<s>
The	O
AT&T	B-Application
Hobbit	I-Application
is	O
a	O
microprocessor	B-Architecture
design	O
that	O
AT&T	O
Corporation	O
developed	O
in	O
the	O
early	O
1990s	O
.	O
</s>
<s>
It	O
was	O
based	O
on	O
the	O
company	O
's	O
CRISP	O
(	O
C-language	B-Application
Reduced	I-Application
Instruction	I-Application
Set	I-Application
Processor	I-Application
)	O
design	O
,	O
which	O
in	O
turn	O
grew	O
out	O
of	O
the	O
C	B-Application
Machine	I-Application
design	O
by	O
Bell	O
Labs	O
of	O
the	O
late	O
1980s	O
.	O
</s>
<s>
All	O
were	O
optimized	O
for	O
running	O
code	O
compiled	B-Language
from	O
the	O
C	B-Language
programming	I-Language
language	I-Language
.	O
</s>
<s>
Its	O
processor	O
is	O
partially	O
RISC-like	O
.	O
</s>
<s>
The	O
C	B-Application
Machine	I-Application
Project	O
at	O
Bell	O
Labs	O
had	O
been	O
underway	O
since	O
1975	O
to	O
develop	O
computer	O
architectures	O
to	O
run	O
C	B-Language
programming	I-Language
language	I-Language
programs	O
efficiently	O
,	O
aiming	O
for	O
a	O
design	O
that	O
would	O
offer	O
an	O
order	O
of	O
magnitude	O
performance	O
improvement	O
over	O
commercially	O
available	O
computers	O
while	O
remaining	O
competitive	O
in	O
terms	O
of	O
cost	O
.	O
</s>
<s>
The	O
design	O
methodology	O
for	O
the	O
C	B-Application
Machine	I-Application
architecture	O
involved	O
an	O
iterative	O
development	O
approach	O
informed	O
by	O
measurements	O
of	O
C	B-Language
program	I-Language
characteristics	O
,	O
involving	O
the	O
formulation	O
and	O
implementation	O
of	O
new	O
computer	O
architecture	O
revisions	O
,	O
the	O
development	O
of	O
a	O
compiler	B-Language
to	O
target	O
each	O
new	O
revision	O
,	O
the	O
compilation	B-Language
of	O
"	O
a	O
large	O
body	O
of	O
UNIX	O
software	O
"	O
,	O
and	O
the	O
analysis	O
of	O
the	O
compiled	B-Language
software	I-Language
.	O
</s>
<s>
Following	O
on	O
from	O
the	O
stabilisation	O
of	O
the	O
C	B-Application
Machine	I-Application
architecture	O
in	O
1981	O
for	O
an	O
uncompleted	O
ECL	B-General_Concept
implementation	O
,	O
a	O
design	O
team	O
was	O
formed	O
for	O
CRISP	O
in	O
April	O
1983	O
,	O
and	O
CRISP	O
was	O
first	O
produced	O
in	O
a	O
silicon	O
implementation	O
in	O
1986	O
.	O
</s>
<s>
This	O
was	O
competitive	O
with	O
the	O
MIPS	B-Device
R2000	I-Device
as	O
delivered	O
in	O
the	O
MIPS	O
M/500	O
Development	O
System	O
(	O
an	O
8MHz	O
device	O
delivering	O
around	O
7.4	O
VAX	O
MIPS	O
)	O
although	O
some	O
benchmarks	O
showed	O
somewhat	O
stronger	O
performance	O
by	O
the	O
CRISP	O
processor	O
.	O
</s>
<s>
Compared	O
to	O
the	O
R2000	B-Device
which	O
required	O
numerous	O
support	O
chips	O
when	O
incorporated	O
into	O
a	O
computer	O
system	O
,	O
the	O
CRISP	O
was	O
a	O
"	O
complete	O
"	O
processor	O
incorporating	O
on-chip	O
caches	O
and	O
had	O
"	O
substantially	O
"	O
reduced	O
board	O
area	O
requirements	O
.	O
</s>
<s>
Apple	O
Computer	O
approached	O
AT&T	O
and	O
paid	O
it	O
to	O
develop	O
a	O
newer	O
version	O
of	O
the	O
CRISP	O
suitable	O
for	O
low-power	O
use	O
in	O
the	O
Newton	B-Device
handheld	O
computer	O
.	O
</s>
<s>
The	O
Hobbit-based	O
Newton	B-Device
was	O
never	O
produced	O
.	O
</s>
<s>
Apple	O
dropped	O
its	O
interest	O
in	O
the	O
Hobbit	O
and	O
adopted	O
the	O
ARM610	O
for	O
the	O
Newton	B-Device
,	O
also	O
partnering	O
with	O
Acorn	O
Computers	O
and	O
VLSI	O
Technology	O
to	O
form	O
Advanced	B-Architecture
RISC	I-Architecture
Machines	I-Architecture
,	O
ARM	O
,	O
in	O
late	O
1990	O
with	O
a	O
$2.5	O
million	O
investment	O
.	O
</s>
<s>
The	O
Active	O
Book	O
Company	O
(	O
founded	O
by	O
Hermann	O
Hauser	O
,	O
who	O
also	O
founded	O
Acorn	O
Computers	O
)	O
,	O
which	O
had	O
been	O
using	O
an	O
ARM	O
in	O
its	O
Active	O
Book	O
personal	B-Application
digital	I-Application
assistant	I-Application
(	O
PDA	B-Application
)	O
,	O
was	O
later	O
purchased	O
by	O
AT&T	O
and	O
was	O
subsumed	O
by	O
T	O
's	O
Eo	B-Application
subsidiary	O
,	O
which	O
produced	O
an	O
early	O
PDA	B-Application
,	O
the	O
EO	B-Application
Personal	I-Application
Communicator	I-Application
,	O
running	O
PenPoint	B-Application
OS	I-Application
from	O
the	O
GO	O
Corporation	O
.	O
</s>
<s>
Hobbit	O
was	O
used	O
in	O
the	O
earliest	O
prototypes	O
of	O
the	O
BeBox	B-Device
until	O
in	O
1993	O
,	O
AT&T	O
announced	O
discontinuation	O
of	O
Hobbit	O
.	O
</s>
<s>
With	O
AT&T	O
closing	O
its	O
Eo	B-Application
operations	O
,	O
these	O
being	O
responsible	O
for	O
the	O
only	O
commercially	O
released	O
product	O
using	O
the	O
Hobbit	O
,	O
despite	O
early	O
announcements	O
of	O
broad	O
vendor	O
adoption	O
,	O
AT&T	O
finally	O
discontinued	O
the	O
Hobbit	O
in	O
1994	O
.	O
</s>
<s>
In	O
a	O
traditional	O
RISC	B-Architecture
design	O
implementing	O
a	O
load-store	B-Architecture
architecture	I-Architecture
,	O
memory	O
is	O
accessed	O
through	O
instructions	O
that	O
explicitly	O
load	O
data	O
into	O
registers	B-General_Concept
and	O
store	O
data	O
back	O
to	O
memory	O
,	O
with	O
instructions	O
that	O
manipulate	O
data	O
working	O
solely	O
on	O
the	O
registers	B-General_Concept
.	O
</s>
<s>
By	O
seeking	O
to	O
limit	O
the	O
data	O
processing	O
operations	O
to	O
a	O
single	O
clock	O
cycle	O
,	O
a	O
simpler	O
control	O
mechanism	O
can	O
be	O
employed	O
to	O
dispatch	O
instructions	O
,	O
making	O
it	O
easier	O
to	O
tune	O
the	O
instruction	B-General_Concept
pipelines	I-General_Concept
,	O
and	O
add	O
superscalar	B-General_Concept
support	O
.	O
</s>
<s>
Generally	O
they	O
use	O
a	O
stack	B-Application
containing	O
local	O
variables	O
and	O
other	O
information	O
for	O
subroutines	O
known	O
as	O
a	O
stack	B-Application
frame	O
or	O
activation	O
record	O
.	O
</s>
<s>
The	O
compiler	B-Language
writes	O
code	O
to	O
create	O
activation	O
records	O
using	O
the	O
underlying	O
processor	O
's	O
load-store	O
design	O
.	O
</s>
<s>
The	O
C	B-Application
Machine	I-Application
in	O
its	O
CRISP	O
implementation	O
,	O
and	O
the	O
Hobbit	O
that	O
followed	O
directly	O
,	O
both	O
aim	O
to	O
support	O
the	O
types	O
of	O
memory	O
access	O
that	O
programming	O
languages	O
use	O
,	O
with	O
the	O
C	B-Language
programming	I-Language
language	I-Language
being	O
a	O
particular	O
consideration	O
.	O
</s>
<s>
Although	O
this	O
memory-to-memory	O
model	O
was	O
typical	O
of	O
the	O
earlier	O
CISC	B-Architecture
designs	O
,	O
the	O
C	B-Application
Machine	I-Application
as	O
implemented	O
by	O
CRISP	O
differs	O
from	O
both	O
CISC	B-Architecture
and	O
RISC	B-Architecture
designs	O
,	O
including	O
the	O
earlier	O
Bellmac	B-General_Concept
32	I-General_Concept
,	O
by	O
providing	O
no	O
directly	O
accessible	O
registers	B-General_Concept
.	O
</s>
<s>
Instead	O
,	O
a	O
"	O
stack	B-Application
cache	O
"	O
of	O
32-bit	O
register	O
entries	O
is	O
provided	O
,	O
32	O
entries	O
in	O
CRISP	O
but	O
extended	O
to	O
64	O
entries	O
in	O
Hobbit	O
,	O
mapped	O
to	O
the	O
address	O
space	O
corresponding	O
to	O
the	O
top	O
of	O
the	O
program	O
stack	B-Application
,	O
these	O
being	O
purely	O
accessible	O
using	O
a	O
stack-relative	O
addressing	O
mode	O
.	O
</s>
<s>
The	O
CRISP	O
architecture	O
was	O
described	O
as	O
a	O
"	O
2½	O
address	O
memory-to-memory	O
machine	O
"	O
,	O
where	O
instructions	O
could	O
employ	O
zero	O
,	O
one	O
or	O
two	O
memory	O
addresses	O
as	O
well	O
as	O
employing	O
a	O
stack	B-Application
entry	O
called	O
the	O
accumulator	O
for	O
computation	O
results	O
.	O
</s>
<s>
Reminiscent	O
of	O
the	O
Bellmac	B-General_Concept
32	I-General_Concept
architecture	O
,	O
various	O
instructions	O
designed	O
to	O
support	O
procedure	O
calling	O
are	O
provided	O
by	O
the	O
CRISP	O
architecture	O
:	O
call	O
saves	O
the	O
return	O
address	O
and	O
branches	O
to	O
a	O
routine	O
;	O
enter	O
allocates	O
a	O
stack	B-Application
frame	O
for	O
a	O
routine	O
,	O
flushing	O
stack	B-Application
cache	O
entries	O
if	O
necessary	O
;	O
return	O
deallocates	O
the	O
stack	B-Application
frame	O
and	O
branches	O
to	O
the	O
caller	O
's	O
return	O
address	O
;	O
catch	O
restores	O
stack	B-Application
entries	O
from	O
memory	O
.	O
</s>
<s>
One	O
side	O
effect	O
of	O
the	O
Hobbit	O
design	O
is	O
that	O
it	O
inspired	O
designers	O
of	O
the	O
Dis	B-Application
virtual	B-Architecture
machine	I-Architecture
(	O
an	O
offshoot	O
of	O
Plan	B-Operating_System
9	I-Operating_System
from	I-Operating_System
Bell	I-Operating_System
Labs	I-Operating_System
)	O
to	O
use	O
a	O
memory-to-memory-based	O
system	O
that	O
more	O
closely	O
matches	O
the	O
internal	O
register-based	B-Application
workings	O
of	O
real-world	O
processors	O
.	O
</s>
<s>
They	O
found	O
,	O
as	O
RISC	B-Architecture
designers	O
would	O
have	O
expected	O
,	O
that	O
without	O
a	O
load-store	O
design	O
it	O
was	O
difficult	O
to	O
improve	O
the	O
instruction	B-General_Concept
pipeline	I-General_Concept
and	O
thereby	O
operate	O
at	O
higher	O
speeds	O
.	O
</s>
<s>
They	O
decided	O
that	O
all	O
future	O
processors	O
would	O
thus	O
move	O
to	O
a	O
load-store	O
design	O
,	O
and	O
built	O
Inferno	B-Operating_System
to	O
reflect	O
this	O
.	O
</s>
<s>
In	O
contrast	O
,	O
Java	B-Language
and	O
.NET	O
virtual	B-Architecture
machines	I-Architecture
are	O
stack-based	O
,	O
a	O
side	O
effect	O
of	O
being	O
designed	O
by	O
language	O
programmers	O
as	O
opposed	O
to	O
chip	O
designers	O
.	O
</s>
<s>
Translating	O
from	O
a	O
stack-based	O
language	O
to	O
a	O
register-based	B-Application
assembly	B-Language
language	I-Language
is	O
a	O
"	O
heavyweight	O
"	O
operation	O
;	O
Java	B-Language
's	O
virtual	B-Architecture
machine	I-Architecture
(	O
VM	O
)	O
and	O
compiler	B-Language
are	O
many	O
times	O
larger	O
and	O
slower	O
than	O
the	O
Dis	B-Application
VM	O
and	O
the	O
Limbo	B-Application
(	O
the	O
most	O
common	O
language	O
compiled	B-Language
for	O
Dis	B-Application
)	O
compiler	B-Language
.	O
</s>
<s>
The	O
VMs	O
for	O
Android	B-Application
(	O
Dalvik	B-Application
)	O
,	O
Parrot	B-Language
,	O
and	O
Lua	O
are	O
also	O
register-based	B-Application
.	O
</s>
