<s>
The	O
ICL	B-Device
2900	I-Device
Series	I-Device
was	O
a	O
range	O
of	O
mainframe	B-Architecture
computer	I-Architecture
systems	O
announced	O
by	O
the	O
British	O
manufacturer	O
ICL	O
on	O
9	O
October	O
1974	O
.	O
</s>
<s>
In	O
marketing	O
terms	O
,	O
the	O
2900	O
Series	O
was	O
superseded	O
by	O
Series	B-Device
39	I-Device
in	O
the	O
mid-1980s	O
;	O
however	O
,	O
Series	B-Device
39	I-Device
was	O
essentially	O
a	O
new	O
set	O
of	O
machines	O
implementing	O
the	O
2900	O
Series	O
architecture	O
,	O
as	O
were	O
subsequent	O
ICL	O
machines	O
branded	O
"	O
Trimetra	O
"	O
.	O
</s>
<s>
These	O
included	O
enhancements	O
to	O
either	O
ICT	O
's	O
1900	B-Device
Series	I-Device
or	O
the	O
English	B-Device
Electric	I-Device
System	I-Device
4	I-Device
,	O
</s>
<s>
The	O
design	O
of	O
Burroughs	O
mainframes	B-Architecture
was	O
influential	O
,	O
although	O
ICL	O
rejected	O
the	O
concept	O
of	O
optimising	O
the	O
design	O
for	O
one	O
high-level	O
language	O
.	O
</s>
<s>
The	O
Multics	B-Application
system	O
provided	O
other	O
ideas	O
,	O
notably	O
in	O
the	O
area	O
of	O
protection	O
.	O
</s>
<s>
The	O
2900	O
Series	O
architecture	O
uses	O
the	O
concept	O
of	O
a	O
virtual	B-Architecture
machine	I-Architecture
as	O
the	O
set	O
of	O
resources	O
available	O
to	O
a	O
program	O
.	O
</s>
<s>
The	O
concept	O
of	O
a	O
virtual	B-Architecture
machine	I-Architecture
in	O
the	O
2900	O
Series	O
architecture	O
differs	O
from	O
the	B-Architecture
term	I-Architecture
as	I-Architecture
used	I-Architecture
in	I-Architecture
other	I-Architecture
environments	I-Architecture
.	O
</s>
<s>
Because	O
each	O
program	O
runs	O
in	O
its	O
own	O
virtual	B-Architecture
machine	I-Architecture
,	O
the	O
concept	O
may	O
be	O
likened	O
to	O
a	O
process	B-Operating_System
in	O
other	O
operating	O
systems	O
,	O
while	O
the	O
2900	O
Series	O
process	B-Operating_System
is	O
more	O
like	O
a	O
thread	B-Operating_System
.	O
</s>
<s>
The	O
most	O
obvious	O
resource	O
in	O
a	O
virtual	B-Architecture
machine	I-Architecture
is	O
the	O
virtual	O
store	O
(	O
memory	O
)	O
.	O
</s>
<s>
In	O
a	O
virtual	B-Architecture
machine	I-Architecture
,	O
code	O
can	O
run	O
at	O
up	O
to	O
sixteen	O
different	O
layers	O
of	O
protection	O
,	O
called	O
access	O
levels	O
(	O
or	O
ACR	O
levels	O
,	O
after	O
the	O
Access	O
Control	O
Register	O
which	O
controls	O
the	O
mechanism	O
)	O
.	O
</s>
<s>
The	O
most	O
privileged	O
levels	O
of	O
operating	O
system	O
code	O
(	O
the	O
kernel	O
)	O
operate	O
in	O
the	O
same	O
virtual	B-Architecture
machine	I-Architecture
as	O
the	O
user	O
application	O
,	O
as	O
do	O
intermediate	O
levels	O
such	O
as	O
the	O
subsystems	O
to	O
implement	O
filestore	O
access	O
and	O
networking	O
.	O
</s>
<s>
System	O
calls	O
thus	O
involve	O
a	O
change	O
of	O
protection	O
level	O
,	O
but	O
not	O
an	O
expensive	O
call	O
to	O
invoke	O
code	O
in	O
a	O
different	O
virtual	B-Architecture
machine	I-Architecture
.	O
</s>
<s>
Segments	O
of	O
memory	O
can	O
be	O
shared	O
between	O
virtual	B-Architecture
machines	I-Architecture
.	O
</s>
<s>
There	O
are	O
two	O
kinds	O
of	O
shared	O
memory	O
:	O
public	O
segments	O
used	O
by	O
the	O
operating	O
system	O
(	O
which	O
are	O
present	O
in	O
all	O
virtual	B-Architecture
machines	I-Architecture
)	O
,	O
and	O
global	O
segments	O
used	O
for	O
application-level	O
shared	O
data	O
:	O
this	O
latter	O
mechanism	O
is	O
used	O
only	O
when	O
there	O
is	O
an	O
application	O
requirement	O
for	O
two	O
virtual	B-Architecture
machines	I-Architecture
to	O
communicate	O
.	O
</s>
<s>
A	O
minor	O
curiosity	O
is	O
that	O
two	O
virtual	B-Architecture
machines	I-Architecture
sharing	O
a	O
global	O
segment	O
will	O
use	O
different	O
virtual	O
addresses	O
for	O
the	O
same	O
memory	O
locations	O
,	O
which	O
means	O
that	O
virtual	O
addresses	O
cannot	O
safely	O
be	O
passed	O
from	O
one	O
VM	O
to	O
another	O
.	O
</s>
<s>
The	O
term	O
used	O
in	O
the	O
ICL	B-Device
2900	I-Device
Series	I-Device
and	O
ICL	B-Device
Series	I-Device
39	I-Device
machines	O
for	O
central	B-General_Concept
processing	I-General_Concept
unit	I-General_Concept
(	O
CPU	O
)	O
is	O
"	O
Order	O
Code	O
Processor	O
"	O
(	O
OCP	O
)	O
.	O
</s>
<s>
The	O
2900	O
architecture	O
supports	O
a	O
hardware-based	O
call	B-General_Concept
stack	I-General_Concept
,	O
providing	O
an	O
efficient	O
vehicle	O
for	O
executing	O
high-level	O
language	O
programs	O
,	O
especially	O
those	O
allowing	O
recursive	O
function	O
calls	O
.	O
</s>
<s>
This	O
was	O
a	O
forward-looking	O
decision	O
at	O
the	O
time	O
,	O
since	O
it	O
was	O
expected	O
that	O
the	O
dominant	O
programming	O
languages	O
would	O
initially	O
be	O
COBOL	B-Application
and	O
FORTRAN	B-Application
.	O
</s>
<s>
Strings	O
are	O
stored	O
as	O
arrays	O
of	O
8-bit	O
characters	O
,	O
conventionally	O
encoded	O
in	O
EBCDIC	B-Language
(	O
although	O
ICL	O
's	O
EBCDIC	B-Language
has	O
minor	O
variations	O
from	O
IBM	O
's	O
version	O
)	O
.	O
</s>
<s>
It	O
is	O
possible	O
to	O
use	O
ISO	O
(	O
essentially	O
ASCII	O
)	O
instead	O
of	O
EBCDIC	B-Language
by	O
setting	O
a	O
control	O
bit	O
in	O
a	O
privileged	O
register	O
;	O
among	O
other	O
things	O
,	O
this	O
affects	O
certain	O
decimal	O
conversion	O
instructions	O
.	O
</s>
<s>
ICL	O
marketing	O
literature	O
tended	O
to	O
use	O
the	O
concept	O
of	O
"	O
IBM	O
equivalent	O
MIPS	O
"	O
,	O
being	O
the	O
MIPS	O
rating	O
of	O
an	O
IBM	O
mainframe	B-Architecture
that	O
achieved	O
the	O
same	O
throughput	O
in	O
application	O
benchmarks	O
.	O
</s>
<s>
The	O
2980	O
was	O
initially	O
the	O
most	O
powerful	O
of	O
ICL	O
's	O
New	O
Range	O
mainframe	B-Architecture
computers	I-Architecture
.	O
</s>
<s>
It	O
supported	O
the	O
VME/B	B-General_Concept
,	I-General_Concept
VME/K	I-General_Concept
and	O
Edinburgh	B-Application
Multiple	I-Application
Access	I-Application
System	I-Application
(	O
EMAS	O
)	O
operating	O
systems	O
.	O
</s>
<s>
Unlike	O
the	O
2980	O
,	O
the	O
2970	O
and	O
the	O
subsequent	O
2960	O
were	O
microcoded	O
,	O
and	O
thus	O
allowed	O
emulation	O
of	O
instruction	O
sets	O
such	O
as	O
that	O
of	O
the	O
older	O
1900	B-Device
Series	I-Device
or	O
the	O
System	O
4	O
.	O
</s>
<s>
The	O
boot	O
process	B-Operating_System
for	O
the	O
2960	O
Series	O
merits	O
special	O
mention	O
:	O
the	O
OCP	O
contained	O
a	O
mini	O
OPER	O
terminal	O
and	O
a	O
cassette	O
deck	O
.	O
</s>
<s>
This	O
process	B-Operating_System
was	O
called	O
a	O
GROPE	O
or	O
General	O
Reconnaissance	O
Of	O
Peripheral	O
Equipment	O
.	O
</s>
<s>
The	O
first	O
machines	O
were	O
subsequently	O
replaced	O
by	O
a	O
family	O
of	O
machines	O
based	O
on	O
the	O
2966	O
mid-range	O
design	O
,	O
which	O
was	O
less	O
costly	O
to	O
build	O
and	O
used	O
serial	B-Protocol
rather	O
than	O
parallel	O
interconnections	O
.	O
</s>
<s>
The	O
2966	O
was	O
extended	O
upward	O
in	O
performance	O
to	O
the	O
2988	O
and	O
downward	O
to	O
the	O
2958	O
,	O
augmented	O
by	O
dual	B-Architecture
processor	I-Architecture
versions	O
,	O
to	O
cover	O
the	O
entire	O
performance	O
range	O
.	O
</s>
