<s>
the	O
only	O
370	B-Device
introduced	O
that	O
year	O
.	O
</s>
<s>
The	O
135	O
was	O
IBM	O
's	O
fifth	O
System	B-Device
370	I-Device
,	O
and	O
it	O
was	O
withdrawn	O
October	O
16	O
,	O
1979	O
.	O
</s>
<s>
Although	O
microcode	B-Device
was	O
not	O
a	O
uniquely	O
new	O
feature	O
at	O
the	O
time	O
of	O
the	O
135	O
's	O
introduction	O
,	O
the	O
ability	O
to	O
upgrade	O
a	O
system	O
's	O
microcode	B-Device
without	O
changing	O
hardware	O
was	O
not	O
common	O
at	O
that	O
time	O
.	O
</s>
<s>
"	O
Later	O
models	O
like	O
the	O
85	B-Device
and	O
25	O
had	O
writable	O
control	O
stores	O
,	O
the	O
85	B-Device
using	O
a	O
volatile	O
SRAM	O
array	O
and	O
the	O
25	O
using	O
a	O
part	O
of	O
core	O
memory.	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
145	B-Device
,	O
introduced	O
the	O
prior	O
year	O
,	O
also	O
had	O
this	O
feature	O
.	O
</s>
<s>
The	O
Model	O
135	O
was	O
the	O
last	O
of	O
the	O
370s	O
to	O
be	O
introduced	O
without	O
Virtual	B-Architecture
memory	I-Architecture
.	O
</s>
<s>
hardware	O
upgrade	O
to	O
add	O
a	O
DAT	B-General_Concept
box	I-General_Concept
(	O
Dynamic	O
Address	O
Translation	O
)	O
,	O
the	O
135	O
&	O
145	B-Device
could	O
get	O
obtain	O
their	O
virtual	B-Architecture
memory	I-Architecture
upgrade	O
from	O
a	O
floppy	O
disk	O
.	O
</s>
<s>
Customers	O
of	O
the	O
370/135	O
had	O
a	O
choice	O
of	O
four	O
main	O
memory	O
sizes	O
,	O
ranging	O
from	O
96K	O
to	O
256K	O
.	O
</s>
<s>
The	O
370/135	O
was	O
introduced	O
as	O
running	O
"	O
under	O
either	O
OS	B-Application
or	O
DOS	O
.	O
</s>
<s>
Newer	O
versions	O
thereof	O
(	O
DOS/VS	O
and	O
OS/VS1	O
)	O
and	O
Virtual	O
Machine	O
Facility/370	O
(	O
VM/370	B-Application
)	O
subsequently	O
became	O
available	O
options	O
once	O
the	O
135	O
's	O
microcode	B-Device
was	O
upgraded	O
to	O
support	O
virtual	B-Architecture
memory	I-Architecture
.	O
</s>
<s>
The	O
upgrade	O
could	O
be	O
done	O
"	O
in	O
the	O
field	O
"	O
and	O
the	O
resultant	O
system	O
was	O
now	O
deemed	O
a	O
370/135	O
-3	O
.	O
</s>
