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+.\"	$OpenBSD: spl.9,v 1.1 1997/05/29 15:20:49 kstailey Exp $
+.\"	$NetBSD: spl.9,v 1.1 1997/03/11 06:15:05 mikel Exp $
+.\"
+.\" Copyright (c) 1997 Michael Long.
+.\" Copyright (c) 1997 Jonathan Stone.
+.\" All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\" 3. All advertising materials mentioning features or use of this software
+.\"    must display the following acknowledgement:
+.\"        This product includes software developed by Michael Long.
+.\" 4. The name of the author may not be used to endorse or promote products
+.\"    derived from this software without specific prior written permission.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+.\"
+.Dd March 11, 1997
+.Dt SPL 9
+.Os OpenBSD
+.Sh NAME
+.Nm spl
+.Nd modify system interrupt priority level
+.Sh SYNOPSIS
+.Fd #include <machine/intr.h>
+.Ft int
+.Fn splhigh void
+.Ft int
+.Fn splsched void
+.Ft int
+.Fn splserial void
+.Ft int
+.Fn splclock void
+.Ft int
+.Fn splstatclock void
+.Ft int
+.Fn splimp void
+.Ft int
+.Fn spltty void
+.Ft int
+.Fn splsoftserial void
+.Ft int
+.Fn splnet void
+.Ft int
+.Fn splbio void
+.Ft int
+.Fn splsoftnet void
+.Ft int
+.Fn splsoftclock void
+.Ft int
+.Fn spl0 void
+.Ft void
+.Fn splx "int s"
+.Sh DESCRIPTION
+These functions raise and lower the system priority level.
+They are used by kernel code running at any given priority level to
+block higher-priority interrupts, so that it can safely access
+variables or data structures which are used by kernel code that runs
+at a higher priority level.
+.Pp
+A
+.Nm
+function exists for each distinct priority level which can exist in
+the system.  These macros and the corresponding priority levels are
+used for various defined purposes, and may be divided into two main
+types: hard and soft.  Hard interrupts are generated by hardware
+devices, while soft interrupts are generated by callouts and called
+from the kernel's periodic timer interrupt service routine.
+.Pp
+In order of highest to lowest priority, the priority-raising macros
+are:
+.Bl -tag -width splsoftserialXX
+.It Fn splhigh
+blocks all hard and soft interrupts.  It is used for code that cannot
+tolerate any interrupts, like hardware context switching code and 
+the
+.Xr ddb 4
+in-kernel debugger.
+.It Fn splserial
+blocks hard interrupts from serial interfaces.  Code running at this
+level may not access the tty subsystem.
+.It Fn splsched
+blocks interrupts that may access scheduler data structures.  Code
+running at or above this level may not call
+.Fn sleep ,
+.Fn tsleep ,
+or
+.Fn wakeup ,
+nor may it post signals.
+.It Fn splclock
+blocks the hardware clock interrupt.  It is used by
+.Fn hardclock
+to update kernel and process times, and must be used by any other code
+that accesses time-related data.
+.It Fn splstatclock
+blocks the hardware statistics clock interrupt.  It is used by
+.Fn statclock
+to update kernel profiling and other statistics, and must be used by
+any code that accesses that data.
+This level is identical to
+.Fn splclock
+if there is no separate statistics clock.
+.It Fn splimp
+blocks hard interrupts from all devices that are allowed to use the
+kernel
+.Xr malloc 9 .
+That includes all disk, network, and tty device interrupts.
+.It Fn spltty
+blocks hard interrupts from TTY devices.
+.It Fn splsoftserial
+blocks soft interrupts generated by serial devices.
+.It Fn splnet
+blocks hard interrupts from network interfaces.
+.It Fn splbio
+blocks hard interrupts from disks and other mass-storage devices.
+.It Fn splsoftnet
+blocks soft network interrupts.
+.El
+.Pp
+Two macros lower the system priority level.  They are:
+.Bl -tag -width splsoftclockXX
+.It Fn splsoftclock
+unblocks all interrupts but the soft clock interrupt.
+.It Fn spl0
+unblocks all interrupts.
+.El
+.Pp
+The
+.Fn splx
+macro restores the system priority level to the one encoded in
+.Fa s ,
+which must be a value previously returned by one of the other
+.Nm
+macros.