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chap4-elf32abi.sgml
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<!-- Author(s): See <authorgroup> in book-elfabi.sgml -->
<!-- Copyright (c) 2011, Power.org -->
<!-- Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3; with
no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
Texts. A copy of the license is available in the COPYING.txt file. -->
<chapter id="OBJ-FILES"><title>Object Files</title>
<sect1 CONDITION="ATR-VLE" id="ELF-Object-Files-VLE">
<title>EABI Executable and Linking Format (ELF) Object Files</title>
<para>Implementations supporting VLE mark a per-page TLB entry storage control bit to indicate that a
memory page holds either VLE Category <emphasis role="bold">or</emphasis> Embedded Category
instructions. In this way the instructions in both the VLE category (<citetitle>Book VLE</citetitle>)
and the Embedded Category (<citetitle>Book III-E</citetitle>) of the Power ISA can coexist in the same
ELF binary.</para>
<para>Binding of VLE Category and Embedded Category memory pages to different memory bounds
requires separation of VLE Category and Embedded Category encodings into different ELF sections,
allowing easy identification for defining memory management page tables for run-time environments.
Memory pages of VLE Category and Embedded Category instructions can be freely intermixed. </para>
<para>The VLE encodings also require additional relocation types (see relocations 216 - 233 in
<emphasis><xref linkend="RELOCATION-TABLE"></emphasis>), which allow the link editor to resolve
immediate and branch displacement fields in the instruction encoding once a symbol or label address is
known (at link time).</para>
</sect1>
<sect1 CONDITION="ATR-EABI" id="OBJECT-FILE-PROCESSING-EABI">
<title>EABI Object File Processing</title>
<para>An EABI-conforming link editor shall accept as input EABI-conforming and SVR4-conforming relocatable files, and it
shall produce EABI-conforming shared object files.</para>
</sect1>
<sect1 id="ELF-HEAD">
<title>ELF Header</title>
<para>The <emphasis>file class</emphasis> member of the ELF header
identification array, <varname>e_ident[EI_CLASS]</varname>,
identifies the ELF file as 32-bit encoded by holding the value 1, defined as class
<varname>ELFCLASS32</varname>.</para>
<para>For a big-endian encoded ELF file the <emphasis>data
encoding</emphasis> member of the ELF header identification array,
<varname>e_ident[EI_DATA]</varname>, holds the value
<varname>2</varname>, defined as data encoding <varname>ELFDATA2MSB</varname>. For a little-endian encoded ELF
file it holds the value 1, defined as
data encoding <varname>ELFDATA2LSB</varname>.</para>
<para>The ELF header <varname>e_flags</varname> member may
hold the following bit masks that are applicable on the Power Architecture.</para>
<table frame="none"><title>e_flags Bit Masks</title>
<tgroup cols='3' colsep='0' rowsep='0'>
<colspec colwidth='140' colname='c1' align="left">
<colspec colwidth='60' colname='c2' align="left">
<colspec colwidth='200' colname='c3' align="left">
<thead>
<row rowsep='1'>
<entry>Mask</entry>
<entry>Value</entry>
<entry>Description</entry>
</row>
</thead>
<tbody>
<row>
<entry>EF_PPC_EMB</entry>
<entry>0x80000000</entry>
<entry>Power Architecture Embedded Flag.</entry>
</row>
<row>
<entry>EF_PPC_RELOCATABLE_LIB</entry>
<entry>0x00008000</entry>
<entry> Mark ELF file as relocatable (containing Position Independent
Code, see <emphasis><xref linkend="ADDRESSING-MODELS"></emphasis>) and
intended for use in a library. </entry>
</row>
<row>
<entry>EF_PPC_RELOCATABLE</entry>
<entry>0x00010000</entry>
<entry>Mark ELF file as relocatable (containing Position Independent
Code, see <emphasis><xref linkend="ADDRESSING-MODELS"></emphasis>).</entry>
</row>
</tbody>
</tgroup>
</table>
<!--
<para>An executable file (or shared library) is marked <varname>EF_PPC_RELOCATABLE_LIB</varname> if all
of the modules in the executable file have this flag.</para>
<para>Restrictive Text: A module with the <varname>EF_PPC_RELOCATABLE</varname> flag may only be linked
with modules which have the <varname>EF_PPC_RELOCATABLE</varname> or
<varname>EF_PPC_RELOCATABLE_LIB</varname> flags.</para>
<para>Permissive Text: An executable (or shared library) is marked <varname>EF_PPC_RELOCATABLE</varname>
if all of the modules in the executable have either the <varname>EF_PPC_RELOCATABLE</varname> or
<varname>EF_PPC_RELOCATABLE_LIB</varname> flags, but at least one module does not have the
<varname>EF_PPC_RELOCATABLE_LIB</varname> flag.</para>
<para>Modules which are marked <varname>EF_PPC_RELOCATABLE_LIB</varname> may be linked with either
relocatable or nonrelocatable modules.</para>
-->
<para CONDITION="ATR-EABI">EABI-conforming ELF files shall have <varname>EF_PPC_EMB</varname> set in
the <varname>e_flags</varname> member.</para>
<para>The ELF header <varname>e_machine</varname> member identifies the architecture of the ELF file
as the Power Architecture by holding the value 20, defined as machine name
<varname>EM_PPC</varname>.</para>
</sect1>
<sect1 id="SPECIAL-SECTIONS"><title>Special Sections</title>
<para>For the Power Architecture the following special sections with
their corresponding section types and attributes apply:</para>
<!-- <screen>
Name Type Attributes
.got SHT_PROGBITS SHF_ALLOC + SHF_WRITE
.sdata SHT_PROGBITS SHF_ALLOC + SHF_WRITE
.sbss SHT_NOBITS SHF_ALLOC + SHF_WRITE</screen><screen CONDITION="!ATR-LINUX">
.PPC.EMB.apuinfo SHT_NOTE 0 </screen><screen CONDITION="ATR-EABI">
.PPC.EMB.sdata2 SHT_PROGBITS SHF_ALLOC and possibly SHF_WRITE
(See Table 4-4)
.PPC.EMB.sdata0 SHT_PROGBITS SHF_ALLOC + SHF_WRITE
.PPC.EMB.sbss2 SHT_NOBITS SHF_ALLOC + SHF_WRITE
.PPC.EMB.sbss0 SHT_NOBITS SHF_ALLOC + SHF_WRITE
.PPC.EMB.seginfo SHT_PROGBITS 0</screen>
<screen CONDITION="ATR-SECURE-PLT">
.plt SHT_PROGBITS SHF_ALLOC + SHF_WRITE
</screen>
<screen CONDITION="ATR-BSS-PLT">
.plt SHT_NOBITS SHF_ALLOC + SHF_WRITE + SHF_EXECINSTR
</screen>-->
<variablelist id="ELF-SPECIAL-SECTIONS">
<varlistentry>
<term><emphasis role="bold">.got</emphasis></term>
<listitem>
<para> This section holds the <emphasis><link linkend="GLOBAL-OFFSET-TABLE">Global Offset Table</link></emphasis> (GOT). Further information on accessing data in the GOT is contained in <emphasis><xref linkend="DATA-OBJ"></emphasis>. Information on the layout of
the Global Offset Table is in <emphasis><xref linkend="GLOBAL-OFFSET-TABLE"></emphasis>.</para>
<informaltable frame="none">
<tgroup cols='2' colsep='0' rowsep='0'>
<colspec colwidth='60' colname='c1' align="left">
<colspec colwidth='250' colname='c2' align="left">
<thead>
<row rowsep='1'>
<entry>Name</entry>
<entry>Value</entry>
</row>
</thead>
<tbody>
<row>
<entry>sh_name</entry>
<entry>.got</entry>
</row>
<row>
<entry>sh_type</entry>
<entry>SHT_PROGBITS</entry>
</row>
<row>
<entry>sh_flags</entry>
<entry>SHF_ALLOC + SHF_WRITE</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</listitem>
</varlistentry>
<varlistentry>
<term><emphasis role="bold">.plt</emphasis></term>
<listitem>
<para> This section holds the <emphasis><link linkend="PLT">Procedure Linkage Table</link></emphasis> (PLT) (see
<emphasis><xref linkend="PLT"></emphasis>).</para>
<informaltable frame="none" CONDITION="ATR-SECURE-PLT">
<tgroup cols='2' colsep='0' rowsep='0'>
<colspec colwidth='60' colname='c1' align="left">
<colspec colwidth='250' colname='c2' align="left">
<thead>
<row rowsep='1'>
<entry>Name</entry>
<entry>Value</entry>
</row>
</thead>
<tbody>
<row>
<entry>sh_name</entry>
<entry>.plt</entry>
</row>
<row>
<entry>sh_type</entry>
<entry>SHT_PROGBITS</entry>
</row>
<row>
<entry>sh_flags</entry>
<entry>SHF_ALLOC + SHF_WRITE</entry>
</row>
</tbody>
</tgroup>
</informaltable>
<informaltable frame="none" CONDITION="ATR-BSS-PLT">
<tgroup cols='2' colsep='0' rowsep='0'>
<colspec colwidth='60' colname='c1' align="left">
<colspec colwidth='250' colname='c2' align="left">
<thead>
<row rowsep='1'>
<entry>Name</entry>
<entry>Value</entry>
</row>
</thead>
<tbody>
<row>
<entry>sh_name</entry>
<entry>.plt</entry>
</row>
<row>
<entry>sh_type</entry>
<entry>SHT_NOBITS</entry>
</row>
<row>
<entry>sh_flags</entry>
<entry>SHF_ALLOC + SHF_WRITE + SHF_EXECINSTR</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</listitem>
</varlistentry>
<varlistentry>
<term><emphasis role="bold">.sdata</emphasis></term>
<listitem>
<para></para><!-- necessary 'cause stylesheets don't conditionalize the first para. -->
<para CONDITION="ATR-LINUX"> Initialized data can be held in this section, which is part of
the <emphasis><link linkend="SMALL-DATA-AREA">Small Data Area</link></emphasis> (SDA). Further information is found in <emphasis><xref linkend="SMALL-DATA-AREA"></emphasis>.</para>
<para CONDITION="ATR-EABI"> Initialized data can be held in this section, which is part of
the <emphasis><link linkend="SDATA-AND-SBSS-E500">Small Data Area</link></emphasis> (SDA). Further information is found in <emphasis><xref linkend="SDATA-AND-SBSS-E500"></emphasis>.</para>
<informaltable frame="none">
<tgroup cols='2' colsep='0' rowsep='0'>
<colspec colwidth='60' colname='c1' align="left">
<colspec colwidth='250' colname='c2' align="left">
<thead>
<row rowsep='1'>
<entry>Name</entry>
<entry>Value</entry>
</row>
</thead>
<tbody>
<row>
<entry>sh_name</entry>
<entry>.sdata</entry>
</row>
<row>
<entry>sh_type</entry>
<entry>SHT_PROGBITS</entry>
</row>
<row>
<entry>sh_flags</entry>
<entry>SHF_ALLOC + SHF_WRITE</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</listitem>
</varlistentry>
<varlistentry>
<term><emphasis role="bold">.sbss</emphasis></term>
<listitem>
<para></para> <!-- necessary 'cause stylesheets don't conditionalize the first para. -->
<para CONDITION="ATR-LINUX"> Uninitialized data (set to zero on program
execution) can be held in this section, which is part of the SDA (Small Data
Area). Further information is found in <emphasis><xref linkend="SMALL-DATA-AREA"></emphasis>.</para>
<para CONDITION="ATR-EABI"> Uninitialized data (set to zero on program
execution) can be held in this section, which is part of the SDA (Small Data
Area). Further information is found in <emphasis><xref linkend="SDATA-AND-SBSS-E500"></emphasis>.</para>
<informaltable frame="none">
<tgroup cols='2' colsep='0' rowsep='0'>
<colspec colwidth='60' colname='c1' align="left">
<colspec colwidth='250' colname='c2' align="left">
<thead>
<row rowsep='1'>
<entry>Name</entry>
<entry>Value</entry>
</row>
</thead>
<tbody>
<row>
<entry>sh_name</entry>
<entry>.sbss</entry>
</row>
<row>
<entry>sh_type</entry>
<entry>SHT_NOBITS</entry>
</row>
<row>
<entry>sh_flags</entry>
<entry>SHF_ALLOC + SHF_WRITE</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</listitem>
</varlistentry>
<varlistentry>
<term><emphasis role="bold">.PPC.EMB.apuinfo</emphasis></term>
<listitem>
<para> If an APU is required this section will contain records describing which are required for a
program to execute properly. See <emphasis><xref linkend="APU-INFORMATION-SECTION"></emphasis> for further details. </para>
<informaltable frame="none">
<tgroup cols='2' colsep='0' rowsep='0'>
<colspec colwidth='60' colname='c1' align="left">
<colspec colwidth='250' colname='c2' align="left">
<thead>
<row rowsep='1'>
<entry>Name</entry>
<entry>Value</entry>
</row>
</thead>
<tbody>
<row>
<entry>sh_name</entry>
<entry>.PPC.EMB.apuinfo</entry>
</row>
<row>
<entry>sh_type</entry>
<entry>SHT_NOTES</entry>
</row>
<row>
<entry>sh_flags</entry>
<entry>0</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</listitem>
</varlistentry>
</variablelist>
</sect1>
<sect1 CONDITION="ATR-EABI" id="SPECIAL-SECTIONS-EABI">
<title>Special Embedded Sections</title>
<para>In addition to the special sections described in <emphasis><xref linkend="SPECIAL-SECTIONS"></emphasis>, an
EABI-conforming ELF file shall be allowed to contain the following special
sections. The SVR4 ABI has reserved for this document any
section names beginning with .PPC.EMB.</para>
<variablelist id="ELF-SPECIAL-EMBEDDED-SECTIONS">
<varlistentry>
<term><emphasis role="bold">.PPC.EMB.sdata2</emphasis></term>
<listitem>
<para> This section holds initialized read-only small data that contributes
to the program memory image. The section can, however, be used to hold
writable data.</para>
<para>If a link editor creates a .PPC.EMB.sdata2 section that
combines a .PPC.EMB.sdata2 section whose sh_flags is SHF_ALLOC with a
.PPC.EMB.sdata2 section whose <varname>sh_flags</varname> is SHF_ALLOC + SHF_WRITE, then
the resulting .PPC.EMB.sdata2 section's <varname>sh_flags</varname> value shall be
SHF_ALLOC + SHF_WRITE. See <emphasis><xref linkend="SDATA2-AND-SBSS2-E500"></emphasis>
for more details.</para>
<informaltable frame="none">
<tgroup cols='2' colsep='0' rowsep='0'>
<colspec colwidth='60' colname='c1' align="left">
<colspec colwidth='250' colname='c2' align="left">
<thead>
<row rowsep='1'>
<entry>Name</entry>
<entry>Value</entry>
</row>
</thead>
<tbody>
<row>
<entry>sh_name</entry>
<entry>.PPC.EMB.sdata2</entry>
</row>
<row>
<entry>sh_type</entry>
<entry>SHT_PROGBITS</entry>
</row>
<row>
<entry>sh_flags</entry>
<entry>SHF_ALLOC or</entry>
</row>
<row>
<entry></entry>
<entry> or</entry>
</row>
<row>
<entry></entry>
<entry>SHF_ALLOC + SHF_WRITE</entry>
</row>
<row>
<entry>sh_link</entry>
<entry>SHF_UNDEF</entry>
</row>
<row>
<entry>sh_addralign</entry>
<entry>Maximum alignment required by any data item in .PPC.EMB.sdata2</entry>
</row>
<row>
<entry>sh_info</entry>
<entry>0</entry>
</row>
<row>
<entry>sh_entsize</entry>
<entry>0</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</listitem>
</varlistentry>
<varlistentry>
<term><emphasis role="bold">.PPC.EMB.sbss2</emphasis></term>
<listitem>
<para> The special section .PPC.EMB.sbss2 is intended to hold writable
small data that contribute to the program memory image and whose initial
values are 0. See <emphasis><xref linkend="SDATA2-AND-SBSS2-E500"></emphasis>
for more details.</para>
<informaltable frame="none">
<tgroup cols='2' colsep='0' rowsep='0'>
<colspec colwidth='60' colname='c1' align="left">
<colspec colwidth='250' colname='c2' align="left">
<thead>
<row rowsep='1'>
<entry>Name</entry>
<entry>Value</entry>
</row>
</thead>
<tbody>
<row>
<entry>sh_name</entry>
<entry>.PPC.EMB.sbss2</entry>
</row>
<row>
<entry>sh_type</entry>
<entry>SHT_NOBITS</entry>
</row>
<row>
<entry>sh_flags</entry>
<entry>SHF_ALLOC + SHF_WRITE</entry>
</row>
<row>
<entry>sh_link</entry>
<entry>SHF_UNDEF</entry>
</row>
<row>
<entry>sh_addralign</entry>
<entry>Maximum alignment required by any data item in .PPC.EMB.sbss2.</entry>
</row>
<row>
<entry>sh_info</entry>
<entry>0</entry>
</row>
<row>
<entry>sh_entsize</entry>
<entry>0</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</listitem>
</varlistentry>
<varlistentry>
<term><emphasis role="bold">.PPC.EMB.sdata0</emphasis></term>
<listitem>
<para> This section is intended to hold initialized small data that
contribute to the program memory image and whose addresses are all
within a 16-bit signed offset of address 0. See <emphasis><xref linkend="SDATA0-AND-SBSS0-E500"></emphasis>
for more details.</para>
<informaltable frame="none">
<tgroup cols='2' colsep='0' rowsep='0'>
<colspec colwidth='60' colname='c1' align="left">
<colspec colwidth='250' colname='c2' align="left">
<thead>
<row rowsep='1'>
<entry>Name</entry>
<entry>Value</entry>
</row>
</thead>
<tbody>
<row>
<entry>sh_name</entry>
<entry>.PPC.EMB.sdata0</entry>
</row>
<row>
<entry>sh_type</entry>
<entry>SHT_PROGBITS</entry>
</row>
<row>
<entry>sh_flags</entry>
<entry>SHF_ALLOC + SHF_WRITE</entry>
</row>
<row>
<entry>sh_link</entry>
<entry>SHF_UNDEF</entry>
</row>
<row>
<entry>sh_addralign</entry>
<entry>Maximum alignment required by any data item in .PPC.EMB.sdata0</entry>
</row>
<row>
<entry>sh_info</entry>
<entry>0</entry>
</row>
<row>
<entry>sh_entsize</entry>
<entry>0</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</listitem>
</varlistentry>
<varlistentry>
<term><emphasis role="bold">.PPC.EMB.sbss0</emphasis></term>
<listitem>
<para> This section is intended to hold small data that contribute to the
program memory image, whose addresses are all within a 16-bit signed offset of
address 0, and whose initial values are 0. See
<emphasis><xref linkend="SDATA0-AND-SBSS0-E500"></emphasis> for further
details.</para>
<informaltable frame="none">
<tgroup cols='2' colsep='0' rowsep='0'>
<colspec colwidth='60' colname='c1' align="left">
<colspec colwidth='250' colname='c2' align="left">
<thead>
<row rowsep='1'>
<entry>Name</entry>
<entry>Value</entry>
</row>
</thead>
<tbody>
<row>
<entry>sh_name</entry>
<entry>.PPC.EMB.sbss0</entry>
</row>
<row>
<entry>sh_type</entry>
<entry>SHT_NOBITS</entry>
</row>
<row>
<entry>sh_flags</entry>
<entry>SHF_ALLOC + SHF_WRITE</entry>
</row>
<row>
<entry>sh_link</entry>
<entry>SHF_UNDEF</entry>
</row>
<row>
<entry>sh_addralign</entry>
<entry>Maximum alignment required by any data item in .PPC.EMB.sbss0.</entry>
</row>
<row>
<entry>sh_info</entry>
<entry>0</entry>
</row>
<row>
<entry>sh_entsize</entry>
<entry>0</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</listitem>
</varlistentry>
<varlistentry>
<term><emphasis role="bold">.PPC.EMB.seginfo</emphasis></term>
<listitem>
<para> The special section .PPC.EMB.seginfo provides a means of naming
and providing additional information about ELF segments (which are described by
ELF program header table entries). A file shall contain at most one section
named .PPC.EMB.seginfo. See <emphasis><xref linkend="ROM-COPY-SEGMENT-INFORMATION-SECTION"></emphasis> for more details.</para>
<informaltable frame="none">
<tgroup cols='2' colsep='0' rowsep='0'>
<colspec colwidth='60' colname='c1' align="left">
<colspec colwidth='250' colname='c2' align="left">
<thead>
<row rowsep='1'>
<entry>Name</entry>
<entry>Value</entry>
</row>
</thead>
<tbody>
<row>
<entry>sh_name</entry>
<entry>.PPC.EMB.seginfo</entry>
</row>
<row>
<entry>sh_type</entry>
<entry>SHT_PROGBITS</entry>
</row>
<row>
<entry>sh_flags</entry>
<entry>0</entry>
</row>
<row>
<entry>sh_link</entry>
<entry>SHF_UNDEF</entry>
</row>
<row>
<entry></entry>
<entry> or</entry>
</row>
<row>
<entry></entry>
<entry>The section header table index of a section of type SHT_STRTAB whose string table contains the null terminated names to which entries in .PPC.EMB.seginfo refer.</entry>
</row>
<row>
<entry>sh_addr</entry>
<entry>0</entry>
</row>
<row>
<entry>sh_addralign</entry>
<entry>0</entry>
</row>
<row>
<entry>sh_info</entry>
<entry>0</entry>
</row>
<row>
<entry>sh_entsize</entry>
<entry>12</entry>
</row>
</tbody>
</tgroup>
</informaltable>
</listitem>
</varlistentry>
</variablelist>
</sect1>
<sect1 id="SYMBOL-TABLE">
<title>Symbol Table</title>
<sect2 id="SYMBOL-VALUES"><title>Symbol Values</title>
<para>An executable file that contains a symbol reference that is to be resolved dynamically by
an associated shared object will have a symbol table entry for that symbol. This entry will
identify the symbol as undefined by setting the <emphasis role="bold">st_shndx</emphasis> member
to <emphasis role="bold">SHN_UNDEF</emphasis>.</para>
<para>An executable file that needs to compare the value of two symbol references will have a
symbol table entry for that symbol where the <emphasis role="bold">st_value</emphasis> member is
nonzero.</para>
<para>If the <emphasis role="bold">st_value</emphasis> of an undefined symbol is nonzero, the
loader must resolve every reference to the named symbol to the same value. This insures that
all pointers to the symbol will be identical. If <emphasis role="bold">st_value</emphasis> is
zero, the loader may resolve these symbols to different values, for example, to point directly
to the symbol in some cases or into the GOT in other cases. If no PLT entry is allocated for
the symbol, then <emphasis role="bold">st_value</emphasis> is zero.</para>
<para CONDITION="ATR-SECURE-PLT">Under the Secure-PLT ABI, if a PLT entry is allocated for a
symbol reference in the executable file the value of this <emphasis
role="bold">st_value</emphasis> member is the address of an executable PLT call code stub. This
executable stub is used for branching to the virtual address held by the nonexecutable PLT
entry for the symbol. The content of the PLT entry defaults to the address of a PLT symbol
resolver stub, which will direct the dynamic linker to resolve the reference to the symbol.
Following resolution the PLT entry holds the absolute virtual address of the symbol.</para>
<para CONDITION="ATR-BSS-PLT">Under the BSS-PLT ABI this <emphasis
role="bold">st_value</emphasis> member holds the R_PPC_REL32 relocated address into the <emphasis
role="bold">.plt</emphasis> section for the PLT entry used to resolve the undefined symbol.
This PLT entry contains executable code used to dynamically resolve the address of the target
symbol. The number of instructions in this code stub varies on the distance to the
target.</para>
<para>Referencing GOT nonlocal statics is shown in <xref
linkend="SMALL-MODEL-POS-IND-LOAD-STORE"> and <xref linkend="LARGE-MODEL-POS-IND-LOAD-STORE">.
Taking the address of nonstatic function pointers is indicated by
<varname><symbol>@plt</varname>. <xref linkend="SMALL-MODEL-PIC-CALL"> and <xref
linkend="LARGE-MODEL-PIC-CALL"> demonstrate how to perform this action.</para>
</sect2>
</sect1>
<sect1 id="SMALL-DATA-AREA" CONDITION="!ATR-EABI">
<title>Small Data Area</title>
<para>The <emphasis>small data area</emphasis> resides within the <emphasis>Data segment</emphasis>. It is composed of the
.sdata and .sbss sections which contain initialized and uninitialized data items, respectively. The data items
in these sections are addressed by 16-bit signed offsets with respect to the base of the small data
area.</para>
<para>The use of small data areas for data items typically results in smaller programs and faster program execution.</para>
<para>The small data area is adjacent to the initialized and uninitialized data in the Data segment of both executables and shared objects.</para>
<para CONDITION="ATR-BSS-PLT">The typical order of sections in the Data segment (some possibly empty) under the BSS-PLT ABI is shown in <emphasis><xref linkend="BSS-PLT-SECTION-ORDERING"></emphasis>.
<figure id="BSS-PLT-SECTION-ORDERING">
<title>Section Ordering Under the BSS-PLT</title>
<screen>
.data
.got
.sdata
.sbss
.plt
.bss</screen>
</figure></para>
<para CONDITION="ATR-SECURE-PLT">Under the Secure-PLT ABI, for security reasons, the .got and .plt may be
marked read-only after relocation, which requires placing the .got and .plt with other sections that are
similarly made read-only after relocation, before sections that remain read-write as shown in
<emphasis><xref linkend="SECURE-PLT-SECTION-ORDERING"></emphasis>. If an implementation does not mark the .got and .plt sections as read-only after relocation it may still reorder the sections as indicated or it may use the section layout as described in <emphasis><xref linkend="BSS-PLT-SECTION-ORDERING"></emphasis>. See <emphasis><xref linkend="SECURE-PLT"></emphasis> for information on the Secure-PLT ABI.
<figure id="SECURE-PLT-SECTION-ORDERING">
<title>Section Ordering Under the Secure-PLT</title>
<screen>
.got
.plt
.data
.sdata
.sbss
.bss</screen>
</figure></para>
<para>The size of the small data area is limited. A data item is placed in the small data area by
a compiler that supports small data relative addressing based on its size. All data items up to a
certain specified size (with 8 bytes being the typical default size) are placed into the small data
area.</para>
<para>The link editor fails to build the executable file or shared object file if the
default or specified size for the placement of items into the small data area results in the small data
area being too large to be addressed with 16-bit relative offsets. In such a situation, recompilation
with a smaller value for the size criterion must be done.</para>
<sect2 id="SMALL-DATA-AREA-FOR-EXECUTABLES">
<title>Use of the Small Data Area in Executables</title>
<para>In the case of executable files, the small data area may contain up to 64 KB of data
items with local or global scope. The link editor defines the symbol
<varname>_SDA_BASE_</varname> (small data area base) to be an address relative to which all data
in the .sdata and .sbss sections may be addressed with 16-bit signed offsets. In case there
is not a .sdata or a .sbss section, the symbol <varname>_SDA_BASE_</varname> is defined to be
0.</para>
<para>For a data item in the .sdata or .sbss sections, a compiler may generate short-form one
instruction references. In an executable file, such a reference is relative to the address of
<varname>_SDA_BASE_</varname> symbol, which is held in the small data area pointer register,
r13.</para>
<para>At process initialization time, r13 is loaded with the
value of the symbol <varname>_SDA_BASE_</varname>. General-purpose register r13 retains this value subsequently, i.e., its
contents remain intact. </para>
</sect2>
<sect2 id="SMALL-DATA-AREA-FOR-SHARED-OBJECTS" CONDITION="ATR-BSS-PLT">
<title>Use of the Small Data Area in Shared Objects</title>
<para CONDITION="ATR-SECURE-PLT">In a shared object under the Secure-PLT ABI, addressing .sdata
and .sbss using short (16-bit) offsets is not supported and therefore using the small data area
in shared objects is not supported, which is a change from the SYSV ABI.</para>
<para>Because the small data area follows the <emphasis><link linkend="GLOBAL-OFFSET-TABLE">Global Offset Table</link></emphasis> in a shared object, the data in
the small data area can be addressed relative to the GOT pointer. For each shared object, the
symbol <varname>_SDA_BASE_</varname> shall have the same value possessed by the symbol
<varname>_GLOBAL_OFFSET_TABLE_</varname>.</para>
<para>Since the small data area pointer register, r13, holds the value of the executable file's
<varname>_SDA_BASE_</varname> symbol, a shared object may not modify r13 and should not attempt to use
it for referencing the shared object's small data area.</para>
<para>The <varname>_GLOBAL_OFFSET_TABLE_</varname> and <varname>_SDA_BASE_</varname> symbols are
relative to each shared object and therefore the small data area of a shared object may only
contain data items having local (i.e., non global) scope.</para>
<para>When <varname>_GLOBAL_OFFSET_TABLE_</varname> relative addressing is used in a shared
object to access the small data area, the size of the small data area can be 32 KB at the
maximum, although it can be less if it happens that the <emphasis><link linkend="GLOBAL-OFFSET-TABLE">Global Offset Table</link></emphasis> is large.</para>
<para>A compiler may generate short-form one instruction references relative to a register
that contains the address of the shared object's <varname>_SDA_BASE_</varname> symbol.</para>
</sect2>
<!-- <para>The small data segment of a shared object may only contain data items having local (i.e.
nonglobal) scope.</para> -->
</sect1>
<sect1 CONDITION="ATR-EABI" id="SMALL-DATA-AREA-E500">
<title>EABI Small Data Areas</title>
<para>Three distinct small data areas, each possibly containing both initialized and zero-initialized data, are supported by the Embedded
ABI, and are summarized in the following table.</para>
<table id="EABI-SMALL-DATA-AREAS-SUMMARY" frame="none"><title>EABI Small Data Areas Summary</title>
<tgroup cols='4' colsep='0' rowsep='0'>
<colspec colwidth='100' colname='c1' align="left">
<colspec colwidth='80' colname='c2' align="left">
<colspec colwidth='80' colname='c3' align="left">
<colspec colwidth='100' colname='c4' align="left">
<thead>
<row>
<entry namest="c1" rowsep='1'>Section Names</entry>
<entry namest="c2" rowsep='1'>Register or Value</entry>
<entry namest="c3" rowsep='1'>Symbol</entry>
<entry namest="c4" rowsep='1'>Shared Object Addressability?</entry>
</row>
</thead>
<tbody>
<row>
<entry namest="c1">.sdata</entry>
<entry namest="c2">r13</entry>
<entry namest="c3"><varname>_SDA_BASE_</varname></entry>
<entry namest="c4">local data only</entry>
</row>
<row>
<entry namest="c1" nameend="c4" rowsep='1'>.sbss</entry>
</row>
<row>
<entry namest="c1">.PPC.EMB.sdata2</entry>
<entry namest="c2">r2</entry>
<entry namest="c3"><varname>_SDA2_BASE_</varname></entry>
<entry namest="c4">no</entry>
</row>
<row>
<entry namest="c1" nameend="c4" rowsep='1'>.PPC.EMB.sbss2</entry>
</row>
<row>
<entry namest="c1">.PPC.EMB.sdata0</entry>
<entry namest="c2">0</entry>
<entry namest="c3">n/a</entry>
<entry namest="c4">no</entry>
</row>
<row>
<entry namest="c1" nameend="c4" rowsep='1'>.PPC.EMB.sbss0</entry>
</row>
</tbody>
</tgroup>
</table>
<para>In both shared objects and executables, the small data areas straddle the boundary between initialized and
uninitialized data in the Data segment. The usual order of sections in the Data segment, some of which may be
empty, is shown in <xref linkend="EABI-SECTION-ORDERING">.</para>
<figure id="EABI-SECTION-ORDERING"><title>Section Ordering In the EABI</title>
<screen>
.rodata
.PPC.EMB.sdata2
.PPC.EMB.sbss2
.data
.got
.sdata
.sbss
.plt
.bss</screen></figure>
<para>All three small data areas can contain at most 64 KB of data items. All areas may hold both local
and global data items in executables. In shared objects, .sdata/.sbss may only hold local data
items, and the other two areas are not permitted. These areas are not permitted to hold values that
might be changed outside of the program (that is, volatile variables). </para>
<para>Compilers may generate short-form, one-instruction references with 16-bit offsets for all data items that are in these six
sections. Placing more data items in small data areas usually results in smaller and faster program execution.</para>
<para>These areas together provide up to 192 KB of data items that can be addressed in a single
instruction: two 64-KB regions that can be placed anywhere in the address space but typically in
standard locations (see <emphasis><xref linkend="SDATA-AND-SBSS-E500"></emphasis>), and one 64 KB region straddling address 0 (32 KB at
addresses 0xFFFF_8000 through 0xFFFF_FFFF, and 32 KB at addresses 0x0000_0000 through
0x0000_7FFF).</para>
<para>Because the sizes of these areas are limited, compilers that support small data area relative
addressing typically determine whether or not an eligible data item is placed in the small data area
based on its size. Under this scheme, all data items less than or equal to a specified size (the default
is usually 8 bytes) are placed in the small data area. Initialized data items are placed in one of the
.data sections, uninitialized data items in one of the .sbss sections. If the default size results in
a small data area that is too large to be addressed with 16-bit relative offsets, the link editor fails
to build the executable file or shared object file, and some of the code that makes up the file must be recompiled
with a smaller value for the size criterion. </para>
<para>This ABI does not preclude a compiler from using profiling information or some form of heuristics, rather than purely
data item size, to make more informed decisions about which data items should be placed in these regions.</para>
<sect2 id="SDATA-AND-SBSS-E500">
<title>Small Data Area (.sdata and .sbss)</title>
<para>The small data area is part of the data segment of an executable program. It contains data items within the .sdata and
.sbss sections, which can be addressed with 16-bit signed offsets from the base of the small data area.</para>
<para>Only data items with local (nonglobal) scope may appear in the small data area of a
shared object. In a shared object the small data area follows the <emphasis><link linkend="GLOBAL-OFFSET-TABLE">Global Offset Table</link></emphasis>, so data
in the small data area can be addressed relative to the GOT pointer. However, in this case, the
small data area is limited in size to no more than 32 KB, and less if the global offset
table is large.</para>
<para>For executable files, up to 64 KB of data items with local or global scope can be
placed into the small data area. In an executable file, the symbol <varname>_SDA_BASE_</varname>
(small data area base) is defined by the link editor to be an address relative to which all data
in the .sdata and .sbss sections can be addressed with 16-bit signed offsets or, if there is
neither a .sdata nor a .sbss section, the value 0. In a shared object,
<varname>_SDA_BASE_</varname> is defined to have the same value as
<varname>_GLOBAL_OFFSET_TABLE_</varname>. The value of <varname>_SDA_BASE_</varname> in an
executable is normally loaded into r13 at process initialization time, and r13 thereafter
remains unchanged. In particular, shared objects shall not change the value in r13.</para>
<para>In executables, references to data items in the .sdata or .sbss sections are relative to r13; in shared objects,
they are relative to a register that contains the address of the <emphasis><link linkend="GLOBAL-OFFSET-TABLE">Global Offset Table</link></emphasis>.</para>
</sect2>
<sect2 id="SDATA2-AND-SBSS2-E500">