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Subject: Re: [PATCH] introduction: document bitfield notation
On Mon, Mar 05, 2018 at 03:20:35PM +0100, Cornelia Huck wrote: > On Wed, 28 Feb 2018 21:16:32 +0200 > "Michael S. Tsirkin" <mst@redhat.com> wrote: > > > Bitfields are a useful and familiar way to specify sub-byte structure > > layout. The only issue is that bitfield order isn't portable across > > architectures. Document that we list bitfields from least to > > most significant one, and warn about portability issues. > > > > Signed-off-by: Michael S. Tsirkin <mst@redhat.com> > > --- > > introduction.tex | 18 ++++++++++++++++++ > > 1 file changed, 18 insertions(+) > > > > diff --git a/introduction.tex b/introduction.tex > > index 979881e..3cb7a70 100644 > > --- a/introduction.tex > > +++ b/introduction.tex > > @@ -157,5 +157,23 @@ in little-endian byte order. > > in big-endian byte order. > > \end{description} > > > > +When documenting sub-byte data fields, C-like bitfield notation > > +is used. Fields within an integer are always listed in order, > > +from the least significant to the most significant bit. > > + > > +For example: > > +\begin{lstlisting} > > +be16 A : 15; > > +be16 B : 1; > > +\end{lstlisting} > > +documents the value A stored in the low 15 bit of a 16 bit > > +integer and the value B stored in the high bit of the 16 bit > > +integer, the integer in turn using the big-endian byte order. > > + > > +Note that this notation typically matches the way bitfields are > > +packed by C compilers on little-endian architectures but not the > > +way bitfields are packed by C compilers on big-endian > > +architectures. > > + > > \newpage > > > > I must admit that this explanation confuses me a bit. What it is saying is that this is equivalent to CPU_TO_BE16(B << 15 | A) Maybe adding this part will clarify things? > Would some kind > of graphic representation be more helpful? I'm not good at graphics :) > For example, on s390 I would expect the structure to look like the > following: > > |0 .. 14 | 15 | > | A | B | > > If you included another example for little-endian byte order, this > would clear up things more, I think. It's BE so I think it's | 15 |14 .. 0 | | B | A |
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