Spl (Unix)

2023-10-05T18:14:22Z

2601:1C0:7182:E730:AB3:3566:DD74:D9A: Typo fix

{{Short description|Unix interrupt priority control commands}}
{{lowercase}}
'''spl''' (short for '''set priority level''', after the [[PDP-11]] [[Assembly language|assembler]] instruction of the same name<ref name="smp">{{citation|last1=Lehey|first1=Greg|title=Improving the FreeBSD SMP implementation|url=http://www.lemis.com/grog/SMPng/USENIX/|date=2001|access-date=11 May 2018}}</ref>) is the name for a collection of [[Unix]] kernel [[Subroutine|routines]] or [[Macro (computer science)|macros]] used to change the [[interrupt priority level]].<ref name="man page">{{cite web|title=spl(9) - OpenBSD manual pages|url=https://man.openbsd.org/spl.9|access-date=11 May 2018}}</ref><ref name="lions p.43">{{cite book|last1=Lions|first1=John|title=[[Lions' Commentary on UNIX 6th Edition, with Source Code]]|page=43|date=1976}}</ref> This was historically needed to synchronize [[critical section]]s of kernel code that should not be interrupted.<ref>{{cite book|last1=Lions|first1=John|title=[[Lions' Commentary on UNIX 6th Edition, with Source Code]]|page=41|date=1976}}</ref> Newer Unix variants which support [[symmetric multiprocessing]] now mostly use [[Lock (computer science)|mutexes]] for this purpose, which is a more general solution, so multiple processors can execute kernel code at the same time.<ref>{{cite book|last1=McKusick|first1=Marshall Kirk|collaboration=authors|title=The Design and Implementation of the FreeBSD Operating System|publisher=Addison-Wesley|page=93|date=2004}}</ref><ref name="smp" />

On older PDP-11 versions of Unix, there were eight of these routines, ranging from <code>spl0</code> to <code>spl7</code>, each corresponding to one PDP-11 interrupt priority level,<ref name="lions p.43" /> in addition to <code>splx</code>, which restores a previous priority level (returned by one of the other routines).<ref name="man page" /> On [[Berkeley Software Distribution|BSD Unix]] and its derivatives, these are called <code>splhigh</code>, <code>splserial</code>, <code>splsched</code>, <code>splclock</code>, <code>splstatclock</code>, <code>splvm</code>, <code>spltty</code>, <code>splsofttty</code>, <code>splnet</code>, <code>splbio</code>, <code>splsoftnet</code>, <code>splsoftclock</code>, <code>spllowersoftclock</code>, <code>spl0</code>, and <code>splx</code>.<ref name="man page" />

{{As of|2019|03}}, the spl family of primitives is still heavily used in [[OpenBSD]]<ref name="o/net/if"/> and [[NetBSD]],<ref name="n/net/if"/> which is evidenced by the plentiful calls to <code>splnet()</code> within the networking code;<ref name="o/net/if"/><ref name="n/net/if"/> whereas [[FreeBSD]] and [[DragonFly BSD]] use more modern concepts; for example, in DragonFly, [[Serializing tokens|LWKT tokens]] may be used in place of spl.

==See also==
* [[IRQL_(Windows)|IRQL]]
* [[Interrupt]]
* [[Interrupt priority level]]
* [[Programmable Interrupt Controller]]

==References==
{{reflist
|refs =

<ref name = "o/net/if" >{{cite web
|url = http://bxr.su/o/sys/net/if.c
|title = /sys/net/if.c
|website = BSD Cross Reference
|publisher = [[OpenBSD]]
|quote = s = splnet();
|date = 2019-03-01
|access-date = 2019-03-05
}}</ref>

<ref name = "n/net/if" >{{cite web
|url = http://bxr.su/n/sys/net/if.c
|title = /sys/net/if.c
|website = BSD Cross Reference
|publisher = [[NetBSD]]
|quote = s = splnet();
|date = 2019-03-01
|access-date = 2019-03-05
}}</ref>

}}

{{OpenBSD}}
{{NetBSD}}

[[Category:Interrupts]]
[[Category:Unix]]

{{Unix-stub}}

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Spl (Unix)