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diff --git a/introduction.tex b/introduction.tex index 979881e..a4ac01d 100644 --- a/introduction.tex +++ b/introduction.tex @@ -157,5 +157,46 @@ in little-endian byte order. in big-endian byte order. \end{description} +Some of the fields to be defined in this specification don't +start or don't end on a byte boundary. Such fields are called bit-fields. +A set of bit-fields is always a sub-division of an integer typed field. + +Bit-fields within integer fields are always listed in order, +from the least significant to the most significant bit. The +bit-fields are considered unsigned integers of the specified +width with the next in significance relationship of the bits +preserved. + +For example: +\begin{lstlisting} +struct S { + be16 { + A : 15; + B : 1; + } x; + be16 y; +}; +\end{lstlisting} +documents the value A stored in the low 15 bit of \field{x} and +the value B stored in the high bit of \field{x}, the 16-bit +integer \field{x} in turn stored using the big-endian byte order +at the beginning of the structure S, +and being followed immediately by an unsigned integer \field{y} +stored in big-endian byte order at an offset of 2 bytes (16 bits) +from the beginning of the structure. + +Note that this notation somewhat resembles the C bitfield syntax but +should not be naively converted to a bitfield notation for portable +code: it 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. + +Assuming that CPU_TO_BE16 converts a 16-bit integer from a native +CPU to the big-endian byte order, the following is the equivalent +portable C code to generate a value to be stored into \field{x}: +\begin{lstlisting} +CPU_TO_BE16(B << 15 | A) +\end{lstlisting} + \newpage |