
original	O
Z80	B-General_Concept
,	O
but	O
instead	O
primarily	O
extends	O
the	O
16-bit	B-Device
registers	O
of	O
the	O
Z80	B-General_Concept
to	O
24	O
bits	O
wide	O
.	O
</s>
<s>
This	O
enables	O
it	O
to	O
reach	O
256	O
times	O
as	O
much	O
memory	O
,	O
and	O
adds	O
a	O
fully	O
pipelined	O
execution	O
unit	O
that	O
executes	O
Z80	B-General_Concept
opcodes	O
4×	O
as	O
fast	O
as	O
the	O
original	O
.	O
</s>
