<s>
The	O
IBM	B-Device
System/360	I-Device
Model	I-Device
20	I-Device
is	O
the	O
smallest	O
member	O
of	O
the	O
IBM	B-Application
System/360	I-Application
family	O
announced	O
in	O
November	O
1964	O
.	O
</s>
<s>
The	O
Model	O
20	O
supports	O
only	O
a	O
subset	O
of	O
the	O
System/360	B-Application
instruction	O
set	O
,	O
with	O
binary	O
numbers	O
limited	O
to	O
16	O
bits	O
and	O
no	O
floating	O
point	O
.	O
</s>
<s>
In	O
later	O
years	O
it	O
would	O
have	O
been	O
classified	O
as	O
a	O
16-bit	O
minicomputer	B-Architecture
rather	O
than	O
a	O
mainframe	B-Architecture
,	O
but	O
the	O
term	O
"	O
minicomputer	B-Architecture
"	O
was	O
not	O
current	O
,	O
and	O
in	O
any	O
case	O
IBM	O
wanted	O
to	O
emphasize	O
the	O
compatibility	O
of	O
the	O
Model	O
20	O
rather	O
than	O
its	O
differences	O
from	O
the	O
rest	O
of	O
the	O
System/360	B-Application
line	O
.	O
</s>
<s>
It	O
does	O
,	O
however	O
,	O
have	O
the	O
full	O
System/360	B-Application
decimal	O
instruction	O
set	O
,	O
that	O
allows	O
for	O
addition	O
,	O
subtraction	O
,	O
product	O
,	O
and	O
dividend	O
of	O
up	O
to	O
31	O
decimal	O
digits	O
.	O
</s>
<s>
Developed	O
by	O
IBM	O
in	O
Böblingen	O
,	O
Germany	O
,	O
the	O
system	O
was	O
intended	O
for	O
data	B-General_Concept
processing	I-General_Concept
and	O
as	O
a	O
replacement	O
for	O
tab	O
equipment	O
.	O
</s>
<s>
An	O
incompatible	O
small	O
computer	O
,	O
the	O
IBM	B-Device
1130	I-Device
introduced	O
the	O
following	O
year	O
,	O
was	O
designed	O
for	O
scientific	O
and	O
engineering	O
computing	O
.	O
</s>
<s>
John	O
Haanstra	O
had	O
wanted	O
a	O
low-end	O
machine	O
that	O
could	O
execute	O
IBM	B-Device
1401	I-Device
instructions	O
.	O
</s>
<s>
Fred	O
Brooks	O
wanted	O
the	O
machine	O
to	O
have	O
System/360	B-Application
architecture	O
.	O
</s>
<s>
Based	O
on	O
number	O
of	O
systems	O
sold	O
the	O
Model	O
20	O
was	O
the	O
most	O
successful	O
model	O
of	O
System/360	B-Application
.	O
</s>
<s>
As	O
in	O
other	O
models	O
of	O
System/360	B-Application
memory	O
is	O
byte-addressable	O
.	O
</s>
<s>
It	O
has	O
eight	O
16-bit	O
general	B-General_Concept
purpose	I-General_Concept
registers	I-General_Concept
numbered	O
R8	O
through	O
R15	O
which	O
can	O
be	O
used	O
in	O
computations	O
as	O
base	O
registers	O
.	O
</s>
<s>
The	O
instruction	O
set	O
is	O
a	O
subset	O
of	O
System/360	B-Application
consisting	O
of	O
37	O
instructions	O
instead	O
of	O
143	O
,	O
with	O
some	O
incompatible	O
instructions	O
,	O
such	O
as	O
a	O
BASR	O
(	O
Branch	O
And	O
Store	O
Register	O
)	O
rather	O
than	O
BALR	O
(	O
Branch	O
And	O
Link	O
Register	O
)	O
.	O
</s>
<s>
Binary	O
arithmetic	O
uses	O
the	O
S/360	B-Application
"	O
halfword	O
"	O
instructions	O
to	O
operate	O
on	O
16-bit	O
quantities	O
.	O
</s>
<s>
Decimal	O
arithmetic	O
includes	O
the	O
complete	O
S/360	B-Application
decimal	O
instruction	O
set	O
for	O
operations	O
on	O
packed	O
decimal	O
operands	O
of	O
up	O
to	O
31	O
digits	O
plus	O
sign	O
in	O
storage	O
.	O
</s>
<s>
One	O
particularly	O
unusual	O
logical	O
instruction	O
,	O
not	O
present	O
in	O
other	O
System/360	B-Application
models	O
,	O
is	O
"	O
Halt	O
and	O
Proceed	O
"	O
(	O
HPR	O
,	O
operation	O
code	O
X'99'	O
)	O
which	O
will	O
quiesce	O
input/output	O
and	O
stop	O
the	O
processor	O
displaying	O
a	O
code	O
to	O
the	O
operator	O
.	O
</s>
<s>
Branch	O
instructions	O
include	O
standard	O
S/360	B-Application
Branch	O
on	O
Condition	O
.	O
</s>
<s>
Lacking	O
I/O	B-Device
channels	I-Device
all	O
peripherals	O
are	O
directly	O
controlled	O
by	O
the	O
processor	O
.	O
</s>
<s>
Data	O
transfer	O
can	O
optionally	O
be	O
performed	O
by	O
cycle	B-General_Concept
stealing	I-General_Concept
(	O
IBM	O
documentation	O
calls	O
this	O
time-sharing	O
)	O
,	O
that	O
is	O
taking	O
processor	O
cycles	O
from	O
the	O
user	O
program	O
as	O
necessary	O
to	O
perform	O
the	O
operation	O
.	O
</s>
<s>
Like	O
most	O
other	O
models	O
of	O
System/360	B-Application
the	O
Model	O
20	O
is	O
microprogrammed	O
;	O
it	O
uses	O
transformer	B-General_Concept
read-only	I-General_Concept
storage	I-General_Concept
(	O
TROS	O
)	O
.	O
</s>
<s>
IBM	O
2415	O
Magnetic	O
Tape	O
Unit	O
with	O
up	O
to	O
6	O
tape	B-Architecture
drives	I-Architecture
.	O
</s>
<s>
IBM	O
2520	O
Card	O
Read/Punch	O
or	O
Card	B-Device
Punch	I-Device
.	O
</s>
<s>
IBM	B-Device
2311	I-Device
Disk	O
Storage	O
.	O
</s>
<s>
IBM	B-Device
1442	I-Device
Model	O
5	O
Card	B-Device
Punch	I-Device
.	O
</s>
<s>
IBM	B-Device
1403	I-Device
Line	O
Printer	O
.	O
</s>
<s>
IBM	B-Device
2501	I-Device
Card	B-Device
Reader	I-Device
.	O
</s>
<s>
Two	O
adapter	O
versions	O
were	O
available	O
,	O
one	O
for	O
Synchronous	B-Protocol
transmit-receive	I-Protocol
(	O
STR	O
)	O
and	O
one	O
for	O
binary	B-Protocol
synchronous	I-Protocol
communications	I-Protocol
(	O
Bisync	B-Protocol
)	O
.	O
</s>
<s>
Model	O
20	O
disk	O
storage	O
uses	O
IBM	B-Device
2311	I-Device
disk	O
drives	O
,	O
model	O
11	O
or	O
12	O
,	O
attached	O
to	O
an	O
integrated	O
"	O
storage	O
control	O
feature	O
"	O
on	O
the	O
CPU	O
.	O
</s>
<s>
Both	O
models	O
use	O
the	O
1316	O
disk	B-Operating_System
pack	I-Operating_System
which	O
provides	O
a	O
maximum	O
of	O
203	O
cylinders	O
with	O
10	O
tracks	B-Device
per	O
cylinder	O
.	O
</s>
<s>
The	O
2560	O
has	O
two	O
hoppers	O
into	O
which	O
punch	B-Device
cards	O
can	O
be	O
placed	O
.	O
</s>
<s>
Card	O
flow	O
is	O
:	O
a	O
read	O
station	O
,	O
a	O
punch	B-Device
station	O
,	O
and	O
on	O
the	O
A2	O
a	O
print	O
station	O
allowing	O
text	O
to	O
be	O
printed	O
on	O
cards	O
.	O
</s>
<s>
This	O
allows	O
it	O
to	O
function	O
as	O
a	O
card	B-Device
reader	I-Device
and	O
a	O
card	B-Device
punch	I-Device
.	O
</s>
<s>
Service	O
programs	O
for	O
library	O
management	O
,	O
I/O	O
device	O
table	O
maintenance	O
,	O
linkage	B-Application
editing	I-Application
,	O
and	O
for	O
loading	O
and	O
copying	O
the	O
systems	O
residence	O
disk	O
,	O
backing	O
up	O
and	O
restoring	O
other	O
disk	O
drives	O
.	O
</s>
<s>
Tape	O
Programming	O
Support	O
(	O
TPS	O
)	O
supports	O
systems	O
with	O
magnetic	B-Architecture
tape	I-Architecture
drives	I-Architecture
but	O
no	O
disk	O
drives	O
.	O
</s>
<s>
The	O
minimum	O
configuration	O
for	O
TPS	O
is	O
4096	O
bytes	O
of	O
main	O
storage	O
,	O
a	O
card	B-Device
reader	I-Device
,	O
a	O
card	B-Device
punch	I-Device
,	O
a	O
printer	O
and	O
two	O
magnetic	B-Architecture
tape	I-Architecture
drives	I-Architecture
.	O
</s>
<s>
and	O
still	O
more	O
with	O
four	O
or	O
more	O
magnetic	B-Architecture
tape	I-Architecture
drives	I-Architecture
.	O
</s>
<s>
Report	B-Language
Program	I-Language
Generator	I-Language
(	O
RPG	B-Language
)	O
.	O
</s>
<s>
PL/I	B-Language
subset	O
,	O
DPS	O
only	O
.	O
</s>
<s>
Basic	B-Device
Assembler	I-Device
versions	O
were	O
available	O
for	O
CPS	O
,	O
TPS	O
,	O
and	O
DPS	O
including	O
Input/Output	B-Application
Control	I-Application
System	I-Application
(	O
IOCS	B-Application
)	O
macros	O
and	O
processing	O
routines	O
.	O
</s>
<s>
Sort/Merge	B-Application
.	O
</s>
<s>
The	O
UNIVAC	B-Device
9200	I-Device
and	I-Device
9300	I-Device
processors	I-Device
were	O
clones	O
of	O
the	O
Model	O
20	O
.	O
</s>
<s>
Despite	O
having	O
been	O
sold	O
or	O
leased	O
in	O
very	O
large	O
numbers	O
for	O
a	O
mainframe	B-Architecture
system	O
of	O
its	O
era	O
,	O
only	O
a	O
few	O
of	O
System/360	B-Application
Model	O
20	O
computers	O
remain	O
.	O
</s>
<s>
Two	O
Model	O
20	O
processors	O
along	O
with	O
numerous	O
peripherals	O
(	O
forming	O
at	O
least	O
one	O
complete	O
system	O
)	O
located	O
in	O
Nürnberg	O
,	O
Germany	O
were	O
purchased	O
on	O
eBay	B-Application
in	O
April/May	O
2019	O
for	O
€3710	O
by	O
two	O
UK	O
enthusiasts	O
who	O
,	O
over	O
the	O
course	O
of	O
some	O
months	O
,	O
moved	O
the	O
machine	O
to	O
Creslow	O
Park	O
in	O
Buckinghamshire	O
,	O
United	O
Kingdom	O
.	O
</s>
<s>
A	O
running	O
list	O
of	O
remaining	O
System/360s	B-Application
can	O
be	O
found	O
at	O
.	O
</s>
