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lzsabi.tex
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% © Copyright IBM Corporation 2001, 2024
% © Copyright Linux Foundation 2002
%
% SPDX-License-Identifier: GFDL-1.1-no-invariants-only
%
% s390/s390x ABI specification -- LaTeX source
%
% Permission is granted to copy, distribute and/or modify this document
% under the terms of the GNU Free Documentation License, Version 1.1; with
% no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
% Texts.
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\newcommand{\aARCH}{a \ARCH}
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% ------------------------------------------------------------
\begin{document}
\newcommand{\myTitle}{ELF Application Binary Interface \ABINAME{}~Supplement}
\begin{DIFnomarkup}
\title{\myTitle}
\subtitle{Version \Version}
\author{Martin Schwidefsky\and Ulrich Weigand\and Andreas Arnez%
\and Andreas Krebbel}
\publishers{IBM\textregistered{} Corporation}
\lowertitleback{%
\noindent \textbf{\myTitle}
\noindent Version \Version
\noindent © Copyright IBM Corporation 2001, 2024
\noindent © Copyright Linux Foundation 2002
\medskip
\noindent Permission is granted to copy, distribute and/or modify
this document under the terms of the GNU Free Documentation License,
Version 1.1; with no Invariant Sections, with no Front-Cover Texts,
and with no Back-Cover Texts. A copy of the license is included in
the section entitled ``GNU Free Documentation License.''}
\maketitle
\end{DIFnomarkup}
\tableofcontents
\listoffigures
\listoftables
\lstlistoflistings
\chapter*{About This Book}
The \ABINAME{} supplement to the Executable and Linkage
Format Application Binary Interface (or ELF ABI) defines a system
interface for compiled application programs. Its purpose is to
establish a standard binary interface for application programs on
Linux\textregistered{} for \ARCH{}\textregistered{} systems.
This book is a supplement to the generic ``System V Application Binary
Interface'' and should be read in conjunction with it.
\section*{History}
\begin{description}
\item[1.6.1] Minor release to pick up a few fixes. Published at
\url{https://github.com/ibm/s390x-abi}, January 2024.
\item[1.6] Describe vector types and registers (based on input from
Andreas Krebbel); mention condition code and program mask; enhance
exception handling information. Published at
\url{https://github.com/ibm/s390x-abi}, November 2021.
\item[1.5] \emph{``ELF Application Binary Interface
\ABINAME{}~Supplement''} -- Conversion to \LaTeX{}; various
corrections to revision~1.02. Edited by Andreas Arnez. Published at
\url{https://github.com/ibm/s390x-abi}, January~2021.
\item[1.02] \emph{``{\ifzseries zSeries\else S/390\fi} ELF Application
Binary Interface Supplement''} -- Revised edition. Published under
the GNU Free Documentation License~1.1 by The Linux Foundation as a
``referenced specification'' at \url{http://refspecs.linuxbase.org/},
November 2002.
\item[1.0] \emph{``LINUX for {\ifzseries zSeries\else S/390\fi}: ELF
Application Binary Interface Supplement''} -- First edition.
Published by IBM as LNUX-1107-{\ifzseries 01\else 02\fi}, March 2001.
\end{description}
\chapter{Low-Level System Information}
\section{Machine Interface}
This section describes the processor-specific information for
\ARCH{} processors.
\subsection{Processor Architecture}
\index{processor architecture}
\index{instruction set}
{\ifzseries
\cite{sa22} (SA22-7832) defines \theARCH{}.
\else
\cite{sa22} (SA22-7201) defines \theARCH{}.
\fi}
% TODO: Explain micro-architecture levels
% - "based on editions of the z/Architecture"
Programs intended to execute directly on the processor use the
\ARCH{} instruction set and the
instruction encoding and semantics of the architecture.
An application program can assume that all instructions defined by the
architecture and that are neither privileged nor optional, exist and work
as documented.
To be ABI conforming, the processor must implement the instructions of
the architecture, perform the specified operations, and produce the
expected results. The ABI neither places performance constraints on
systems nor specifies what instructions must be implemented in
hardware. A software emulation of the architecture can conform to
the ABI\@.
{\ifzseries\else
There are some instructions in \ARCHarch{}
which are described as ``optional.'' This ABI
requires some of these to be available; in particular:
\begin{itemize}
\item additional floating-point facilities, % checksum instruction
\item compare and move extended,
\item immediate and relative instructions,
\item square root,
\item string instructions.
\end{itemize}
The ABI guarantees that these instructions are present. In order to
comply with the ABI the operating system must emulate these
instructions on machines which do not support them in the hardware.
Other instructions are not available in some current models; programs
using these instructions do not conform to the \ABINAME{} ABI and
executing them on machines without the extra capabilities will result
in undefined behavior. \fi}
In \ARCHarch{} a
processor runs in big-endian mode. (See \cref{byteordering}.)
\subsection{Data Representation}
\subsubsection{Byte Ordering}
\label{byteordering}
\index{byte ordering}
The architecture defines an 8-bit byte\index{byte},
a 16-bit halfword\index{halfword},
a 32-bit word\index{word},{\ifzseries\else{} and\fi}
a 64-bit doubleword\index{doubleword}{\ifzseries ,
and a 128-bit quadword\index{quadword}\fi}.
Byte ordering defines how the bytes that make up halfwords,
words,{\ifzseries\else{} and\fi} doublewords{\ifzseries , and
quadwords\fi} are ordered in memory. Most significant byte (MSB)
ordering, also called ``big-endian,''\index{big-endian} means that
the most significant byte
of a structure is located in the lowest addressed byte position in a
storage unit (byte 0). By contrast, least significant byte (LSB)
ordering, or ``little-endian,''\index{little-endian} refers to the
reverse byte order, where
the lowest addressed byte position holds the least significant byte.
\Crefrange{fig:halfword}{\ifzseries fig:quadwords\else
fig:doublewords\fi} illustrate the conventions for bit and byte
numbering within storage units of various widths. These conventions
apply to both integer data and floating-point data, where the most
significant byte of a floating-point value holds the sign and the
exponent (or at least the start of the exponent). The figures show
big-endian byte numbers in the upper left corners and bit numbers in
the lower corners.
\begin{figure}
\centering
\ifSkipTikZ
\begin{verbatim}
+-------------+-------------+
| 0 | 1 |
| msb | lsb |
| 0 7 | 8 15 |
+-------------+-------------+
\end{verbatim}
\else
\begin{tikzpicture}[x=1.3ex,y=3em]
\path [bitchart box] (0, 0) rectangle (16, 1);
\bitchartfields 0 8/\textrm{msb}/, 16/\textrm{lsb}/;
\bitchartbytes{0}{0,1}
\end{tikzpicture}
\fi
\caption{Bit and byte numbering in halfwords}
\label{fig:halfword}
\end{figure}
\begin{figure}
\centering
\ifSkipTikZ
\begin{verbatim}
+-------------+-------------+-------------+-------------+
| 0 | 1 | 2 | 3 |
| msb | | | lsb |
| 0 7 | 8 15 | 16 23 | 24 31 |
+-------------+-------------+-------------+-------------+
\end{verbatim}
\else
\begin{tikzpicture}[x=1.3ex,y=3em]
\path [bitchart box] (0, 0) rectangle (32, 1);
\bitchartfields 0 8/\textrm{msb}/, 16/~/, 24/~/, 32/\textrm{lsb}/;
\bitchartbytes{0}{0,...,3}
\end{tikzpicture}
\fi
\caption{Bit and byte numbering in words}
\label{fig:words}
\end{figure}
\begin{figure}
\centering
\ifSkipTikZ
\begin{verbatim}
+-------------+-------------+-------------+-------------+
| 0 | 1 | 2 | 3 |
| msb | | | |
| 0 7 | 8 15 | 16 23 | 24 31 |
+-------------+-------------+-------------+-------------+
| 4 | 5 | 6 | 7 |
| | | | lsb |
| 32 39 | 40 47 | 48 55 | 56 63 |
+-------------+-------------+-------------+-------------+
\end{verbatim}
\else
\begin{tikzpicture}[x=1.3ex,y=3em]
\path [bitchart box] (0, -1) rectangle (32, 1);
\bitchartfields 0 8/\textrm{msb}/, 16/~/, 24/~/, 32/~/;
\bitchartbytes{0}{0,...,3}
\bitchartsep
\begin{scope}[shift={(0,-1)}]
\bitchartfields 32 40/~/, 48/~/, 56/~/, 64/\textrm{lsb}/;
\bitchartbytes{4}{4,...,7}
\end{scope}
\end{tikzpicture}
\fi
\caption{Bit and byte numbering in doublewords}
\label{fig:doublewords}
\end{figure}
\ifzseries
\begin{figure}
\centering
\ifSkipTikZ
\begin{verbatim}
+-------------+-------------+-------------+-------------+
| 0 | 1 | 2 | 3 |
| msb | | | |
| 0 7 | 8 15 | 16 23 | 24 31 |
+-------------+-------------+-------------+-------------+
| 4 | 5 | 6 | 7 |
| | | | |
| 32 39 | 40 47 | 48 55 | 56 63 |
+-------------+-------------+-------------+-------------+
| 8 | 9 | 10 | 11 |
| | | | |
| 64 71 | 72 79 | 80 87 | 88 95 |
+-------------+-------------+-------------+-------------+
| 12 | 13 | 14 | 15 |
| | | | lsb |
| 96 103 | 104 111 | 112 119 | 120 127 |
+-------------+-------------+-------------+-------------+
\end{verbatim}
\else
\begin{tikzpicture}[x=1.3ex,y=3.2em]
\path [bitchart box] (0, -3) rectangle (32, 1);
\bitchartfields 0 8/\textrm{msb}/, 16/~/, 24/~/, 32/~/;
\bitchartbytes{0}{0,...,3}
\bitchartsep
\begin{scope}[shift={(0,-1)}]
\bitchartfields 32 40/~/, 48/~/, 56/~/, 64/~/;
\bitchartbytes{4}{4,...,7}
\bitchartsep
\end{scope}
\begin{scope}[shift={(0,-2)}]
\bitchartfields 64 72/~/, 80/~/, 88/~/, 96/~/;
\bitchartbytes{8}{8,...,11}
\bitchartsep
\end{scope}
\begin{scope}[shift={(0,-3)}]
\bitchartfields 96 104/~/, 112/~/, 120/~/, 128/\textrm{lsb}/;
\bitchartbytes{12}{12,...,15}
\end{scope}
\end{tikzpicture}
\fi
\caption{Bit and byte numbering in quadwords}
\label{fig:quadwords}
\end{figure}
\fi
\subsubsection{Fundamental Types}
\index{type!scalar}
\Cref{tab:scalar} shows how ISO C scalar types correspond to those of
\aARCH{} processor. To comply with this ABI, objects stored in memory
must be aligned\index{alignment!scalar} as indicated, even though the
architecture permits unaligned storage operands for most instructions.
For all types, a null pointer\index{null pointer} has the value zero (binary).
A Boolean\index{Boolean} object is represented in memory as a single byte
with a value of 0 or 1. If a byte with any other value is evaluated as a
Boolean, the behavior is undefined.
For each binary floating-point type, there is a corresponding complex
type\index{complex type}. It is represented as a two-element array with
the real part as its first and the imaginary part as its second element.
Some C dialects permit enumeration constants that exceed the range of an
\texttt{int}. Then the enumeration type\index{enumeration type} shall be
encoded as the smallest unsigned or signed C integer type that can
represent all of its enumeration constants and is not smaller than
\texttt{int}.
\begin{table}
\centering
\begin{DIFnomarkup}
\begin{threeparttable}
\begin{tabularx}{\textwidth}{XlrrX}
\toprule
\multirow{2}{*}{Type}
& \multirow{2}{*}{ISO C}
& Size in & Align- & \ARCH{} \\
&
& bytes & ment & type \\
\midrule
\multirow{8}{\hsize}{Unsigned integer}
& \texttt{\_Bool} & 1 & 1
& \multirow{8}{\hsize}{$n$-bit unsigned binary integer\tnote{\dagger}}
\\
& \texttt{unsigned char} & 1 & 1 \\
& \texttt{char} & 1 & 1 \\
& \texttt{unsigned short} & 2 & 2 \\
& \texttt{unsigned int} & 4 & 4 \\
& \texttt{unsigned long} & \NBYTES{} & \NBYTES{} \\
& \texttt{unsigned long long} & 8 & 8 \\
& \texttt{unsigned \_\_int128}\tnote{\dagger\dagger} & 16 & 8 \\
\midrule
\multirow{12}{\hsize}{Signed integer}
& \texttt{signed char} & 1 & 1
& \multirow{12}{\hsize}{$n$-bit signed binary integer\tnote{\dagger}}
\\
& \texttt{signed short} & 2 & 2 \\
& \texttt{short} & 2 & 2 \\
& \texttt{signed int} & 4 & 4 \\
& \texttt{int} & 4 & 4 \\
& \texttt{enum} & 4 & 4 \\
& \texttt{signed long} & \NBYTES{} & \NBYTES{} \\
& \texttt{long} & \NBYTES{} & \NBYTES{} \\
& \texttt{signed long long} & 8 & 8 \\
& \texttt{long long} & 8 & 8 \\
& \texttt{\_\_int128}\tnote{\dagger\dagger} & 16 & 8 \\
& \texttt{signed \_\_int128}\tnote{\dagger\dagger} & 16 & 8 \\
\midrule
\multirow{2}{\hsize}{Pointer}
& \textit{any-type}\texttt{ *} & \NBYTES{} & \NBYTES{}
& \multirow{2}{\hsize}{\ADDRBITS{}-bit address} \\
& \textit{any-type}\texttt{ (*) ()} & \NBYTES{} & \NBYTES{} \\
\midrule
\multirow{3}{\hsize}{Binary floating-point} &
\texttt{float} & 4 & 4 & short BFP \\
& \texttt{double} & 8 & 8 & long BFP \\
& \texttt{long double} & 16 & 8 & extended BFP \\
\midrule
\multirow{3}{\hsize}{Decimal floating-point} &
\texttt{\_Decimal32}\tnote{\dagger\dagger} & 4 & 4 & short DFP \\
& \texttt{\_Decimal64}\tnote{\dagger\dagger} & 8 & 8 & long DFP \\
& \texttt{\_Decimal128}\tnote{\dagger\dagger} & 16 & 8 & extended DFP \\
\bottomrule
\end{tabularx}
\medskip
\begin{tablenotes}
\item [\dagger] Here $n$ denotes the bit size, which equals the byte
size multiplied by 8.
\item [\dagger\dagger] These types are an extension to C (ISO/IEC
9899:2011).
\end{tablenotes}
\end{threeparttable}
\end{DIFnomarkup}
\caption{Scalar types}
\label{tab:scalar}
\end{table}
\subsubsection{Aggregates and Unions}
Aggregates\index{aggregate}\index{type!aggregate}
(structures\index{structure} and arrays\index{array}) and
unions\index{union}\index{type!union} assume the
alignment\index{alignment!aggregate or union} of their most strictly
aligned component---that is, the component with the largest alignment.
The size\index{size!aggregate or union} of any object, including
aggregates and unions, is always a multiple of the alignment of the
object. An array uses the same alignment as its elements. Structure and
union objects may require padding to meet these size and alignment
constraints:
\begin{itemize}
\item An entire structure or union object is aligned on the same
boundary as its most strictly aligned member.
\item Each member is assigned to the lowest available offset with the
appropriate alignment. This may require internal
padding\index{padding}, depending on the previous member.
\item If necessary, a structure's size is increased to make it a
multiple of the structure's alignment. This may require tail padding
if the last member does not end on the appropriate boundary.
\end{itemize}
In the examples shown in \crefrange{fig:struct1}{fig:struct5},
member byte offsets (for the big-endian implementation) appear in the
upper left corners.
\begin{figure}
\centering
\ifSkipTikZ
\begin{verbatim}
Byte aligned, sizeof is 1
+-------------+
struct { | 0 |
char c; | |
}; | c |
+-------------+
\end{verbatim}
\else
\begin{tabular}{>{\texttt\bgroup}l<{\texttt\egroup}}
struct \{\\
~~~~char c;\\
\};\\
\end{tabular}
\quad
\begin{tikzpicture}[baseline=(mid),x=1.3ex,y=3em]
\path [bitchart box] (0, 0) rectangle (8, 1);
\bytechartfields 0 1/c/;
\path (current bounding box.center) coordinate (mid) {};
\path (current bounding box.north)
node [above] {Byte aligned, \texttt{sizeof} is 1};
\end{tikzpicture}
\fi
\caption{Structure smaller than a word}
\label{fig:struct1}
\end{figure}
\begin{figure}
\centering
\ifSkipTikZ
\begin{verbatim}
Word aligned, sizeof is 8
+-------------+-------------+---------------------------+
struct { | 0 | 1 | 2 |
char c; | | | |
char d; | c | d | s |
short s; |-------------+-------------+---------------------------|
int n; | 4 |
}; | |
| n |
+-------------------------------------------------------+
\end{verbatim}
\else
\begin{tabular}{>{\texttt\bgroup}l<{\texttt\egroup}}
struct \{\\
~~~~char c;\\
~~~~char d;\\
~~~~short s;\\
~~~~{\ifzseries int\else long\fi} n;\\
\};\\
\end{tabular}
\quad
\begin{tikzpicture}[baseline=(mid),x=1.3ex,y=3em]
\path [bitchart box] (0, -1) rectangle (32, 1);
\bytechartfields 0 1/c/, 2/d/, 4/s/;
\bitchartsep
\begin{scope}[shift={(0,-1)}]
\bytechartfields 4 8/n/;
\end{scope}
\path (current bounding box.center) coordinate (mid) {};
\path (current bounding box.north)
node [above] {Word aligned, \texttt{sizeof} is 8};
\end{tikzpicture}
\fi
\caption{No padding}
\label{fig:struct2}
\end{figure}
\begin{figure}
\centering
\ifSkipTikZ
\begin{verbatim}
+-------------+-------------+---------------------------+
struct { | 0 | 1 | 2 |
char c; | | | |
short s; | c | pad | s |
}; +-------------+-------------+---------------------------+
\end{verbatim}
\else
\begin{tabular}{>{\texttt\bgroup}l<{\texttt\egroup}}
struct \{\\
~~~~char c;\\
~~~~short s;\\
\};\\
\end{tabular}
\quad
\begin{tikzpicture}[baseline=(mid),x=1.3ex,y=3em]
\path [bitchart box] (0, 0) rectangle (32, 1);
\bytechartfields 0 1/c/, 2//, 4/s/;
\path (current bounding box.center) coordinate (mid) {};
\path (current bounding box.north)
node [above] {Halfword aligned, \texttt{sizeof} is 4};
\end{tikzpicture}
\fi
\caption{Internal padding}
\label{fig:struct3}
\end{figure}
\begin{figure}
\centering
\ifSkipTikZ
\begin{verbatim}
Doubleword aligned, sizeof is 24
+-------------+-----------------------------------------+
| 0 | 1 |
| | |
| c | pad |
|-------------+-----------------------------------------|
| 4 |
| |
| pad |
|-------------------------------------------------------|
struct { | 8 |
char c; | |
double d; | d |
short s; |-------------------------------------------------------|
}; | 12 |
| |
| d |
|---------------------------+---------------------------|
| 16 | 18 |
| | |
| s | pad |
|---------------------------+---------------------------|
| 20 |
| |
| pad |
+-------------------------------------------------------+
\end{verbatim}
\else
\begin{tabular}{>{\texttt\bgroup}l<{\texttt\egroup}}
struct \{\\
~~~~char c;\\
~~~~double d;\\
~~~~short s;\\
\};\\
\end{tabular}
\quad
\begin{tikzpicture}[baseline=(mid),x=1.3ex,y=2.5em]
\path [bitchart box] (0, -5) rectangle (32, 1);
\bytechartfields 0 1/c/, 4//;
\bitchartsep
\begin{scope}[shift={(0,-1)}]
\bytechartfields 4 8//;
\bitchartsep
\end{scope}
\begin{scope}[shift={(0,-2)}]
\bytechartfields 8 12/d/;
\bitchartsep
\end{scope}
\begin{scope}[shift={(0,-3)}]
\bytechartfields 12 16/d/;
\bitchartsep
\end{scope}
\begin{scope}[shift={(0,-4)}]
\bytechartfields 16 18/s/, 20//;
\bitchartsep
\end{scope}
\begin{scope}[shift={(0,-5)}]
\bytechartfields 20 24//;
\bitchartsep
\end{scope}
\path (current bounding box.center) coordinate (mid) {};
\path (current bounding box.north)
node [above] {Doubleword aligned, \texttt{sizeof} is 24};
\end{tikzpicture}
\fi
\caption{Internal and tail padding}
\label{fig:struct4}
\end{figure}
\begin{figure}
\centering
\ifSkipTikZ
\begin{verbatim}
Word aligned, sizeof is 4
+-------------+-----------------------------------------+
| 0 | 1 |
| | |
| c | pad |
+-------------+-----------------------------------------+
union { +---------------------------+---------------------------+
char c; | 0 | 2 |
short s; | | |
int j; | s | pad |
}; +---------------------------+---------------------------+
+-------------------------------------------------------+
| 0 |
| |
| j |
+-------------------------------------------------------+
\end{verbatim}
\else
\begin{tabular}{>{\texttt\bgroup}l<{\texttt\egroup}}
union \{\\
~~~~char c;\\
~~~~short s;\\
~~~~int j;\\
\};\\
\end{tabular}
\quad
\begin{tikzpicture}[baseline=(mid),x=1.3ex,y=2.5em]
\path [bitchart box] (0, 0) rectangle (32, 1);
\bytechartfields 0 1/c/, 4//;
\begin{scope}[shift={(0,-1.2)}]
\path [bitchart box] (0, 0) rectangle (32, 1);
\bytechartfields 0 2/s/, 4//;
\end{scope}
\begin{scope}[shift={(0,-2.4)}]
\path [bitchart box] (0, 0) rectangle (32, 1);
\bytechartfields 0 4/j/;
\end{scope}
\path (current bounding box.south west) coordinate (sw) {}
(current bounding box.north west) coordinate (nw) {};
\path (current bounding box.center) coordinate (mid) {};
\path (current bounding box.north)
node [above] {Word aligned, \texttt{sizeof} is 4};
\path (nw) +(-1em,0) coordinate (nw) {};
\draw [decorate,decoration=brace, thick] (sw) + (-1em,0) -- (nw);
\end{tikzpicture}
\fi
\caption{Union padding}
\label{fig:struct5}
\end{figure}
\subsubsection{Bit-Fields}
C struct and union definitions may have ``bit-fields,'' defining
integral objects with a specified number of bits
(see \cref{tab:bitfields}).
\begin{table}
\centering
\begin{DIFnomarkup}
\begin{tabular}[t]{lcr@{~\ldots~}l}
\toprule
Bit-field type & Width $n$ & \multicolumn{2}{c}{Range} \\
\midrule
\texttt{signed char} & \multirow{3}{*}{1…8} & $-2^{n-1}$ & $2^{n-1}-1$ \\
\texttt{char} & & $0$ & $2^n-1$ \\
\texttt{unsigned char} & & $0$ & $2^n-1$ \\
\midrule
\texttt{signed short} & \multirow{3}{*}{1…16} & -$2^{n-1}$ & $2^{n-1}-1$ \\
\texttt{short} & & -$2^{n-1}$ & $2^{n-1}-1$ \\
\texttt{unsigned short} & & $0$ & $2^n-1$ \\
\midrule
\texttt{signed int} & \multirow{\ifzseries 3\else 6\fi}{*}{1…32} &
$-2^{n-1}$ & $2^{n-1}-1$ \\
\texttt{int} & & $-2^{n-1}$ & $2^{n-1}-1$ \\
\texttt{unsigned int} & & $0$ & $2^n-1$ \\
\ifzseries \midrule \fi
\texttt{signed long} & {\ifzseries\multirow{3}{*}{1…64}\fi} &
$-2^{n-1}$ & $2^{n-1}-1$ \\
\texttt{long} & & $-2^{n-1}$ & $2^{n-1}-1$ \\
\texttt{unsigned long} & & $0$ & 2$^n-1$ \\
\midrule
\texttt{signed long long} & \multirow{3}{*}{1…64} & $-2^{n-1}$ & $2^{n-1}-1$ \\
\texttt{long long} & & $-2^{n-1}$ & $2^{n-1}-1$ \\
\texttt{unsigned long long} & & $0$ & $2^n-1$ \\
\bottomrule
\end{tabular}
\end{DIFnomarkup}
\caption{Bit-fields}
\label{tab:bitfields}
\end{table}
Bit-fields\index{bit-field} have the signedness of their underlying type.
For example, a bit-field of type \texttt{long} is signed, whereas a
bit-field of type \texttt{char} is unsigned.
Bit-fields obey the same size and alignment rules as other structure and
union members, with the following additions:
\begin{itemize}
\item Bit-fields are allocated from left to right (most to least
significant).
\item A bit-field must entirely reside in a storage unit appropriate
for its declared type. Thus, a bit-field never crosses its unit
boundary.
\item Bit-fields must share a storage unit with other structure and
union members (either bit-field or non-bit-field) if and only if
there is sufficient space within the storage unit.
\item Unnamed bit-fields' types do not affect the alignment of a
structure or union, although an individual bit-field's member
offsets obey the alignment constraints. An unnamed, zero-width
bit-field shall prevent any further member, bit-field or other, from
residing in the storage unit corresponding to the type of the
zero-width bit-field.
\end{itemize}
The examples in \crefrange{fig:bitnum}{fig:unnbitf} show
structure and union member byte offsets in the upper left corners.
Bit numbers appear in the lower corners.
\begin{figure}
\centering