<s>
The	O
Atlas	B-Device
Computer	I-Device
was	O
one	O
of	O
the	O
world	O
's	O
first	O
supercomputers	B-Architecture
,	O
in	O
use	O
from	O
1962	O
(	O
when	O
it	O
was	O
claimed	O
to	O
be	O
the	O
most	O
powerful	O
computer	O
in	O
the	O
world	O
)	O
to	O
1972	O
.	O
</s>
<s>
Atlas	B-Device
 '	O
capacity	O
promoted	O
the	O
saying	O
that	O
when	O
it	O
went	O
offline	O
,	O
half	O
of	O
the	O
United	O
Kingdom	O
's	O
computer	O
capacity	O
was	O
lost	O
.	O
</s>
<s>
It	O
is	O
notable	O
for	O
being	O
the	O
first	O
machine	O
with	O
virtual	B-Architecture
memory	I-Architecture
(	O
at	O
that	O
time	O
referred	O
to	O
as	O
'	O
one-level	O
store	O
 '	O
)	O
using	O
paging	B-Architecture
techniques	O
;	O
this	O
approach	O
quickly	O
spread	O
,	O
and	O
is	O
now	O
ubiquitous	O
.	O
</s>
<s>
Atlas	B-Device
was	O
a	O
second-generation	O
computer	O
,	O
using	O
discrete	O
germanium	O
transistors	B-Application
.	O
</s>
<s>
Atlas	B-Device
was	O
created	O
in	O
a	O
joint	O
development	O
effort	O
among	O
the	O
University	O
of	O
Manchester	O
,	O
Ferranti	O
International	O
plc	O
and	O
the	O
Plessey	O
Co.	O
,	O
plc	O
.	O
</s>
<s>
Two	O
other	O
Atlas	B-Device
machines	O
were	O
built	O
:	O
one	O
for	O
British	O
Petroleum	O
and	O
the	O
University	O
of	O
London	O
,	O
and	O
one	O
for	O
the	O
Atlas	B-Device
Computer	I-Device
Laboratory	O
at	O
Chilton	O
near	O
Oxford	O
.	O
</s>
<s>
Called	O
the	O
Titan	B-Device
,	O
or	O
Atlas	B-Device
2	I-Device
,	O
it	O
had	O
a	O
different	O
memory	O
organisation	O
and	O
ran	O
a	O
time-sharing	B-General_Concept
operating	I-General_Concept
system	I-General_Concept
developed	O
by	O
Cambridge	O
University	O
Computer	O
Laboratory	O
.	O
</s>
<s>
Two	O
further	O
Atlas	B-Device
2s	O
were	O
delivered	O
:	O
one	O
to	O
the	O
CAD	B-Application
Centre	O
in	O
Cambridge	O
(	O
later	O
called	O
CADCentre	O
,	O
then	O
AVEVA	O
)	O
,	O
and	O
the	O
other	O
to	O
the	O
Atomic	O
Weapons	O
Research	O
Establishment	O
(	O
AWRE	O
)	O
,	O
Aldermaston	O
.	O
</s>
<s>
The	O
University	O
of	O
Manchester	O
's	O
Atlas	B-Device
was	O
decommissioned	O
in	O
1971	O
.	O
</s>
<s>
The	O
final	O
Atlas	B-Device
,	O
the	O
CADCentre	O
machine	O
,	O
was	O
switched	O
off	O
in	O
late	O
1976	O
.	O
</s>
<s>
Parts	O
of	O
the	O
Chilton	B-Device
Atlas	I-Device
are	O
preserved	O
by	O
National	O
Museums	O
Scotland	O
in	O
Edinburgh	O
;	O
the	O
main	O
console	O
itself	O
was	O
rediscovered	O
in	O
July	O
2014	O
and	O
is	O
at	O
Rutherford	O
Appleton	O
Laboratory	O
in	O
Chilton	O
,	O
near	O
Oxford	O
.	O
</s>
<s>
Through	O
this	O
period	O
,	O
Tom	O
Kilburn	O
's	O
team	O
at	O
Manchester	O
University	O
had	O
been	O
experimenting	O
with	O
transistor-based	O
systems	O
,	O
building	O
two	O
small	O
machines	O
to	O
test	O
various	O
techniques	O
.	O
</s>
<s>
This	O
was	O
clearly	O
the	O
way	O
forward	O
,	O
and	O
in	O
the	O
autumn	O
of	O
1956	O
,	O
Kilburn	O
began	O
canvassing	O
possible	O
customers	O
on	O
what	O
features	O
they	O
would	O
want	O
in	O
a	O
new	O
transistor-based	O
machine	O
.	O
</s>
<s>
A	O
program	B-Application
that	O
processes	O
data	O
from	O
a	O
card	B-Device
reader	I-Device
,	O
for	O
instance	O
,	O
will	O
spend	O
the	O
vast	O
majority	O
of	O
its	O
time	O
waiting	O
for	O
the	O
reader	O
to	O
send	O
in	O
the	O
next	O
bit	O
of	O
data	O
.	O
</s>
<s>
To	O
support	O
these	O
devices	O
while	O
still	O
making	O
efficient	O
use	O
of	O
the	O
central	B-General_Concept
processing	I-General_Concept
unit	I-General_Concept
(	O
CPU	O
)	O
,	O
the	O
new	O
system	O
would	O
need	O
to	O
have	O
additional	O
memory	O
to	O
buffer	O
data	O
and	O
have	O
an	O
operating	B-General_Concept
system	I-General_Concept
that	O
could	O
coordinate	O
the	O
flow	O
of	O
data	O
around	O
the	O
system	O
.	O
</s>
<s>
When	O
the	O
Brunt	O
Committee	O
heard	O
of	O
new	O
and	O
much	O
faster	O
US	O
designs	O
,	O
the	O
Univac	B-Device
LARC	I-Device
and	O
IBM	B-Device
STRETCH	I-Device
,	O
they	O
were	O
able	O
to	O
gain	O
the	O
attention	O
of	O
the	O
National	O
Research	O
Development	O
Corporation	O
(	O
NRDC	O
)	O
,	O
responsible	O
for	O
moving	O
technologies	O
from	O
war-era	O
research	O
groups	O
into	O
the	O
market	O
.	O
</s>
<s>
At	O
some	O
point	O
during	O
this	O
process	O
,	O
the	O
machine	O
was	O
renamed	O
Atlas	B-Device
.	O
</s>
<s>
The	O
detailed	O
design	O
was	O
completed	O
by	O
the	O
end	O
of	O
1959	O
,	O
and	O
the	O
construction	O
of	O
the	O
compilers	B-Language
was	O
proceeding	O
.	O
</s>
<s>
However	O
,	O
the	O
Supervisor	O
operating	B-General_Concept
system	I-General_Concept
was	O
already	O
well	O
behind	O
.	O
</s>
<s>
This	O
led	O
to	O
David	O
Howarth	O
,	O
newly	O
hired	O
at	O
Ferranti	O
,	O
expanding	O
the	O
operating	B-General_Concept
system	I-General_Concept
team	O
from	O
two	O
to	O
six	O
programmers	O
.	O
</s>
<s>
In	O
what	O
is	O
described	O
as	O
a	O
Herculean	O
effort	O
,	O
led	O
by	O
the	O
tireless	O
and	O
energetic	O
Howarth	O
,	O
the	O
team	O
eventually	O
delivered	O
a	O
Supervisor	O
consisting	O
of	O
35,000	O
lines	O
of	O
assembler	B-Language
language	I-Language
which	O
had	O
support	O
for	O
multiprogramming	O
to	O
solve	O
the	O
problem	O
of	O
peripheral	O
handling	O
.	O
</s>
<s>
The	O
first	O
Atlas	B-Device
was	O
built	O
up	O
at	O
the	O
university	O
throughout	O
1962	O
.	O
</s>
<s>
The	O
schedule	O
was	O
further	O
constrained	O
by	O
the	O
planned	O
shutdown	O
of	O
the	B-Device
Ferranti	I-Device
Mercury	I-Device
machine	O
at	O
the	O
end	O
of	O
December	O
.	O
</s>
<s>
Atlas	B-Device
met	O
this	O
goal	O
,	O
and	O
was	O
officially	O
commissioned	O
on	O
7	O
December	O
by	O
John	O
Cockcroft	O
,	O
director	O
of	O
the	O
AEA	O
.	O
</s>
<s>
This	O
system	O
had	O
only	O
an	O
early	O
version	O
of	O
Supervisor	O
,	O
and	O
the	O
only	O
compiler	B-Language
was	O
for	O
Autocode	B-Language
.	O
</s>
<s>
It	O
was	O
not	O
until	O
January	O
1964	O
that	O
the	O
final	O
version	O
of	O
Supervisor	O
was	O
installed	O
,	O
along	O
with	O
compilers	B-Language
for	O
ALGOL	B-Language
60	I-Language
and	O
Fortran	B-Application
.	O
</s>
<s>
Ferranti	O
sold	O
two	O
other	O
Atlas	B-Device
installations	O
,	O
one	O
to	O
a	O
joint	O
consortium	O
of	O
London	O
University	O
and	O
British	O
Petroleum	O
in	O
1963	O
,	O
and	O
another	O
to	O
the	O
Atomic	O
Energy	O
Research	O
Establishment	O
(	O
Harwell	O
)	O
in	O
December	O
1964	O
.	O
</s>
<s>
The	O
AEA	O
machine	O
was	O
later	O
moved	O
to	O
the	O
Atlas	B-Device
Computer	I-Device
Laboratory	O
at	O
Chilton	O
,	O
a	O
few	O
yards	O
outside	O
the	O
boundary	O
fence	O
of	O
Harwell	O
,	O
which	O
placed	O
it	O
on	O
civilian	O
lands	O
and	O
thus	O
made	O
it	O
much	O
easier	O
to	O
access	O
.	O
</s>
<s>
This	O
installation	O
grew	O
to	O
be	O
the	O
largest	O
Atlas	B-Device
,	O
containing	O
48kWords	O
of	O
48-bit	O
core	B-General_Concept
memory	I-General_Concept
and	O
32	O
tape	O
drives	O
.	O
</s>
<s>
In	O
February	O
1962	O
,	O
Ferranti	O
gave	O
some	O
parts	O
of	O
an	O
Atlas	B-Device
machine	O
to	O
Cambridge	O
University	O
,	O
and	O
in	O
return	O
,	O
the	O
University	O
would	O
use	O
these	O
to	O
develop	O
a	O
cheaper	O
version	O
of	O
the	O
system	O
.	O
</s>
<s>
The	O
result	O
was	O
the	O
Titan	B-Device
machine	O
,	O
which	O
became	O
operational	O
in	O
the	O
summer	O
of	O
1963	O
.	O
</s>
<s>
Ferranti	O
sold	O
two	O
more	O
of	O
this	O
design	O
under	O
the	O
name	O
Atlas	B-Device
2	I-Device
,	O
one	O
to	O
the	O
Atomic	O
Weapons	O
Research	O
Establishment	O
(	O
Aldermaston	O
)	O
in	O
1963	O
,	O
and	O
another	O
to	O
the	O
government-sponsored	O
Computer	B-Application
Aided	I-Application
Design	I-Application
Center	O
in	O
1966	O
.	O
</s>
<s>
Atlas	B-Device
had	O
been	O
designed	O
as	O
a	O
response	O
to	O
the	O
US	O
LARC	O
and	O
STRETCH	O
programs	O
.	O
</s>
<s>
Both	O
ultimately	O
beat	O
Atlas	B-Device
into	O
official	O
use	O
,	O
LARC	O
in	O
1961	O
,	O
and	O
STRETCH	O
a	O
few	O
months	O
before	O
Atlas	B-Device
.	O
</s>
<s>
Atlas	B-Device
was	O
much	O
faster	O
than	O
LARC	O
,	O
about	O
four	O
times	O
,	O
and	O
ran	O
slightly	O
slower	O
than	O
STRETCH	O
-	O
Atlas	B-Device
added	O
two	O
floating-point	B-Algorithm
numbers	I-Algorithm
in	O
about	O
1.59	O
microseconds	O
,	O
while	O
STRETCH	O
did	O
the	O
same	O
in	O
1.38	O
to	O
1.5	O
microseconds	O
.	O
</s>
<s>
It	O
was	O
not	O
until	O
1964	O
's	O
arrival	O
of	O
the	O
CDC	B-Device
6600	I-Device
that	O
the	O
Atlas	B-Device
was	O
significantly	O
bested	O
.	O
</s>
<s>
This	O
led	O
to	O
it	O
winning	O
a	O
contract	O
for	O
the	O
CSIRO	O
in	O
Australia	O
,	O
which	O
had	O
originally	O
been	O
in	O
discussions	O
to	O
buy	O
an	O
Atlas	B-Device
.	O
</s>
<s>
ICT	O
decided	O
to	O
focus	O
on	O
the	O
mid-range	O
market	O
with	O
their	O
ICT	B-Device
1900	I-Device
series	I-Device
,	O
a	O
flexible	O
range	O
of	O
machines	O
based	O
on	O
the	O
Canadian	O
Ferranti-Packard	B-Device
6000	I-Device
.	O
</s>
<s>
The	O
Atlas	B-Device
was	O
highly	O
regarded	O
by	O
many	O
in	O
the	O
computer	O
industry	O
.	O
</s>
<s>
A	O
word	O
could	O
hold	O
one	O
floating-point	B-Algorithm
number	I-Algorithm
,	O
one	O
instruction	O
,	O
two	O
24-bit	O
addresses	O
or	O
signed	O
integers	O
,	O
or	O
eight	O
6-bit	O
characters	O
.	O
</s>
<s>
8K	O
words	O
of	O
read-only	B-Device
memory	I-Device
(	O
referred	O
to	O
as	O
the	O
fixed	O
store	O
)	O
.	O
</s>
<s>
This	O
contained	O
the	O
supervisor	O
and	O
extracode	B-Device
routines	O
.	O
</s>
<s>
96K	O
words	O
of	O
drum	B-General_Concept
store	I-General_Concept
(	O
eqv	O
.	O
</s>
<s>
to	O
576	O
KB	O
)	O
,	O
split	O
across	O
four	O
drums	O
but	O
integrated	O
with	O
the	O
core	O
store	O
using	O
virtual	B-Architecture
memory	I-Architecture
.	O
</s>
<s>
128	O
high-speed	O
index	B-General_Concept
registers	I-General_Concept
(	O
B-lines	O
)	O
that	O
could	O
be	O
used	O
for	O
address	B-General_Concept
modification	O
in	O
the	O
mostly	O
double-modified	O
instructions	O
.	O
</s>
<s>
The	O
register	B-General_Concept
address	B-General_Concept
space	I-General_Concept
also	O
included	O
special	O
registers	O
such	O
as	O
the	O
extracode	B-Device
operand	O
address	B-General_Concept
and	O
the	O
exponent	O
of	O
the	O
floating-point	B-Algorithm
accumulator	B-General_Concept
.	O
</s>
<s>
Three	O
of	O
the	O
128	O
registers	O
were	O
program	B-General_Concept
counter	I-General_Concept
registers	O
:	O
125	O
was	O
supervisor	O
(	O
interrupt	B-Application
)	O
control	O
,	O
126	O
was	O
extracode	B-Device
control	O
,	O
and	O
127	O
was	O
user	O
control	O
.	O
</s>
<s>
Register	B-General_Concept
0	O
always	O
held	O
value	O
0	O
.	O
</s>
<s>
Peripheral	O
control	O
through	O
V-store	O
addresses	O
(	O
memory-mapped	B-Architecture
I/O	I-Architecture
)	O
,	O
interrupts	B-Application
and	O
extracode	B-Device
routines	O
,	O
by	O
reading	O
and	O
writing	O
special	O
wired-in	O
store	O
addresses	O
.	O
</s>
<s>
Atlas	B-Device
did	O
not	O
use	O
a	O
synchronous	O
clocking	O
mechanism	O
—	O
it	O
was	O
an	O
asynchronous	O
processor	O
—	O
so	O
performance	O
measurements	O
were	O
not	O
easy	O
,	O
but	O
as	O
an	O
example	O
:	O
</s>
<s>
One	O
feature	O
of	O
the	O
Atlas	B-Device
was	O
"	O
Extracode	B-Device
"	O
,	O
a	O
technique	O
that	O
allowed	O
complex	O
instructions	O
to	O
be	O
implemented	O
in	O
software	O
.	O
</s>
<s>
Dedicated	O
hardware	B-Architecture
expedited	O
entry	O
to	O
and	O
return	O
from	O
the	O
extracode	B-Device
routine	O
and	O
operand	O
access	O
;	O
also	O
,	O
the	O
code	O
of	O
the	O
extracode	B-Device
routines	O
was	O
stored	O
in	O
ROM	B-Device
,	O
which	O
could	O
be	O
accessed	O
faster	O
than	O
the	O
core	O
store	O
.	O
</s>
<s>
The	O
uppermost	O
ten	O
bits	O
of	O
a	O
48-bit	O
Atlas	B-Device
machine	B-Language
instruction	I-Language
were	O
the	O
operation	B-Language
code	I-Language
.	O
</s>
<s>
If	O
the	O
most	O
significant	O
bit	O
was	O
set	O
to	O
zero	O
,	O
this	O
was	O
an	O
ordinary	O
machine	B-Language
instruction	I-Language
executed	O
directly	O
by	O
the	O
hardware	B-Architecture
.	O
</s>
<s>
If	O
the	O
uppermost	O
bit	O
was	O
set	O
to	O
one	O
,	O
this	O
was	O
an	O
Extracode	B-Device
and	O
was	O
implemented	O
as	O
a	O
special	O
kind	O
of	O
subroutine	O
jump	O
to	O
a	O
location	O
in	O
the	O
fixed	O
store	O
(	O
ROM	B-Device
)	O
,	O
its	O
address	B-General_Concept
being	O
determined	O
by	O
the	O
other	O
nine	O
bits	O
.	O
</s>
<s>
Extracodes	B-Device
were	O
what	O
would	O
be	O
called	O
software	B-Application
interrupts	I-Application
or	O
traps	B-Application
today	O
.	O
</s>
<s>
They	O
were	O
used	O
to	O
call	O
mathematical	O
procedures	O
which	O
would	O
have	O
been	O
too	O
inefficient	O
to	O
implement	O
in	O
hardware	B-Architecture
,	O
for	O
example	O
sine	O
,	O
logarithm	O
,	O
and	O
square	O
root	O
.	O
</s>
<s>
But	O
about	O
half	O
of	O
the	O
codes	O
were	O
designated	O
as	O
Supervisor	O
functions	O
,	O
which	O
invoked	O
operating	B-General_Concept
system	I-General_Concept
procedures	O
.	O
</s>
<s>
Extracodes	B-Device
were	O
the	O
only	O
means	O
by	O
which	O
a	O
program	B-Application
could	O
communicate	O
with	O
the	O
Supervisor	O
.	O
</s>
<s>
Other	O
UK	O
machines	O
of	O
the	O
era	O
,	O
such	O
as	O
the	O
Ferranti	B-Device
Orion	I-Device
,	O
had	O
similar	O
mechanisms	O
for	O
calling	O
on	O
the	O
services	O
of	O
their	O
operating	B-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
Atlas	B-Device
pioneered	O
many	O
software	O
concepts	O
still	O
in	O
common	O
use	O
today	O
,	O
including	O
the	O
Atlas	B-Application
Supervisor	I-Application
,	O
"	O
considered	O
by	O
many	O
to	O
be	O
the	O
first	O
recognisable	O
modern	O
operating	B-General_Concept
system	I-General_Concept
"	O
.	O
</s>
<s>
One	O
of	O
the	O
first	O
high-level	B-Language
languages	I-Language
available	O
on	O
Atlas	B-Device
was	O
named	O
Atlas	B-Device
Autocode	B-Language
,	O
which	O
was	O
contemporary	O
to	O
Algol60	B-Language
and	O
created	O
specifically	O
to	O
address	B-General_Concept
what	O
Tony	O
Brooker	O
perceived	O
to	O
be	O
some	O
defects	O
in	O
Algol60	B-Language
.	O
</s>
<s>
The	O
Atlas	B-Device
did	O
however	O
support	O
Algol	B-Language
60	I-Language
,	O
as	O
well	O
as	O
Fortran	B-Application
and	O
COBOL	B-Application
,	O
and	O
ABL	O
(	O
Atlas	B-Device
Basic	O
Language	O
,	O
a	O
symbolic	O
input	O
language	O
close	O
to	O
machine	B-Language
language	I-Language
)	O
.	O
</s>
<s>
Being	O
a	O
university	O
computer	O
it	O
was	O
patronised	O
by	O
a	O
large	O
number	O
of	O
the	O
student	O
population	O
,	O
who	O
had	O
access	O
to	O
a	O
protected	O
machine	B-Language
code	I-Language
development	O
environment	O
.	O
</s>
<s>
Several	O
of	O
the	O
compilers	B-Language
were	O
written	O
using	O
the	O
Compiler	B-Language
Compiler	I-Language
,	O
considered	O
to	O
be	O
the	O
first	O
of	O
its	O
type	O
.	O
</s>
<s>
It	O
also	O
had	O
a	O
programming	O
language	O
called	O
SPG	O
(	O
System	O
Program	B-Application
Generator	O
)	O
.	O
</s>
<s>
At	O
run	O
time	O
an	O
SPG	O
program	B-Application
could	O
compile	B-Language
more	O
program	B-Application
for	O
itself	O
.	O
</s>
<s>
Its	O
variables	O
were	O
in	O
<angle brackets>	O
and	O
it	O
had	O
a	O
text	O
parser	O
,	O
giving	O
SPG	O
program	B-Application
text	O
a	O
resemblance	O
to	O
Backus	O
–	O
Naur	O
form	O
.	O
</s>
<s>
From	O
the	O
outset	O
,	O
Atlas	B-Device
was	O
conceived	O
as	O
a	O
supercomputer	B-Architecture
that	O
would	O
include	O
a	O
comprehensive	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
The	O
hardware	B-Architecture
included	O
specific	O
features	O
that	O
facilitated	O
the	O
work	O
of	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
For	O
example	O
,	O
the	O
extracode	B-Device
routines	O
and	O
the	O
interrupt	B-Application
routines	O
each	O
had	O
dedicated	O
storage	O
,	O
registers	O
and	O
program	B-General_Concept
counters	I-General_Concept
;	O
a	O
context	B-Operating_System
switch	I-Operating_System
from	O
user	O
mode	O
to	O
extracode	B-Device
mode	O
or	O
executive	O
mode	O
,	O
or	O
from	O
extracode	B-Device
mode	O
to	O
executive	O
mode	O
,	O
was	O
therefore	O
very	O
fast	O
.	O
</s>
