<s>
spl	B-Operating_System
(	O
short	O
for	O
set	O
priority	O
level	O
,	O
after	O
the	O
PDP-11	B-Device
assembler	B-Language
instruction	O
of	O
the	O
same	O
name	O
)	O
is	O
the	O
name	O
for	O
a	O
collection	O
of	O
Unix	B-Application
kernel	O
routines	O
or	O
macros	O
used	O
to	O
change	O
the	O
interrupt	B-Operating_System
priority	I-Operating_System
level	I-Operating_System
.	O
</s>
<s>
This	O
was	O
historically	O
needed	O
to	O
synchronize	O
critical	B-Operating_System
sections	I-Operating_System
of	O
kernel	O
code	O
that	O
should	O
not	O
be	O
interrupted	O
.	O
</s>
<s>
Newer	O
Unix	B-Application
variants	O
which	O
support	O
symmetric	B-Operating_System
multiprocessing	I-Operating_System
now	O
mostly	O
use	O
mutexes	B-Operating_System
for	O
this	O
purpose	O
,	O
which	O
is	O
a	O
more	O
general	O
solution	O
,	O
so	O
multiple	O
processors	O
can	O
execute	O
kernel	O
code	O
at	O
the	O
same	O
time	O
.	O
</s>
<s>
On	O
older	O
PDP-11	B-Device
versions	O
of	O
Unix	B-Application
,	O
there	O
were	O
eight	O
of	O
these	O
routines	O
,	O
ranging	O
from	O
spl0	O
to	O
spl7	O
,	O
each	O
corresponding	O
to	O
one	O
PDP-11	B-Device
interrupt	B-Operating_System
priority	I-Operating_System
level	I-Operating_System
,	O
in	O
addition	O
to	O
splx	O
,	O
which	O
restores	O
a	O
previous	O
priority	O
level	O
(	O
returned	O
by	O
one	O
of	O
the	O
other	O
routines	O
)	O
.	O
</s>
<s>
On	O
BSD	B-Operating_System
Unix	I-Operating_System
and	O
its	O
derivatives	O
,	O
these	O
are	O
called	O
splhigh	O
,	O
splserial	O
,	O
splsched	O
,	O
splclock	O
,	O
splstatclock	O
,	O
splvm	O
,	O
spltty	O
,	O
splsofttty	O
,	O
splnet	O
,	O
splbio	O
,	O
splsoftnet	O
,	O
splsoftclock	O
,	O
spllowersoftclock	O
,	O
spl0	O
,	O
and	O
splx	O
.	O
</s>
<s>
,	O
the	O
spl	B-Operating_System
family	O
of	O
primitives	O
is	O
still	O
heavily	O
used	O
in	O
OpenBSD	B-Operating_System
and	O
NetBSD	B-Device
,	O
which	O
is	O
evidenced	O
by	O
the	O
plentiful	O
of	O
calls	O
to	O
splnet( )	O
within	O
the	O
networking	O
code	O
;	O
whereas	O
FreeBSD	B-Operating_System
and	O
DragonFly	B-Application
BSD	I-Application
use	O
more	O
modern	O
concepts	O
;	O
for	O
example	O
,	O
in	O
DragonFly	O
,	O
LWKT	B-Operating_System
tokens	I-Operating_System
may	O
be	O
used	O
in	O
place	O
of	O
spl	B-Operating_System
.	O
</s>
