<s>
DUCS	B-Application
(	O
Display	O
Unit	O
Control	O
System	O
)	O
was	O
a	O
teleprocessing	B-General_Concept
monitor	O
from	O
CFS	O
Inc	O
.	O
</s>
<s>
It	O
was	O
one	O
of	O
two	O
early	O
local	O
teleprocessing	B-General_Concept
packages	O
for	O
IBM	O
's	O
DOS/VSE	B-Application
environment	O
.	O
</s>
<s>
DUCS	B-Application
provided	O
an	O
interface	B-Application
and	O
access	B-Application
method	I-Application
for	O
programmers	O
to	O
'	O
talk	O
 '	O
to	O
monitors	O
.	O
</s>
<s>
Such	O
access	B-Application
methods	I-Application
later	O
became	O
known	O
as	O
APIs	B-Application
.	O
</s>
<s>
Initially	O
written	O
for	O
the	O
IBM	B-Device
2260	I-Device
running	O
under	O
DOS	O
on	O
IBM	O
mainframes	B-Architecture
,	O
the	O
original	O
product	O
was	O
free	O
for	O
IBM	O
users	O
.	O
</s>
<s>
With	O
the	O
advent	O
of	O
DOS/VS	B-Application
and	O
the	O
IBM	B-Operating_System
3270	I-Operating_System
series	O
terminals	O
,	O
the	O
original	O
author	O
commercialized	O
the	O
product	O
,	O
circa	O
1970	O
.	O
</s>
<s>
The	O
product	O
is	O
believed	O
to	O
be	O
the	O
first	O
non-IBM	O
publicly	O
available	O
commercial	O
software	B-Application
package	I-Application
to	O
transmit	O
data	O
via	O
satellite	B-Application
.	O
</s>
<s>
DUCS	B-Application
differed	O
from	O
competing	O
products	O
such	O
as	O
Westi	B-Application
and	O
IBM	O
's	O
own	O
CICS	B-Application
in	O
that	O
it	O
was	O
subordinate	O
to	O
the	O
application	B-Application
's	O
mainline	O
program	O
.	O
</s>
<s>
Westi	B-Application
,	O
for	O
example	O
,	O
was	O
the	O
mainline	O
program	O
and	O
users	O
wrote	O
subroutines	O
to	O
read	O
and	O
write	O
data	O
to	O
and	O
from	O
terminals	O
and	O
discs	O
.	O
</s>
<s>
This	O
real	O
time	O
paradigm	O
became	O
known	O
as	O
transaction	B-General_Concept
processing	I-General_Concept
.	O
</s>
<s>
DUCS	B-Application
reversed	O
that	O
model	O
in	O
that	O
it	O
was	O
,	O
in	O
fact	O
,	O
a	O
subroutine	O
package	O
that	O
read	O
from	O
and	O
wrote	O
to	O
monitors	O
,	O
both	O
local	O
and	O
remote	O
.	O
</s>
<s>
While	O
DUCS	B-Application
was	O
considerably	O
easier	O
to	O
program	O
and	O
use	O
,	O
it	O
also	O
placed	O
the	O
onus	O
of	O
task	O
management	O
upon	O
the	O
programmer	O
.	O
</s>
<s>
Correctly	O
designed	O
,	O
a	O
DUCS	B-Application
program	O
was	O
faster	O
than	O
any	O
competing	O
package	O
or	O
access	B-Application
method	I-Application
.	O
</s>
<s>
In	O
1972	O
,	O
IBM	O
released	O
DOS/VS	B-Application
with	O
the	O
IBM/370	B-Device
and	O
the	O
first	O
IBM	B-Operating_System
3270	I-Operating_System
terminals	O
,	O
and	O
CFS	O
began	O
a	O
rewrite	O
for	O
the	O
new	O
products	O
.	O
</s>
<s>
Former	O
New	O
York	O
City	O
IBMer	O
,	O
Leigh	O
Lundin	O
,	O
wrote	O
DUCS	B-Application
Remote	O
,	O
a	O
bi-sync	B-Protocol
module	O
to	O
handle	O
remote	O
teleprocessing	B-General_Concept
.	O
</s>
<s>
The	O
bi-sync	B-Protocol
handler	O
was	O
only	O
4k	O
,	O
in	O
contrast	O
to	O
IBM	O
's	O
BTAM	B-Application
at	O
28k	O
,	O
QTAM	B-Application
at	O
36k	O
,	O
and	O
TCAM	B-Application
at	O
42k	O
,	O
and	O
VTAM	B-Application
which	O
started	O
at	O
48k	O
.	O
</s>
<s>
Lundin	O
wrote	O
games	O
in	O
Fortran	B-Application
and	O
Assembler	B-Language
and	O
Goran	O
in	O
COBOL	B-Application
to	O
demonstrate	O
the	O
API	B-Application
for	O
programmers	O
.	O
</s>
<s>
To	O
model	O
IBM	O
's	O
new	O
light	B-Device
pen	I-Device
,	O
programmers	O
contributed	O
a	O
simple	O
tic-tac-toe	O
(	O
noughts	B-Application
and	I-Application
crosses	I-Application
)	O
,	O
possibly	O
the	O
only	O
practical	O
use	O
of	O
the	O
subsequently	O
discontinued	O
light	B-Device
pen	I-Device
.	O
</s>
<s>
DUCS	B-Application
was	O
sold	O
in	O
North	O
America	O
by	O
CFS	O
,	O
Inc	O
,	O
Brookline	O
,	O
Ma	O
.	O
</s>
