xref: /netbsd-src/share/man/man3/bits.3 (revision a5d6f6fd6aaa10abeee5065d9fc11b1326b78d15)

.\"	$NetBSD: bits.3,v 1.22 2022/01/22 09:22:41 wiz Exp $
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.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 .