<s>
The	O
figure	O
shows	O
a	O
high-level	O
architecture	O
of	O
the	O
OS	B-Operating_System
2200	I-Operating_System
system	O
identifying	O
major	O
hardware	O
and	O
software	O
components	O
.	O
</s>
<s>
Also	O
see	O
UNIVAC	B-Device
1100/2200	I-Device
series	I-Device
.	O
</s>
<s>
It	O
was	O
defined	O
by	O
the	O
Army	O
Signal	O
Corps	O
and	O
called	O
Fieldata	B-Device
(	O
data	O
returned	O
from	O
the	O
field	O
)	O
.	O
</s>
<s>
The	O
1108	O
provided	O
a	O
9-bit	O
character	O
format	O
in	O
order	O
to	O
support	O
ASCII	B-Protocol
and	O
later	O
the	O
ISO	O
8-bit	O
sets	O
,	O
but	O
they	O
were	O
not	O
extensively	O
used	O
until	O
the	O
1980s	O
again	O
because	O
of	O
space	O
constraints	O
.	O
</s>
<s>
The	O
2200	O
Series	O
architecture	O
provides	O
many	O
registers	B-General_Concept
.	O
</s>
<s>
Base	O
registers	B-General_Concept
logically	O
contain	O
a	O
virtual	B-General_Concept
address	I-General_Concept
that	O
points	O
to	O
a	O
word	O
in	O
a	O
code	O
or	O
data	O
bank	O
(	O
segment	O
)	O
.	O
</s>
<s>
Index	B-General_Concept
registers	I-General_Concept
are	O
used	O
by	O
instructions	O
to	O
modify	O
the	O
offset	O
of	O
the	O
specified	O
or	O
assumed	O
base	O
register	O
.	O
</s>
<s>
Simple	O
arithmetic	O
(	O
add	O
,	O
subtract	O
)	O
may	O
be	O
performed	O
on	O
all	O
index	B-General_Concept
registers	I-General_Concept
.	O
</s>
<s>
In	O
addition	O
,	O
index	B-General_Concept
registers	I-General_Concept
consist	O
of	O
a	O
lower	O
offset	O
portion	O
and	O
an	O
upper	O
increment	O
portion	O
.	O
</s>
<s>
An	O
instruction	O
may	O
both	O
use	O
the	O
offset	O
value	O
in	O
an	O
index	B-General_Concept
register	I-General_Concept
as	O
part	O
of	O
an	O
address	O
and	O
specify	O
that	O
the	O
increment	O
is	O
to	O
be	O
added	O
to	O
the	O
offset	O
.	O
</s>
<s>
Arithmetic	O
registers	B-General_Concept
allow	O
the	O
full	O
set	O
of	O
computational	O
instructions	O
including	O
all	O
floating	O
point	O
operations	O
.	O
</s>
<s>
Some	O
of	O
those	O
instructions	O
work	O
on	O
adjacent	O
pairs	O
of	O
registers	B-General_Concept
to	O
perform	O
double-precision	O
operations	O
.	O
</s>
<s>
Any	O
two	O
registers	B-General_Concept
may	O
be	O
used	O
as	O
a	O
double-precision	O
value	O
.	O
</s>
<s>
Four	O
of	O
the	O
arithmetic	O
registers	B-General_Concept
are	O
also	O
index	B-General_Concept
registers	I-General_Concept
(	O
the	O
sets	O
overlap	O
–	O
index	B-General_Concept
register	I-General_Concept
X12	O
is	O
arithmetic	O
register	O
A0	O
)	O
.	O
</s>
<s>
The	O
rest	O
of	O
the	O
registers	B-General_Concept
,	O
known	O
as	O
R	O
registers	B-General_Concept
,	O
are	O
used	O
as	O
fast	O
temporary	O
storage	O
and	O
for	O
certain	O
special	O
functions	O
.	O
</s>
<s>
There	O
are	O
two	O
full	O
sets	O
of	O
registers	B-General_Concept
(	O
A	O
,	O
X	O
,	O
R	O
,	O
and	O
B	O
)	O
.	O
</s>
<s>
One	O
set	O
,	O
the	O
user	O
registers	B-General_Concept
,	O
is	O
used	O
by	O
all	O
applications	O
and	O
most	O
portions	O
of	O
the	O
operating	O
system	O
.	O
</s>
<s>
The	O
other	O
set	O
,	O
the	O
Exec	O
registers	B-General_Concept
,	O
is	O
used	O
by	O
interrupt	B-Application
processing	O
routines	O
and	O
some	O
other	O
portions	O
of	O
the	O
operating	O
system	O
that	O
want	O
to	O
avoid	O
having	O
to	O
save	O
and	O
restore	O
user	O
registers	B-General_Concept
.	O
</s>
<s>
The	O
Exec	O
registers	B-General_Concept
are	O
not	O
writable	O
by	O
user	O
applications	O
although	O
some	O
user	O
code	O
can	O
read	O
them	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
the	O
Exec	O
is	O
carefully	O
designed	O
never	O
to	O
leave	O
private	O
,	O
secure	O
,	O
or	O
confidential	O
information	O
in	O
registers	B-General_Concept
.	O
</s>
<s>
This	O
bit	O
is	O
always	O
set	O
(	O
changed	O
to	O
privileged	O
)	O
on	O
an	O
interrupt	B-Application
.	O
</s>
<s>
All	O
registers	B-General_Concept
are	O
also	O
visible	O
in	O
the	O
address	O
space	O
,	O
but	O
the	O
Exec	O
portion	O
is	O
protected	O
and	O
a	O
reference	O
by	O
non-privileged	O
code	O
will	O
result	O
in	O
a	O
fault	O
interrupt	B-Application
.	O
</s>
<s>
The	O
2200	O
Series	O
uses	O
a	O
36-bit	O
segmented	O
virtual	B-General_Concept
address	I-General_Concept
space	I-General_Concept
.	O
</s>
<s>
The	O
2200	O
Series	O
is	O
a	O
CISC	B-Architecture
architecture	I-Architecture
system	O
.	O
</s>
<s>
Many	O
instructions	O
also	O
perform	O
very	O
complex	O
functions	O
such	O
as	O
one	O
that	O
implements	O
a	O
large	O
part	O
of	O
the	O
COBOL	B-Application
EDIT	O
verb	O
.	O
</s>
<s>
As	O
was	O
mentioned	O
earlier	O
the	O
2200	O
Series	O
uses	O
a	O
36-bit	O
segmented	O
virtual	B-General_Concept
address	I-General_Concept
.	O
</s>
<s>
A	O
virtual	B-General_Concept
address	I-General_Concept
consists	O
of	O
three	O
parts	O
.	O
</s>
<s>
Basic-mode	O
activities	O
do	O
not	O
start	O
out	O
with	O
any	O
such	O
banks	O
as	O
basic-mode	O
programs	O
are	O
not	O
aware	O
of	O
the	O
virtual	B-General_Concept
address	I-General_Concept
space	I-General_Concept
,	O
but	O
any	O
calls	O
to	O
extended-mode	O
subsystems	O
will	O
cause	O
those	O
banks	O
to	O
be	O
created	O
.	O
</s>
<s>
OS	B-Operating_System
2200	I-Operating_System
does	O
not	O
implement	O
programs	O
in	O
exactly	O
the	O
same	O
way	O
that	O
UNIX	B-Application
,	O
Linux	B-Application
,	O
and	O
Windows	B-Application
implement	O
processes	O
,	O
but	O
that	O
is	O
the	O
closest	O
analogy	O
.	O
</s>
<s>
The	O
most	O
obvious	O
difference	O
is	O
that	O
OS	B-Operating_System
2200	I-Operating_System
only	O
permits	O
a	O
single	O
program	O
per	O
Run	O
(	O
Job	O
,	O
Session	O
)	O
to	O
be	O
executing	O
at	O
a	O
time	O
.	O
</s>
<s>
Calls	O
made	O
to	O
the	O
OS	O
are	O
always	O
via	O
"	O
gates	O
,	O
"	O
instructions	O
that	O
exist	O
for	O
that	O
purpose	O
(	O
ER	O
=	O
Executive	O
Request	O
)	O
,	O
or	O
via	O
interrupts	B-Application
.	O
</s>
<s>
The	O
next	O
part	O
of	O
the	O
virtual	B-General_Concept
address	I-General_Concept
is	O
the	O
BDI	O
or	O
Bank	O
Descriptor	O
Index	O
.	O
</s>
<s>
Base	O
registers	B-General_Concept
B16-B23	O
are	O
part	O
of	O
the	O
activity	O
state	O
and	O
are	O
maintained	O
by	O
the	O
Exec	O
with	O
no	O
direct	O
access	O
by	O
the	O
activity	O
.	O
</s>
<s>
Each	O
such	O
entry	O
describes	O
up	O
to	O
1MB	O
(	O
256KW	O
)	O
of	O
virtual	B-General_Concept
address	I-General_Concept
space	I-General_Concept
.	O
</s>
<s>
They	O
only	O
use	O
as	O
many	O
as	O
needed	O
to	O
provide	O
the	O
virtual	B-General_Concept
address	I-General_Concept
space	I-General_Concept
required	O
.	O
</s>
<s>
Virtual	O
addresses	O
that	O
are	O
outside	O
the	O
limits	O
generate	O
a	O
fault	O
interrupt	B-Application
.	O
</s>
<s>
The	O
Exec	O
sets	O
a	O
key	O
value	O
in	O
the	O
state	O
of	O
each	O
activity	O
which	O
may	O
be	O
changed	O
by	O
gate	O
and	O
interrupt	B-Application
transitions	O
.	O
</s>
<s>
Processor	O
Privilege	O
controls	O
the	O
ability	O
to	O
execute	O
privileged	O
instructions	O
and	O
access	O
protected	O
registers	B-General_Concept
and	O
other	O
state	O
.	O
</s>
<s>
Exec	O
activities	O
and	O
user	O
activities	O
that	O
have	O
used	O
a	O
gate	O
to	O
access	O
an	O
Exec	O
API	B-General_Concept
run	O
at	O
PP	O
=	O
0	O
.	O
</s>
<s>
PP	O
=	O
1	O
restricts	O
most	O
privileged	O
instructions	O
but	O
does	O
allow	O
reading	O
of	O
the	O
day	O
clocks	O
and	O
reading	O
the	O
contents	O
of	O
some	O
of	O
the	O
privileged	O
registers	B-General_Concept
.	O
</s>
<s>
None	O
of	O
the	O
privileged	O
registers	B-General_Concept
contain	O
any	O
truly	O
sensitive	O
information	O
,	O
but	O
allowing	O
general	O
read	O
access	O
could	O
easily	O
lead	O
to	O
undetected	O
errors	O
in	O
user	O
programs	O
.	O
</s>
<s>
OS	B-Operating_System
2200	I-Operating_System
is	O
designed	O
to	O
handle	O
up	O
to	O
32	O
instruction	O
processors	O
(	O
or	O
CPUs	B-General_Concept
)	O
.	O
</s>
<s>
For	O
example	O
,	O
OS	B-Operating_System
2200	I-Operating_System
makes	O
almost	O
no	O
use	O
of	O
critical	B-Operating_System
sections	I-Operating_System
in	O
its	O
design	O
.	O
</s>
<s>
Registers	B-General_Concept
appear	O
to	O
exist	O
in	O
the	O
first	O
128	O
words	O
(	O
2008	O
)	O
of	O
the	O
current	O
instruction	O
bank	O
(	O
B0	O
)	O
when	O
referenced	O
as	O
a	O
data	O
item	O
.	O
</s>
<s>
Typical	O
instructions	O
contain	O
a	O
function	O
code	O
,	O
the	O
target	O
(	O
or	O
source	O
)	O
register	O
,	O
an	O
index	B-General_Concept
register	I-General_Concept
,	O
a	O
base	O
register	O
and	O
a	O
displacement	O
field	O
.	O
</s>
<s>
Processor	O
state	O
as	O
captured	O
on	O
a	O
stack	O
at	O
an	O
interrupt	B-Application
contains	O
the	O
information	O
needed	O
to	O
both	O
return	O
control	O
to	O
the	O
interrupted	O
activity	O
and	O
to	O
determine	O
the	O
type	O
of	O
the	O
interrupt	B-Application
.	O
</s>
<s>
Interrupts	B-Application
may	O
occur	O
in	O
the	O
middle	O
of	O
long	O
instructions	O
and	O
the	O
state	O
deals	O
with	O
that	O
possibility	O
.	O
</s>
<s>
The	O
most	O
obvious	O
difference	O
in	O
basic	O
mode	O
is	O
the	O
lack	O
of	O
explicit	O
B	O
registers	B-General_Concept
in	O
instructions	O
.	O
</s>
<s>
Instead	O
there	O
are	O
four	O
implicitly	O
used	O
B	O
registers	B-General_Concept
(	O
B12-B15	O
)	O
.	O
</s>
<s>
There	O
is	O
a	O
complex	O
algorithm	O
using	O
the	O
limits	O
of	O
the	O
banks	O
represented	O
by	O
those	O
B	O
registers	B-General_Concept
,	O
the	O
operand	O
address	O
and	O
the	O
B	O
register	O
within	O
which	O
the	O
current	O
instruction	O
is	O
found	O
.	O
</s>
<s>
Conditional	O
replace	O
is	O
familiar	O
and	O
quite	O
similar	O
to	O
Compare	B-Operating_System
and	I-Operating_System
Swap	I-Operating_System
in	O
the	O
Intel	O
architecture	O
.	O
</s>
<s>
If	O
the	O
bit	O
is	O
set	O
,	O
they	O
either	O
interrupt	B-Application
(	O
TS	O
)	O
or	O
fall	O
through	O
to	O
the	O
next	O
instruction(TSS )	O
.	O
</s>
<s>
On	O
a	O
TS	O
interrupt	B-Application
the	O
OS	O
takes	O
one	O
of	O
several	O
actions	O
depending	O
on	O
the	O
instruction	O
sequence	O
and	O
activity	O
priority	O
.	O
</s>
<s>
All	O
I/O	O
on	O
2200	O
Series	O
systems	O
is	O
handled	O
by	O
I/O	B-Device
processors	I-Device
.	O
</s>
<s>
These	O
processors	O
offload	O
large	O
portions	O
of	O
the	O
I/O	O
path	O
length	O
and	O
recovery	O
,	O
and	O
by	O
fully	O
isolating	O
the	O
main	O
system	O
from	O
I/O	O
faults	O
,	O
interrupts	B-Application
,	O
bus	O
errors	O
,	O
etc	O
.	O
</s>
<s>
The	O
I/O	B-Device
processors	I-Device
come	O
in	O
three	O
different	O
types	O
(	O
Storage	O
,	O
Communications	O
,	O
Clustering	O
)	O
but	O
the	O
only	O
real	O
difference	O
is	O
the	O
firmware	B-Application
load	O
.	O
</s>
<s>
All	O
I/O	B-Device
processors	I-Device
are	O
controlled	O
by	O
the	O
operating	O
system	O
.	O
</s>
<s>
OS	B-Operating_System
2200	I-Operating_System
does	O
provide	O
a	O
raw	O
mode	O
for	O
I/O	O
called	O
"	O
arbitrary	O
device	O
I/O	O
,	O
"	O
but	O
even	O
there	O
the	O
OS	O
validates	O
that	O
the	O
program	O
is	O
accessing	O
an	O
allowed	O
device	O
and	O
handles	O
all	O
interrupts	B-Application
and	O
faults	O
before	O
passing	O
appropriate	O
status	O
on	O
to	O
the	O
program	O
.	O
</s>
<s>
The	O
OS	O
takes	O
very	O
general	O
calls	O
from	O
programs	O
and	O
generates	O
command	O
packets	O
with	O
real	O
memory	O
and	O
device	O
addresses	O
which	O
are	O
then	O
passed	O
to	O
the	O
I/O	B-Device
processor	I-Device
.	O
</s>
<s>
Firmware	B-Application
in	O
the	O
I/O	B-Device
processor	I-Device
actually	O
creates	O
the	O
device	O
specific	O
(	O
e.g.	O
,	O
SCSI	B-Architecture
)	O
packets	O
,	O
sets	O
up	O
the	O
DMA	B-General_Concept
,	O
issues	O
the	O
I/O	O
,	O
and	O
services	O
the	O
interrupts	B-Application
.	O
</s>
