<s>
The	O
IBM	B-Device
System/370	I-Device
(	O
S/370	B-Device
)	O
is	O
a	O
model	O
range	O
of	O
IBM	B-Device
mainframe	I-Device
computers	O
announced	O
on	O
June	O
30	O
,	O
1970	O
,	O
as	O
the	O
successors	O
to	O
the	O
System/360	B-Application
family	O
.	O
</s>
<s>
The	O
series	O
mostly	O
maintains	O
backward	B-General_Concept
compatibility	I-General_Concept
with	O
the	O
S/360	B-Application
,	O
allowing	O
an	O
easy	O
migration	O
path	O
for	O
customers	O
;	O
this	O
,	O
plus	O
improved	O
performance	O
,	O
were	O
the	O
dominant	O
themes	O
of	O
the	O
product	O
announcement	O
.	O
</s>
<s>
In	O
September	O
1990	O
,	O
the	O
System/370	B-Device
line	O
was	O
replaced	O
with	O
the	O
System/390	B-Device
.	O
</s>
<s>
The	O
original	O
System/370	B-Device
line	O
was	O
announced	O
on	O
June	O
30	O
,	O
1970	O
,	O
with	O
first	O
customer	O
shipment	O
of	O
the	O
Models	O
155	O
and	O
165	O
planned	O
for	O
February	O
1971	O
and	O
April	O
1971	O
respectively	O
.	O
</s>
<s>
System/370	B-Device
underwent	O
several	O
architectural	O
improvements	O
during	O
its	O
roughly	O
20-year	O
lifetime	O
.	O
</s>
<s>
are	O
either	O
optional	O
on	O
S/360	B-Application
but	O
standard	O
on	O
S/370	B-Device
,	O
introduced	O
with	O
S/370	B-Device
or	O
added	O
to	O
S/370	B-Device
after	O
announcement	O
.	O
</s>
<s>
The	O
first	O
System/370	B-Device
machines	O
,	O
the	O
Model	B-Device
155	I-Device
and	O
the	O
Model	B-Device
165	I-Device
,	O
incorporated	O
only	O
a	O
small	O
number	O
of	O
changes	O
to	O
the	O
System/360	B-Application
architecture	O
.	O
</s>
<s>
support	O
for	O
the	O
block	O
multiplexer	O
channel	O
introduced	O
in	O
the	O
System/360	B-Device
Model	I-Device
85	I-Device
.	O
</s>
<s>
All	O
of	O
the	O
emulator	B-Application
features	O
were	O
designed	O
to	O
run	O
under	O
the	O
control	O
of	O
the	O
standard	O
operating	O
systems	O
.	O
</s>
<s>
IBM	O
documented	O
the	O
S/370	B-Device
emulator	B-Application
programs	O
as	O
integrated	O
emulators	B-Application
.	O
</s>
<s>
These	O
models	O
had	O
core	B-General_Concept
memory	I-General_Concept
and	O
did	O
not	O
include	O
support	O
for	O
virtual	B-Architecture
storage	I-Architecture
.	O
</s>
<s>
All	O
models	O
of	O
the	O
System/370	B-Device
used	O
IBM	O
's	O
form	O
of	O
monolithic	O
integrated	O
circuits	O
called	O
MST	O
(	O
Monolithic	O
System	O
Technology	O
)	O
making	O
them	O
third	O
generation	O
computers	O
.	O
</s>
<s>
MST	O
provided	O
System/370	B-Device
with	O
four	O
to	O
eight	O
times	O
the	O
circuit	O
density	O
and	O
over	O
ten	O
times	O
the	O
reliability	O
when	O
compared	O
to	O
the	O
previous	O
second	O
generation	O
SLT	B-Algorithm
technology	O
of	O
the	O
System/360	B-Application
.	O
</s>
<s>
On	O
September	O
23	O
,	O
1970	O
,	O
IBM	O
announced	O
the	O
Model	B-Device
145	I-Device
,	O
a	O
third	O
model	O
of	O
the	O
System/370	B-Device
,	O
which	O
was	O
the	O
first	O
model	O
to	O
feature	O
semiconductor	B-Architecture
main	I-Architecture
memory	I-Architecture
made	O
from	O
monolithic	O
integrated	O
circuits	O
and	O
was	O
scheduled	O
for	O
delivery	O
in	O
the	O
late	O
summer	O
of	O
1971	O
.	O
</s>
<s>
All	O
subsequent	O
S/370	B-Device
models	O
used	O
such	O
memory	O
.	O
</s>
<s>
In	O
1972	O
,	O
a	O
very	O
significant	O
change	O
was	O
made	O
when	O
support	O
for	O
virtual	B-Architecture
storage	I-Architecture
was	O
introduced	O
with	O
IBM	O
's	O
"	O
System/370	B-Device
Advanced	O
Function	O
"	O
announcement	O
.	O
</s>
<s>
IBM	O
had	O
initially	O
(	O
and	O
controversially	O
)	O
chosen	O
to	O
exclude	O
virtual	B-Architecture
storage	I-Architecture
from	O
the	O
S/370	B-Device
line	O
.	O
</s>
<s>
Virtual	B-Architecture
storage	I-Architecture
had	O
in	O
fact	O
been	O
delivered	O
on	O
S/370	B-Device
hardware	O
before	O
this	O
announcement	O
:	O
</s>
<s>
In	O
June	O
1971	O
,	O
on	O
the	O
S/370	B-Device
-145	O
(	O
one	O
of	O
which	O
had	O
to	O
be	O
"	O
smuggled	O
"	O
into	O
Cambridge	O
Scientific	O
Center	O
to	O
prevent	O
anybody	O
noticing	O
the	O
arrival	O
of	O
an	O
S/370	B-Device
at	O
that	O
hotbed	O
of	O
virtual	B-Architecture
memory	I-Architecture
development	O
since	O
this	O
would	O
have	O
signaled	O
that	O
the	O
S/370	B-Device
was	O
about	O
to	O
receive	O
address	O
relocation	O
technology	O
)	O
.	O
</s>
<s>
(	O
Varian	O
1997:p29	O
)	O
The	O
S/370	B-Device
-145	O
had	O
an	O
associative	O
memory	O
used	O
by	O
the	O
microcode	B-Device
for	O
the	O
DOS	O
compatibility	O
feature	O
from	O
its	O
first	O
shipments	O
in	O
June	O
1971	O
;	O
the	O
same	O
hardware	O
was	O
used	O
by	O
the	O
microcode	B-Device
for	O
DAT	O
.	O
</s>
<s>
Although	O
IBM	O
famously	O
chose	O
to	O
exclude	O
virtual	B-Architecture
storage	I-Architecture
from	O
the	O
S/370	B-Device
announcement	O
,	O
that	O
decision	O
was	O
being	O
reconsidered	O
during	O
the	O
completion	O
of	O
the	O
145	O
engineering	O
,	O
partly	O
because	O
of	O
virtual	B-Architecture
memory	I-Architecture
experience	O
at	O
CSC	O
and	O
elsewhere	O
.	O
</s>
<s>
The	O
145	O
microcode	B-Device
architecture	O
simplified	O
the	O
addition	O
of	O
virtual	B-Architecture
storage	I-Architecture
,	O
allowing	O
this	O
capability	O
to	O
be	O
present	O
in	O
early	O
145s	O
without	O
the	O
extensive	O
hardware	O
modifications	O
needed	O
in	O
other	O
models	O
.	O
</s>
<s>
However	O
,	O
IBM	O
did	O
not	O
document	O
the	O
145	O
's	O
virtual	B-Architecture
storage	I-Architecture
capability	O
,	O
nor	O
annotate	O
the	O
relevant	O
bits	O
in	O
the	O
control	O
registers	O
and	O
PSW	O
that	O
were	O
displayed	O
on	O
the	O
operator	O
control	O
panel	O
when	O
selected	O
using	O
the	O
roller	O
switches	O
.	O
</s>
<s>
The	O
Reference	O
and	O
Change	O
bits	O
of	O
the	O
Storage-protection	O
Keys	O
,	O
however	O
,	O
were	O
labeled	O
on	O
the	O
rollers	O
,	O
a	O
dead	O
giveaway	O
to	O
anyone	O
who	O
had	O
worked	O
with	O
the	O
earlier	O
360/67	B-Device
.	O
</s>
<s>
Existing	O
S/370	B-Device
-145	O
customers	O
were	O
happy	O
to	O
learn	O
that	O
they	O
did	O
not	O
have	O
to	O
purchase	O
a	O
hardware	O
upgrade	O
in	O
order	O
to	O
run	O
DOS/VS	B-Application
or	O
OS/VS1	B-Application
(	O
or	O
OS/VS2	B-Application
Release	O
1	O
which	O
was	O
possible	O
,	O
but	O
not	O
common	O
because	O
of	O
the	O
limited	O
amount	O
of	O
main	O
storage	O
available	O
on	O
the	O
S/370	B-Device
-145	O
)	O
.	O
</s>
<s>
Shortly	O
after	O
the	O
August	O
2	O
,	O
1972	O
announcement	O
,	O
DAT	O
box	O
(	O
address	O
relocation	O
hardware	O
)	O
upgrades	O
for	O
the	O
S/370	B-Device
-155	O
and	O
S/370	B-Device
-165	O
were	O
quietly	O
announced	O
,	O
but	O
were	O
available	O
only	O
for	O
purchase	O
by	O
customers	O
who	O
already	O
owned	O
a	O
Model	B-Device
155	I-Device
or	O
165	O
.	O
</s>
<s>
After	O
installation	O
,	O
these	O
models	O
were	O
known	O
as	O
the	O
S/370	B-Device
-155-II	O
and	O
S/370	B-Device
-165-II	O
.	O
</s>
<s>
IBM	O
wanted	O
customers	O
to	O
upgrade	O
their	O
155	O
and	O
165	O
systems	O
to	O
the	O
widely	O
sold	O
S/370	B-Device
-158	O
and	O
-168	O
.	O
</s>
<s>
This	O
led	O
to	O
the	O
original	O
S/370	B-Device
-155	O
and	O
S/370	B-Device
-165	O
models	O
being	O
described	O
as	O
"	O
boat	O
anchors	O
"	O
.	O
</s>
<s>
The	O
upgrade	O
,	O
required	O
to	O
run	O
OS/VS1	B-Application
or	O
OS/VS2	B-Application
,	O
was	O
not	O
cost	O
effective	O
for	O
most	O
customers	O
by	O
the	O
time	O
IBM	O
could	O
actually	O
deliver	O
and	O
install	O
it	O
,	O
so	O
many	O
customers	O
were	O
stuck	O
with	O
these	O
machines	O
running	O
MVT	O
until	O
their	O
lease	O
ended	O
.	O
</s>
<s>
It	O
was	O
not	O
unusual	O
for	O
this	O
to	O
be	O
another	O
four	O
,	O
five	O
or	O
even	O
six	O
years	O
for	O
the	O
more	O
unfortunate	O
ones	O
,	O
and	O
turned	O
out	O
to	O
be	O
a	O
significant	O
factor	O
in	O
the	O
slow	O
adoption	O
of	O
OS/VS2	B-Application
MVS	B-Application
,	O
not	O
only	O
by	O
customers	O
in	O
general	O
,	O
but	O
for	O
many	O
internal	O
IBM	O
sites	O
as	O
well	O
.	O
</s>
<s>
Later	O
architectural	O
changes	O
primarily	O
involved	O
expansions	O
in	O
memory	O
(	O
central	O
storage	O
)	O
both	O
physical	O
memory	O
and	O
virtual	O
address	B-General_Concept
space	I-General_Concept
to	O
enable	O
larger	O
workloads	O
and	O
meet	O
client	O
demands	O
for	O
more	O
storage	O
.	O
</s>
<s>
As	O
with	O
all	O
IBM	B-Device
mainframe	I-Device
development	O
,	O
preserving	O
backward	B-General_Concept
compatibility	I-General_Concept
was	O
paramount	O
.	O
</s>
<s>
extended	O
facility	O
and	O
extension	O
features	O
for	O
OS/VS1	B-Application
,	O
MVS	B-Application
and	O
VM	B-Application
.	O
</s>
<s>
Exploiting	O
levels	O
of	O
these	O
operating	O
systems	O
,	O
e.g.	O
,	O
MVS/System	O
Extensions	O
(	O
MVS/SE	O
)	O
,	O
reduce	O
path	O
length	O
for	O
some	O
frequent	O
functions	O
.	O
</s>
<s>
The	O
Dual	O
Address	B-General_Concept
Space	I-General_Concept
(	O
DAS	O
)	O
facility	O
allows	O
a	O
privileged	O
program	O
to	O
move	O
data	O
between	O
two	O
address	B-General_Concept
spaces	I-General_Concept
without	O
the	O
overhead	O
of	O
allocating	O
a	O
buffer	O
in	O
common	O
storage	O
,	O
moving	O
the	O
data	O
to	O
the	O
buffer	O
,	O
scheduling	O
an	O
SRB	O
in	O
the	O
target	O
address	B-General_Concept
space	I-General_Concept
,	O
moving	O
the	O
data	O
to	O
their	O
final	O
destination	O
and	O
freeing	O
the	O
buffer	O
.	O
</s>
<s>
IBM	O
introduced	O
DAS	O
in	O
1981	O
for	O
the	O
3033	O
,	O
but	O
later	O
made	O
it	O
available	O
for	O
some	O
43xx	B-Device
,	O
3031	O
and	O
3032	O
processors	O
.	O
</s>
<s>
MVS/System	O
Product	O
(	O
MVS/SP	O
)	O
Version	O
1	O
exploited	O
DAS	O
if	O
it	O
was	O
available	O
.	O
</s>
<s>
In	O
October	O
1981	O
,	O
the	O
3033	O
and	O
3081	O
processors	O
added	O
"	O
extended	O
real	O
addressing	O
"	O
,	O
which	O
allowed	O
26-bit	O
addressing	O
for	O
physical	O
storage	O
(	O
but	O
still	O
imposed	O
a	O
24-bit	O
limit	O
for	O
any	O
individual	O
address	B-General_Concept
space	I-General_Concept
)	O
.	O
</s>
<s>
This	O
capability	O
appeared	O
later	O
on	O
other	O
systems	O
,	O
such	O
as	O
the	O
4381	B-Device
and	O
3090	B-Device
.	O
</s>
<s>
The	O
System/370	B-Device
Extended	O
Architecture	O
(	O
S/370	B-Device
-XA	I-Device
)	O
,	O
first	O
available	O
in	O
early	O
1983	O
on	O
the	O
3081	O
and	O
3083	O
processors	O
,	O
provided	O
a	O
number	O
of	O
major	O
enhancements	O
,	O
including	O
:	O
expansion	O
of	O
the	O
address	B-General_Concept
space	I-General_Concept
from	O
24-bits	O
to	O
31-bits	O
;	O
facilitating	O
movement	O
of	O
data	O
between	O
two	O
address	B-General_Concept
spaces	I-General_Concept
;	O
and	O
a	O
complete	O
redesign	O
of	O
the	O
I/O	O
architecture	O
.	O
</s>
<s>
The	O
cross-memory	O
services	O
capability	O
which	O
facilitated	O
movement	O
of	O
data	O
between	O
address	B-General_Concept
spaces	I-General_Concept
was	O
actually	O
available	O
just	O
prior	O
to	O
S/370	B-Device
-XA	I-Device
architecture	O
on	O
the	O
3031	O
,	O
3032	O
and	O
3033	O
processors	O
.	O
</s>
<s>
In	O
February	O
1988	O
,	O
IBM	O
announced	O
the	O
Enterprise	O
Systems	O
Architecture/370	O
(	O
ESA/370	O
)	O
for	O
enhanced	O
(	O
E	O
)	O
3090	B-Device
and	O
4381	B-Device
models	O
.	O
</s>
<s>
It	O
added	O
sixteen	O
32-bit	O
access	B-Application
registers	I-Application
,	O
more	O
addressing	O
modes	O
,	O
and	O
various	O
facilities	O
for	O
working	O
with	O
multiple	O
address	B-General_Concept
spaces	I-General_Concept
simultaneously	O
.	O
</s>
<s>
On	O
September	O
5	O
,	O
1990	O
,	O
IBM	O
announced	O
the	O
Enterprise	B-Device
Systems	I-Device
Architecture/390	I-Device
(	O
ESA/390	O
)	O
,	O
upward	O
compatible	O
with	O
ESA/370	O
.	O
</s>
<s>
As	O
described	O
above	O
,	O
the	O
S/370	B-Device
product	O
line	O
underwent	O
a	O
major	O
architectural	O
change	O
:	O
expansion	O
of	O
its	O
address	B-General_Concept
space	I-General_Concept
from	O
24	O
to	O
31	O
bits	O
.	O
</s>
<s>
The	O
evolution	O
of	O
S/370	B-Device
addressing	O
was	O
always	O
complicated	O
by	O
the	O
basic	O
S/360	B-Application
instruction	B-General_Concept
set	I-General_Concept
design	O
,	O
and	O
its	O
large	O
installed	O
code	O
base	O
,	O
which	O
relied	O
on	O
a	O
24-bit	O
logical	B-General_Concept
address	I-General_Concept
.	O
</s>
<s>
(	O
In	O
particular	O
,	O
a	O
heavily	O
used	O
machine	O
instruction	O
,	O
"	O
Load	O
Address	O
"	O
(	O
LA	O
)	O
,	O
explicitly	O
cleared	O
the	O
top	O
eight	O
bits	O
of	O
the	O
address	O
being	O
placed	O
in	O
a	O
register	B-General_Concept
.	O
</s>
<s>
Since	O
the	O
core	O
S/360	B-Application
instruction	B-General_Concept
set	I-General_Concept
remained	O
geared	O
to	O
a	O
24-bit	O
universe	O
,	O
this	O
third	O
step	O
would	O
require	O
a	O
real	O
break	O
from	O
the	O
status	O
quo	O
;	O
existing	O
assembly	B-Language
language	I-Language
applications	O
would	O
of	O
course	O
not	O
benefit	O
,	O
and	O
new	O
compilers	O
would	O
be	O
needed	O
before	O
non-assembler	O
applications	O
could	O
be	O
migrated	O
.	O
</s>
<s>
This	O
evolutionary	O
implementation	O
(	O
repeated	O
in	O
z/Architecture	B-Device
)	O
had	O
the	O
characteristic	O
of	O
solving	O
the	O
most	O
urgent	O
problems	O
first	O
:	O
relief	O
for	O
real	O
memory	O
addressing	O
being	O
needed	O
sooner	O
than	O
virtual	B-Architecture
memory	I-Architecture
addressing	O
.	O
</s>
<s>
The	O
System/360	B-Device
Model	I-Device
67	I-Device
had	O
included	O
a	O
full	O
32-bit	O
addressing	O
mode	O
,	O
but	O
this	O
feature	O
was	O
not	O
carried	O
forward	O
to	O
the	O
System/370	B-Device
series	O
,	O
which	O
began	O
with	O
only	O
24-bit	O
addressing	O
.	O
</s>
<s>
When	O
IBM	O
later	O
expanded	O
the	O
S/370	B-Device
address	B-General_Concept
space	I-General_Concept
in	O
S/370	B-Device
-XA	I-Device
,	O
several	O
reasons	O
are	O
cited	O
for	O
the	O
choice	O
of	O
31	O
bits	O
:	O
</s>
<s>
Interaction	O
between	O
32-bit	O
addresses	O
and	O
two	O
instructions	O
(	O
BXH	O
and	O
BXLE	O
)	O
that	O
treated	O
their	O
arguments	O
as	O
signed	O
numbers	O
(	O
and	O
which	O
was	O
said	O
to	O
be	O
the	O
reason	O
TSS	O
used	O
31-bit	O
addressing	O
on	O
the	O
Model	O
67	B-Device
)	O
.	O
</s>
<s>
Input	O
from	O
key	O
initial	O
Model	O
67	B-Device
sites	O
,	O
which	O
had	O
debated	O
the	O
alternatives	O
during	O
the	O
initial	O
system	O
design	O
period	O
,	O
and	O
had	O
recommended	O
31	O
bits	O
(	O
instead	O
of	O
the	O
32-bit	O
design	O
that	O
was	O
ultimately	O
chosen	O
at	O
the	O
time	O
)	O
.	O
</s>
<s>
Enterprise	O
Systems	O
Architecture/370	O
(	O
ESA/370	O
)	O
for	O
3090	B-Device
enhanced	O
(	O
"	O
E	O
"	O
)	O
models	O
and	O
for	O
4381	B-Device
model	O
groups	O
91E	O
and	O
92E	O
.	O
</s>
<s>
In	O
additional	O
to	O
the	O
primary	O
and	O
secondary	O
addressing	O
modes	O
that	O
System/370	B-Device
Extended	O
Architecture	O
(	O
S/370	B-Device
-XA	I-Device
)	O
supports	O
,	O
ESA	O
has	O
an	O
access	B-Application
register	I-Application
mode	O
in	O
which	O
each	O
use	O
of	O
general	O
register	B-General_Concept
1-15	O
as	O
a	O
base	O
register	B-General_Concept
uses	O
an	O
associated	O
access	B-Application
register	I-Application
to	O
select	O
an	O
address	B-General_Concept
space	I-General_Concept
.	O
</s>
<s>
In	O
addition	O
to	O
the	O
normal	O
address	B-General_Concept
spaces	I-General_Concept
that	O
XA	O
supports	O
,	O
ESA	O
also	O
allows	O
data	O
spaces	O
,	O
which	O
contain	O
no	O
executable	O
code	O
.	O
</s>
<s>
The	O
following	O
table	O
summarizes	O
the	O
major	O
S/370	B-Device
series	O
and	O
models	O
.	O
</s>
<s>
Many	O
models	O
implemented	O
more	O
than	O
one	O
architecture	O
;	O
thus	O
,	O
308x	B-Device
processors	O
initially	O
shipped	O
as	O
S/370	B-Device
architecture	O
,	O
but	O
later	O
offered	O
XA	O
;	O
and	O
many	O
processors	O
,	O
such	O
as	O
the	O
4381	B-Device
,	O
had	O
microcode	B-Device
that	O
allowed	O
customer	O
selection	O
between	O
S/370	B-Device
or	O
XA	O
(	O
later	O
,	O
ESA	O
)	O
operation	O
.	O
</s>
<s>
Note	O
also	O
the	O
confusing	O
term	O
"	O
System/370	B-Device
-compatible	O
"	O
,	O
which	O
appeared	O
in	O
IBM	O
source	O
documents	O
to	O
describe	O
certain	O
products	O
.	O
</s>
<s>
Outside	O
IBM	O
,	O
this	O
term	O
would	O
more	O
often	O
describe	O
systems	O
from	O
Amdahl	O
Corporation	O
,	O
Hitachi	O
Ltd.	O
,	O
and	O
others	O
,	O
that	O
could	O
run	O
the	O
same	O
S/370	B-Device
software	O
.	O
</s>
<s>
This	O
choice	O
of	O
terminology	O
by	O
IBM	O
may	O
have	O
been	O
a	O
deliberate	O
attempt	O
to	O
ignore	O
the	O
existence	O
of	O
those	O
plug	B-Device
compatible	I-Device
manufacturers	I-Device
(	O
PCMs	O
)	O
,	O
because	O
they	O
competed	O
aggressively	O
against	O
IBM	O
hardware	O
dominance	O
.	O
</s>
<s>
IBM	O
used	O
the	O
name	O
System/370	B-Device
to	O
announce	O
the	O
following	O
eleven	O
(	O
3	O
digit	O
)	O
offerings	O
:	O
</s>
<s>
The	O
IBM	B-Device
System/370	I-Device
Model	O
115	O
was	O
announced	O
March	O
13	O
,	O
1973	O
as	O
"	O
an	O
ideal	O
System/370	B-Device
entry	O
system	O
for	O
users	O
of	O
IBM	B-Device
's	I-Device
System/3	I-Device
,	O
1130	B-Device
computing	I-Device
system	I-Device
and	O
System/360	B-Device
Models	I-Device
20	I-Device
,	O
22	B-Device
and	O
25.	O
"	O
</s>
<s>
It	O
was	O
delivered	O
with	O
"	O
a	O
minimum	O
of	O
two	O
(	O
of	O
IBM	O
's	O
newly	O
announced	O
)	O
directly-attached	O
IBM	B-Device
3340	I-Device
disk	O
drives.	O
"	O
</s>
<s>
The	O
CPU	B-General_Concept
could	O
be	O
configured	O
with	O
65,536	O
(	O
64K	O
)	O
or	O
98,304	O
(	O
96K	O
)	O
bytes	O
of	O
main	O
memory	O
.	O
</s>
<s>
An	O
optional	O
360/20	B-Device
emulator	B-Application
was	O
available	O
.	O
</s>
<s>
The	O
IBM	B-Device
System/370	I-Device
Model	O
125	O
was	O
announced	O
Oct	O
4	O
,	O
1972	O
.	O
</s>
<s>
The	O
IBM	B-Device
System/370	I-Device
Model	O
135	O
was	O
announced	O
Mar	O
8	O
,	O
1971	O
.	O
</s>
<s>
Options	O
for	O
the	O
370/135	O
included	O
a	O
choice	O
of	O
four	O
main	O
memory	O
sizes	O
;	O
IBM	B-Device
1400	I-Device
series	I-Device
(	O
1401	O
,	O
1440	O
and	O
1460	O
)	O
emulation	B-Application
was	O
also	O
offered	O
.	O
</s>
<s>
A	O
"	O
reading	O
device	O
located	O
in	O
the	O
Model	O
135	O
console	O
"	O
allowed	O
updates	O
and	O
adding	O
features	O
to	O
the	O
Model	O
135	O
's	O
microcode	B-Device
.	O
</s>
<s>
The	O
IBM	B-Device
System/370	I-Device
Model	O
138	O
which	O
was	O
announced	O
Jun	O
30	O
,	O
1976	O
was	O
offered	O
with	O
either	O
524,288	O
(	O
512K	O
)	O
or	O
1,048,576	O
(	O
1	O
MB	O
)	O
of	O
memory	O
.	O
</s>
<s>
The	O
IBM	B-Device
System/370	I-Device
Model	I-Device
145	I-Device
was	O
announced	O
Sep	O
23	O
,	O
1970	O
,	O
three	O
months	O
after	O
the	O
155	O
and	O
165	O
models	O
.	O
</s>
<s>
The	O
first	O
System/370	B-Device
to	O
use	O
monolithic	O
main	O
memory	O
,	O
the	O
Model	B-Device
145	I-Device
was	O
offered	O
in	O
six	O
memory	O
sizes	O
.	O
</s>
<s>
A	O
portion	O
of	O
the	O
main	O
memory	O
,	O
the	O
"	O
Reloadable	O
Control	O
Storage	O
"	O
(	O
RCS	O
)	O
was	O
loaded	O
from	O
a	O
prewritten	O
disk	O
cartridge	O
containing	O
microcode	B-Device
to	O
implement	O
,	O
for	O
example	O
,	O
all	O
needed	O
instructions	O
,	O
I/O	O
channels	O
,	O
and	O
optional	O
instructions	O
to	O
enable	O
the	O
system	O
to	O
emulate	O
earlier	O
IBM	O
machines	O
.	O
</s>
<s>
The	O
IBM	B-Device
System/370	I-Device
Model	O
148	O
had	O
the	O
same	O
announcement	O
and	O
withdrawal	O
dates	O
as	O
the	O
Model	O
138	O
.	O
</s>
<s>
The	O
IBM	B-Device
System/370	I-Device
Model	I-Device
155	I-Device
and	O
the	O
Model	B-Device
165	I-Device
were	O
announced	O
Jun	O
30	O
,	O
1970	O
,	O
the	O
first	O
of	O
the	O
370s	O
introduced	O
.	O
</s>
<s>
Neither	O
had	O
a	O
DAT	O
box	O
;	O
they	O
were	O
limited	O
to	O
running	O
the	O
same	O
non-virtual-memory	O
operating	O
systems	O
available	O
for	O
the	O
System/360	B-Application
.	O
</s>
<s>
The	O
OS/DOS	O
(	O
DOS/360	O
programs	O
under	O
OS/360	B-Application
)	O
,	O
1401/1440/1460	O
and	O
1410/7010	O
and	O
7070/7074	O
compatibility	O
features	O
were	O
included	O
,	O
and	O
the	O
supporting	O
integrated	O
emulator	B-Application
programs	O
could	O
operate	O
concurrently	O
with	O
standard	O
System/370	B-Device
workloads	O
.	O
</s>
<s>
In	O
August	O
1972	O
IBM	O
announced	O
,	O
as	O
a	O
field	O
upgrade	O
only	O
,	O
the	O
IBM	B-Device
System/370	I-Device
Model	I-Device
155	I-Device
II	O
,	O
which	O
added	O
a	O
DAT	O
box	O
.	O
</s>
<s>
The	O
IBM	B-Device
System/370	I-Device
Model	O
158	O
and	O
the	O
370/168	O
were	O
announced	O
Aug	O
2	O
,	O
1972	O
.	O
</s>
<s>
It	O
included	O
dynamic	O
address	O
translation	O
(	O
DAT	O
)	O
hardware	O
,	O
a	O
prerequisite	O
for	O
the	O
new	O
virtual	B-Architecture
memory	I-Architecture
operating	O
systems	O
(	O
DOS/VS	B-Application
,	O
OS/VS1	B-Application
,	O
OS/VS2	B-Application
)	O
.	O
</s>
<s>
The	O
IBM	B-Device
System/370	I-Device
Model	I-Device
165	I-Device
was	O
described	O
by	O
IBM	O
as	O
"	O
more	O
powerful	O
"	O
compared	O
to	O
the	O
"	O
medium-scale	O
"	O
370/155	O
.	O
</s>
<s>
Compatibility	O
features	O
included	O
emulation	B-Application
for	O
7070/7074	B-Device
,	I-Device
7080	I-Device
,	I-Device
and	I-Device
709/7090/7094/7094	I-Device
II	I-Device
.	O
</s>
<s>
Some	O
have	O
described	O
the	O
360/85	B-Device
'	O
s	O
use	O
of	O
microcoded	B-Device
vs	O
hardwired	O
as	O
a	O
bridge	O
to	O
the	O
370/165	B-Device
.	O
</s>
<s>
In	O
August	O
1972	O
IBM	O
announced	O
,	O
as	O
a	O
field	O
upgrade	O
only	O
,	O
the	O
IBM	B-Device
System/370	I-Device
Model	I-Device
165	I-Device
II	O
which	O
added	O
a	O
DAT	O
box	O
.	O
</s>
<s>
It	O
included	O
dynamic	O
address	O
translation	O
(	O
DAT	O
)	O
hardware	O
,	O
a	O
pre-requisite	O
for	O
the	O
new	O
virtual	B-Architecture
memory	I-Architecture
operating	O
systems	O
.	O
</s>
<s>
The	O
IBM	B-Device
System/370	I-Device
Model	O
195	B-Device
was	O
announced	O
Jun	O
30	O
,	O
1970	O
and	O
,	O
at	O
that	O
time	O
,	O
it	O
was	O
"	O
IBM	O
's	O
most	O
powerful	O
computing	O
system.	O
"	O
</s>
<s>
Its	O
introduction	O
came	O
about	O
14	O
months	O
after	O
the	O
announcement	O
of	O
the	O
360/195	B-Device
.	O
</s>
<s>
Both	O
195	B-Device
machines	O
were	O
withdrawn	O
Feb	O
.	O
9	O
,	O
1977	O
.	O
</s>
<s>
Beginning	O
in	O
1977	O
,	O
IBM	O
began	O
to	O
introduce	O
new	O
systems	O
,	O
using	O
the	O
description	O
"	O
A	O
compatible	O
member	O
of	O
the	O
System/370	B-Device
family.	O
"	O
</s>
<s>
The	O
first	O
of	O
the	O
initial	O
high	O
end	O
machines	O
,	O
IBM	O
's	O
3033	O
,	O
was	O
announced	O
March	O
25	B-Device
,	O
1977	O
and	O
was	O
delivered	O
the	O
following	O
March	O
,	O
at	O
which	O
time	O
a	O
multiprocessor	O
version	O
of	O
the	O
3033	O
was	O
announced	O
.	O
</s>
<s>
IBM	O
noted	O
about	O
the	O
3033	O
,	O
looking	O
back	O
,	O
that	O
"	O
When	O
it	O
was	O
rolled	O
out	O
on	O
March	O
25	B-Device
,	O
1977	O
,	O
the	O
3033	O
eclipsed	O
the	O
internal	O
operating	O
speed	O
of	O
the	O
company	O
's	O
previous	O
flagship	O
the	O
System/370	B-Device
Model	O
168-3	O
...	O
"	O
</s>
<s>
Three	O
systems	O
comprised	O
the	O
next	O
series	O
of	O
high	O
end	O
machines	O
,	O
IBM	O
's	O
308X	B-Device
systems	O
:	O
</s>
<s>
These	O
models	O
introduced	O
IBM	O
's	O
Extended	O
Architecture	O
's	O
31-bit	O
address	O
capability	O
and	O
a	O
set	O
of	O
backward	B-General_Concept
compatible	I-General_Concept
MVS/Extended	O
Architecture	O
(	O
MVS/XA	O
)	O
software	O
replacing	O
previous	O
products	O
and	O
part	O
of	O
OS/VS2	B-Application
R3.8	O
:	O
</s>
<s>
All	O
three	O
308x	B-Device
systems	O
were	O
withdrawn	O
on	O
August	O
4	O
,	O
1987	O
.	O
</s>
<s>
The	O
next	O
series	O
of	O
high-end	O
machines	O
,	O
the	O
IBM	B-Device
3090	I-Device
,	O
began	O
with	O
models	O
200	O
and	O
400	O
.	O
</s>
<s>
Starting	O
with	O
the	O
E	O
models	O
,	O
and	O
continuing	O
with	O
the	O
J	O
and	O
S	O
models	O
,	O
IBM	O
offered	O
Enterprise	O
Systems	O
Architecture/370	O
(	O
ESA/370	O
)	O
,	O
Processor	B-Device
Resource/System	I-Device
Manager	I-Device
(	O
PR/SM	B-Device
)	O
and	O
a	O
set	O
of	O
backward	B-General_Concept
compatible	I-General_Concept
MVS/Enterprise	O
System	O
Architecture	O
(	O
MVS/ESA	O
)	O
software	O
replacing	O
previous	O
products	O
:	O
</s>
<s>
IBM	O
's	O
offering	O
of	O
an	O
optional	O
vector	B-Operating_System
facility	I-Operating_System
(	O
VF	O
)	O
extension	O
for	O
the	O
3090	B-Device
came	O
at	O
a	O
time	O
when	O
Vector	O
processing/Array	O
processing	O
suggested	O
names	O
like	O
Cray	O
and	O
Control	O
Data	O
Corporation	O
(	O
CDC	O
)	O
.	O
</s>
<s>
The	O
first	O
pair	O
of	O
IBM	B-Device
4300	I-Device
processors	O
were	O
Mid/Low	O
end	O
systems	O
announced	O
Jan	O
30	O
,	O
1979	O
as	O
"	O
compact	O
(	O
and	O
)	O
..	O
compatible	O
with	O
System/370.	O
"	O
</s>
<s>
Other	O
models	O
were	O
the	O
4321	O
,	O
4361	O
and	O
4381	B-Device
.	O
</s>
<s>
The	O
4361	O
has	O
"	O
Programmable	O
Power-Off	O
--	O
enables	O
the	O
user	O
to	O
turn	O
off	O
the	O
processor	O
under	O
program	O
control	O
"	O
;	O
"	O
Unit	O
power	O
off	O
"	O
is	O
(	O
also	O
)	O
part	O
of	O
the	O
4381	B-Device
feature	O
list	O
.	O
</s>
<s>
IBM	O
offered	O
many	O
Model	O
Groups	O
and	O
models	O
of	O
the	O
4300	O
family	O
,	O
ranging	O
from	O
the	O
entry	O
level	O
4331	O
to	O
the	O
4381	B-Device
,	O
described	O
as	O
"	O
one	O
of	O
the	O
most	O
powerful	O
and	O
versatile	O
intermediate	O
system	O
processors	O
ever	O
produced	O
by	O
IBM.	O
"	O
</s>
<s>
The	O
4381	B-Device
Model	O
Group	O
3	O
was	O
dual-CPU	O
.	O
</s>
<s>
This	O
low-end	O
system	O
,	O
announced	O
October	O
7	O
,	O
1986	O
,	O
was	O
"	O
designed	O
to	O
satisfy	O
the	O
computing	O
requirements	O
of	O
IBM	O
customers	O
who	O
value	O
System/370	B-Device
affinity	O
"	O
and	O
"	O
small	O
enough	O
and	O
quiet	O
enough	O
to	O
operate	O
in	O
an	O
office	O
environment.	O
"	O
</s>
<s>
Furthermore	O
,	O
it	O
stated	O
its	O
awareness	O
of	O
the	O
needs	O
of	O
small-to-medium	O
size	O
businesses	O
to	O
be	O
able	O
to	O
respond	O
,	O
as	O
"	O
computing	O
requirements	O
grow	O
,	O
"	O
adding	O
that	O
"	O
the	O
IBM	B-Device
9370	I-Device
system	O
can	O
be	O
easily	O
expanded	O
by	O
adding	O
additional	O
features	O
and	O
racks	O
to	O
accommodate	O
...	O
"	O
</s>
<s>
This	O
came	O
at	O
a	O
time	O
when	O
Digital	O
Equipment	O
Corporation	O
(	O
DEC	O
)	O
and	O
its	O
VAX	B-Device
systems	O
were	O
strong	O
competitors	O
in	O
both	O
hardware	O
and	O
software	O
;	O
the	O
media	O
of	O
the	O
day	O
carried	O
IBM	O
's	O
alleged	O
"	O
VAX	B-Device
Killer	O
"	O
phrase	O
,	O
albeit	O
often	O
skeptically	O
.	O
</s>
<s>
In	O
the	O
360	O
era	O
,	O
a	O
number	O
of	O
manufacturers	O
had	O
already	O
standardized	O
upon	O
the	O
IBM/360	B-Application
instruction	B-General_Concept
set	I-General_Concept
and	O
,	O
to	O
a	O
degree	O
,	O
360	O
architecture	O
.	O
</s>
<s>
Notable	O
computer	O
makers	O
included	O
Univac	O
with	O
the	O
UNIVAC	B-Device
9000	I-Device
series	I-Device
,	O
RCA	O
with	O
the	O
RCA	B-Device
Spectra	I-Device
70	I-Device
series	O
,	O
English	O
Electric	O
with	O
the	O
English	B-Device
Electric	I-Device
System	I-Device
4	I-Device
,	O
and	O
the	O
Soviet	O
ES	B-Device
EVM	I-Device
.	O
</s>
<s>
That	O
changed	O
in	O
the	O
1970s	O
with	O
the	O
introduction	O
of	O
the	O
IBM/370	B-Device
and	O
Gene	O
Amdahl	O
's	O
launch	O
of	O
his	O
own	O
company	O
.	O
</s>
<s>
The	O
latter	O
efforts	O
were	O
abandoned	O
and	O
eventually	O
all	O
Japanese	O
efforts	O
focused	O
on	O
the	O
IBM	B-Device
mainframe	I-Device
lines	O
.	O
</s>
<s>
S/370	B-Device
also	O
refers	O
to	O
a	O
computer	B-General_Concept
system	I-General_Concept
architecture	I-General_Concept
specification	O
,	O
and	O
is	O
a	O
direct	O
and	O
mostly	O
backward	B-General_Concept
compatible	I-General_Concept
evolution	O
of	O
the	O
System/360	B-Application
architecture	O
from	O
which	O
it	O
retains	O
most	O
aspects	O
.	O
</s>
<s>
An	O
instruction	B-General_Concept
set	I-General_Concept
.	O
</s>
<s>
A	O
feature	O
to	O
enhance	O
performance	O
for	O
the	O
DOS/VSE	B-Application
operating	O
system	O
.	O
</s>
<s>
This	O
philosophy	O
predates	O
the	O
definition	O
of	O
the	O
S/370	B-Device
architecture	O
and	O
started	O
with	O
the	O
S/360	B-Application
architecture	O
.	O
</s>
<s>
Such	O
an	O
example	O
is	O
that	O
the	O
S/370	B-Device
architecture	O
specifies	O
that	O
the	O
64-bit	O
PSW	O
register	B-General_Concept
bit	O
number	O
32	O
has	O
to	O
be	O
set	O
to	O
0	O
and	O
that	O
doing	O
otherwise	O
leads	O
to	O
an	O
exception	O
.	O
</s>
<s>
Subsequently	O
,	O
when	O
the	O
S/370	B-Device
-XA	I-Device
architecture	O
was	O
defined	O
,	O
it	O
was	O
stated	O
that	O
this	O
bit	O
would	O
indicate	O
whether	O
the	O
program	O
was	O
a	O
program	O
expecting	O
a	O
24-bit	O
address	O
architecture	O
or	O
31-bit	O
address	O
architecture	O
.	O
</s>
<s>
Thus	O
,	O
most	O
programs	O
that	O
ran	O
on	O
the	O
24-bit	O
architecture	O
can	O
still	O
run	O
on	O
31-bit	O
systems	O
;	O
the	O
64-bit	O
z/Architecture	B-Device
has	O
an	O
additional	O
mode	O
bit	O
for	O
64-bit	O
addresses	O
,	O
so	O
that	O
those	O
programs	O
,	O
and	O
programs	O
that	O
ran	O
on	O
the	O
31-bit	O
architecture	O
,	O
can	O
still	O
run	O
on	O
64-bit	O
systems	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
I/O	O
interface	O
was	O
redesigned	O
in	O
S/370	B-Device
-XA	I-Device
making	O
S/370	B-Device
program	O
issuing	O
I/O	O
operations	O
unusable	O
as-is	O
.	O
</s>
<s>
IBM	O
replaced	O
the	O
System/370	B-Device
line	O
with	O
the	O
System/390	B-Device
in	O
the	O
1990s	O
,	O
and	O
similarly	O
extended	O
the	O
architecture	O
from	O
ESA/370	O
to	O
ESA/390	O
.	O
</s>
<s>
In	O
2000	O
,	O
the	O
System/390	B-Device
was	O
replaced	O
with	O
the	O
zSeries	B-Device
(	O
now	O
called	O
IBM	B-Device
System	I-Device
z	I-Device
)	O
.	O
</s>
<s>
The	O
zSeries	B-Device
mainframes	O
introduced	O
the	O
64-bit	O
z/Architecture	B-Device
,	O
the	O
most	O
significant	O
design	O
improvement	O
since	O
the	O
31-bit	O
transition	O
.	O
</s>
<s>
All	O
have	O
retained	O
essential	O
backward	B-General_Concept
compatibility	I-General_Concept
with	O
the	O
original	O
S/360	B-Application
architecture	O
and	O
instruction	B-General_Concept
set	I-General_Concept
.	O
</s>
<s>
The	O
GNU	B-Application
Compiler	I-Application
Collection	I-Application
(	O
GCC	B-Application
)	O
had	O
a	O
back	O
end	O
for	O
S/370	B-Device
,	O
but	O
it	O
became	O
obsolete	O
over	O
time	O
and	O
was	O
finally	O
replaced	O
with	O
the	O
S/390	B-Device
backend	O
.	O
</s>
<s>
Although	O
the	O
S/370	B-Device
and	O
S/390	B-Device
instruction	B-General_Concept
sets	I-General_Concept
are	O
essentially	O
the	O
same	O
(	O
and	O
have	O
been	O
consistent	O
since	O
the	O
introduction	O
of	O
the	O
S/360	B-Application
)	O
,	O
GCC	B-Application
operability	O
on	O
older	O
systems	O
has	O
been	O
abandoned	O
.	O
</s>
<s>
GCC	B-Application
currently	O
works	O
on	O
machines	O
that	O
have	O
the	O
full	O
instruction	B-General_Concept
set	I-General_Concept
of	O
System/390	B-Device
Generation	O
5	O
(	O
G5	O
)	O
,	O
the	O
hardware	O
platform	O
for	O
the	O
initial	O
release	O
of	O
Linux/390	B-Application
.	O
</s>
<s>
However	O
,	O
a	O
separately	O
maintained	O
version	O
of	O
GCC	B-Application
3.2.3	O
that	O
works	O
for	O
the	O
S/370	B-Device
is	O
available	O
,	O
known	O
as	O
GCCMVS	O
.	O
</s>
<s>
The	O
block	O
multiplexer	O
channel	O
,	O
previously	O
available	O
only	O
on	O
the	O
360/85	B-Device
and	O
360/195	B-Device
,	O
was	O
a	O
standard	O
part	O
of	O
the	O
architecture	O
.	O
</s>
<s>
System/370	B-Device
-XA	I-Device
introduced	O
a	O
channel	O
subsystem	O
that	O
performed	O
I/O	O
queuing	O
previously	O
done	O
by	O
the	O
operating	O
system	O
.	O
</s>
<s>
The	O
System/390	B-Device
introduced	O
the	O
ESCON	B-Device
channel	O
,	O
an	O
optical	B-Architecture
fiber	I-Architecture
,	O
half-duplex	O
,	O
serial	B-Protocol
channel	O
with	O
a	O
maximum	O
distance	O
of	O
43	O
kilometers	O
.	O
</s>
<s>
Subsequently	O
,	O
FICON	B-Architecture
became	O
the	O
standard	O
IBM	B-Device
mainframe	I-Device
channel	O
;	O
FIbre	B-Architecture
CONnection	I-Architecture
(	O
FICON	B-Architecture
)	O
is	O
the	O
IBM	O
proprietary	O
name	O
for	O
the	O
ANSI	O
FC-SB-3	O
Single-Byte	O
Command	O
Code	O
Sets-3	O
Mapping	O
Protocol	O
for	O
Fibre	B-Architecture
Channel	I-Architecture
(	O
FC	O
)	O
protocol	O
used	O
to	O
map	O
both	O
IBM	O
's	O
antecedent	O
(	O
either	O
ESCON	B-Device
or	O
parallel	O
Bus	O
and	O
Tag	O
)	O
channel-to-control-unit	O
cabling	O
infrastructure	O
and	O
protocol	O
onto	O
standard	O
FC	O
services	O
and	O
infrastructure	O
at	O
data	O
rates	O
up	O
to	O
16	O
Gigabits/sec	O
at	O
distances	O
up	O
to	O
100km	O
.	O
</s>
<s>
Fibre	B-Architecture
Channel	I-Architecture
Protocol	O
(	O
FCP	O
)	O
allows	O
attaching	O
SCSI	O
devices	O
using	O
the	O
same	O
infrastructure	O
as	O
FICON	B-Architecture
.	O
</s>
