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+.\"	$NetBSD: bits.3,v 1.22 2022/01/22 09:22:41 wiz Exp $
+.\"
+.\" Copyright (c) 2006, 2010 David Young.  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.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY DAVID YOUNG ``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 DAVID
+.\" YOUNG 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 January 22, 2022
+.Dt BITS 3
+.Os
+.Sh NAME
+.Nm __BIT ,
+.Nm __BITS ,
+.Nm __MASK ,
+.Nm __SHIFTIN ,
+.Nm __SHIFTOUT ,
+.Nm __SHIFTOUT_MASK
+.Nd "macros for preparing bitmasks and operating on bit fields"
+.Sh SYNOPSIS
+.In sys/param.h
+.In sys/cdefs.h
+.Ft uintmax_t
+.Fn __BIT "n"
+.Ft uintmax_t
+.Fn __BITS "m" "n"
+.Ft uintmax_t
+.Fn __MASK "n"
+.Ft uintmax_t
+.Fn __SHIFTIN "v" "mask"
+.Ft uintmax_t
+.Fn __SHIFTOUT "v" "mask"
+.Ft uintmax_t
+.Fn __SHIFTOUT_MASK "mask"
+.Sh DESCRIPTION
+These macros prepare bitmasks, extract bitfields from words, and
+insert bitfields into words.
+A
+.Dq bitfield
+is a span of consecutive bits defined by a bitmask, where 1s select
+the bits in the bitfield.
+.Pp
+Use
+.Fn __BIT ,
+.Fn __BITS ,
+and
+.Fn __MASK
+to define bitmasks:
+.Bl -tag -width __BITS -offset indent
+.It Fn __BIT "n"
+Return a bitmask with bit
+.Fa n
+set, where the least significant bit is bit 0.
+.It Fn __BITS "m" "n"
+Return a bitmask with bits
+.Fa m
+through
+.Fa n ,
+inclusive, set.
+It does not matter whether
+.Fa m No > Fa n
+or
+.Fa m No <= Fa n .
+The least significant bit is bit 0.
+.It Fn __MASK "n"
+Return a bitmask with the first
+.Fa n
+bits set.
+That is, bits 0 through
+.Fa n
+- 1, inclusive, set.
+.El
+.Pp
+.Fn __SHIFTIN ,
+.Fn __SHIFTOUT ,
+and
+.Fn __SHIFTOUT_MASK
+help read and write bitfields from words:
+.Bl -tag -width __SHIFTOUT_MASK -offset indent
+.It Fn __SHIFTIN "v" "mask"
+Left-shift bits
+.Fa v
+into the bitfield defined by
+.Fa mask ,
+and return them.
+No side-effects.
+.It Fn __SHIFTOUT "v" "mask"
+Extract and return the bitfield selected by
+.Fa mask
+from
+.Fa v ,
+right-shifting the bits so that the rightmost selected bit is at
+bit 0.
+No side-effects.
+.It Fn __SHIFTOUT_MASK "mask"
+Right-shift the bits in
+.Fa mask
+so that the rightmost non-zero bit is at bit 0.
+This is useful for finding the greatest unsigned value that a
+bitfield can hold.
+No side-effects.
+Note that
+.Fn __SHIFTOUT_MASK "m"
+=
+.Fn __SHIFTOUT "m" "m" .
+.El
+.Sh EXAMPLES
+The following example demonstrates basic usage of the
+.Nm bits
+macros:
+.Bd -literal -offset indent
+uint32_t bits, mask, val;
+
+bits = __BITS(2, 3);			/* 00001100 */
+mask = __BIT(2) | __BIT(3);		/* 00001100 */
+
+val = __SHIFTIN(0x03, mask);		/* 00001100 */
+val = __SHIFTOUT(0xf, mask);		/* 00000011 */
+.Ed
+.Sh SEE ALSO
+.Xr bitops 3 ,
+.Xr cdefs 3
+.Sh HISTORY
+The
+.Nm bits
+macros first appeared in
+.Xr atw 4 ,
+with different names and implementation.
+In their current form these macros appeared in
+.Nx 4.0 .
+.Sh AUTHORS
+The
+.Nm bits
+macros were written by
+.An David Young Aq Mt dyoung@NetBSD.org .
+.An Matt Thomas Aq Mt matt@NetBSD.org
+suggested important improvements to the implementation, and
+contributed the macro names
+.Fn SHIFTIN
+and
+.Fn SHIFTOUT .