<s>
Systems	B-Application
programming	I-Application
,	O
or	O
system	B-Application
programming	I-Application
,	O
is	O
the	O
activity	O
of	O
programming	O
computer	O
system	B-Application
software	I-Application
.	O
</s>
<s>
The	O
primary	O
distinguishing	O
characteristic	O
of	O
systems	B-Application
programming	I-Application
when	O
compared	O
to	O
application	B-General_Concept
programming	I-General_Concept
is	O
that	O
application	B-General_Concept
programming	I-General_Concept
aims	O
to	O
produce	O
software	O
which	O
provides	O
services	O
to	O
the	O
user	O
directly	O
(	O
e.g.	O
</s>
<s>
word	B-General_Concept
processor	I-General_Concept
)	O
,	O
whereas	O
systems	B-Application
programming	I-Application
aims	O
to	O
produce	O
software	O
and	O
software	B-Device
platforms	I-Device
which	O
provide	O
services	O
to	O
other	O
software	O
,	O
are	O
performance	O
constrained	O
,	O
or	O
both	O
(	O
e.g.	O
</s>
<s>
operating	B-General_Concept
systems	I-General_Concept
,	O
computational	O
science	O
applications	O
,	O
game	O
engines	O
,	O
industrial	O
automation	O
,	O
and	O
software	B-Architecture
as	I-Architecture
a	I-Architecture
service	I-Architecture
applications	O
)	O
.	O
</s>
<s>
Systems	B-Application
programming	I-Application
requires	O
a	O
great	O
degree	O
of	O
hardware	O
awareness	O
.	O
</s>
<s>
The	O
following	O
attributes	O
characterize	O
systems	B-Application
programming	I-Application
:	O
</s>
<s>
The	O
programmer	B-Application
can	O
make	O
assumptions	O
about	O
the	O
hardware	O
and	O
other	O
properties	O
of	O
the	O
system	O
that	O
the	O
program	O
runs	O
on	O
,	O
and	O
will	O
often	O
exploit	O
those	O
properties	O
,	O
for	O
example	O
by	O
using	O
an	O
algorithm	O
that	O
is	O
known	O
to	O
be	O
efficient	O
when	O
used	O
with	O
specific	O
hardware	O
.	O
</s>
<s>
Usually	O
a	O
low-level	B-Language
programming	I-Language
language	I-Language
or	O
programming	O
language	O
dialect	O
is	O
used	O
so	O
that	O
:	O
</s>
<s>
Often	O
systems	O
programs	O
cannot	O
be	O
run	O
in	O
a	O
debugger	B-Application
.	O
</s>
<s>
Running	O
the	O
program	O
in	O
a	O
simulated	B-Application
environment	I-Application
can	O
sometimes	O
be	O
used	O
to	O
reduce	O
this	O
problem	O
.	O
</s>
<s>
Systems	B-Application
programming	I-Application
is	O
sufficiently	O
different	O
from	O
application	B-General_Concept
programming	I-General_Concept
that	O
programmers	B-Application
tend	O
to	O
specialize	O
in	O
one	O
or	O
the	O
other	O
.	O
</s>
<s>
In	O
systems	B-Application
programming	I-Application
,	O
often	O
limited	O
programming	O
facilities	O
are	O
available	O
.	O
</s>
<s>
The	O
use	O
of	O
automatic	B-General_Concept
garbage	I-General_Concept
collection	I-General_Concept
is	O
not	O
common	O
and	O
debugging	O
is	O
sometimes	O
hard	O
to	O
do	O
.	O
</s>
<s>
The	O
runtime	B-Library
library	I-Library
,	O
if	O
available	O
at	O
all	O
,	O
is	O
usually	O
far	O
less	O
powerful	O
,	O
and	O
does	O
less	O
error	O
checking	O
.	O
</s>
<s>
Because	O
of	O
those	O
limitations	O
,	O
monitoring	B-Application
and	O
logging	O
are	O
often	O
used	O
;	O
operating	B-General_Concept
systems	I-General_Concept
may	O
have	O
extremely	O
elaborate	O
logging	O
subsystems	O
.	O
</s>
<s>
Implementing	O
certain	O
parts	O
in	O
operating	B-General_Concept
systems	I-General_Concept
and	O
networking	O
requires	O
systems	B-Application
programming	I-Application
,	O
for	O
example	O
implementing	O
paging	O
(	O
virtual	B-Architecture
memory	I-Architecture
)	O
or	O
a	O
device	B-Application
driver	I-Application
for	O
an	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Originally	O
systems	B-Application
programmers	I-Application
invariably	O
wrote	O
in	O
assembly	B-Language
language	I-Language
.	O
</s>
<s>
Experiments	O
with	O
hardware	O
support	O
in	O
high	B-Language
level	I-Language
languages	I-Language
in	O
the	O
late	O
1960s	O
led	O
to	O
such	O
languages	O
as	O
PL/S	B-Language
,	O
BLISS	B-Language
,	O
BCPL	B-Language
,	O
and	O
extended	O
ALGOL	B-Language
for	O
Burroughs	B-Device
large	I-Device
systems	I-Device
.	O
</s>
<s>
Forth	B-Application
also	O
has	O
applications	O
as	O
a	O
systems	O
language	O
.	O
</s>
<s>
In	O
the	O
1970s	O
,	O
C	B-Language
became	O
widespread	O
,	O
aided	O
by	O
the	O
growth	O
of	O
Unix	B-Application
.	O
</s>
<s>
More	O
recently	O
a	O
subset	O
of	O
C++	B-Language
called	O
Embedded	B-Language
C++	I-Language
has	O
seen	O
some	O
use	O
,	O
for	O
instance	O
it	O
is	O
used	O
in	O
the	O
I/O	O
Kit	O
drivers	O
of	O
macOS	B-Application
.	O
</s>
<s>
In	O
2015	O
Rust	B-Application
came	O
out	O
,	O
a	O
new	O
systems	B-Language
programming	I-Language
language	I-Language
as	O
performant	O
as	O
C	B-Language
and	O
C++	B-Language
,	O
with	O
memory	O
safety	O
in	O
mind	O
.	O
</s>
<s>
For	O
historical	O
reasons	O
,	O
some	O
organizations	O
use	O
the	O
term	O
systems	B-Application
programmer	I-Application
to	O
describe	O
a	O
job	O
function	O
which	O
would	O
be	O
more	O
accurately	O
termed	O
systems	B-Application
administrator	I-Application
.	O
</s>
<s>
This	O
is	O
particularly	O
true	O
in	O
organizations	O
whose	O
computer	O
resources	O
have	O
historically	O
been	O
dominated	O
by	O
mainframes	B-Architecture
,	O
although	O
the	O
term	O
is	O
even	O
used	O
to	O
describe	O
job	O
functions	O
which	O
do	O
not	O
involve	O
mainframes	B-Architecture
.	O
</s>
<s>
This	O
usage	O
arose	O
because	O
administration	O
of	O
IBM	B-Device
mainframes	I-Device
often	O
involved	O
the	O
writing	O
of	O
custom	O
assembler	B-Language
code	I-Language
(	O
IBM	B-Device
's	I-Device
Basic	I-Device
Assembly	I-Device
Language	I-Device
(	O
BAL	O
)	O
)	O
,	O
which	O
integrated	O
with	O
the	O
operating	B-General_Concept
system	I-General_Concept
such	O
as	O
OS/MVS	B-Application
,	O
DOS/VSE	B-Application
or	O
VM/CMS	B-Application
.	O
</s>
<s>
This	O
type	O
of	O
programming	O
is	O
progressively	O
less	O
common	O
,	O
but	O
the	O
term	O
systems	B-Application
programmer	I-Application
is	O
still	O
the	O
de	O
facto	O
job	O
title	O
for	O
staff	O
directly	O
administering	O
IBM	B-Device
mainframes	I-Device
.	O
</s>
