rtems-docs/bsp_howto/linker_script.rst

Linker Script
#############

What is a "linkcmds" file?
==========================

The ``linkcmds`` file is a script which is passed to the linker at linking
time.  This file describes the memory configuration of the board as needed
to link the program.  Specifically it specifies where the code and data
for the application will reside in memory.

The format of the linker script is defined by the GNU Loader ``ld``
which is included as a component of the GNU Binary Utilities.  If you
are using GNU/Linux, then you probably have the documentation installed
already and are using these same tools configured for *native* use.
Please visit the Binutils project http://sourceware.org/binutils/
if you need more information.

Program Sections
================

An embedded systems programmer must be much more aware of the
placement of their executable image in memory than the average
applications programmer.  A program destined to be embedded as well
as the target system have some specific properties that must be
taken into account. Embedded machines often mean average performances
and small memory usage.  It is the memory usage that concerns us
when examining the linker command file.

Two types of memories have to be distinguished:

- RAM - volatile offering read and write access

- ROM - non-volatile but read only

Even though RAM and ROM can be found in every personal computer,
one generally doesn't care about them.  In a personal computer,
a program is nearly always stored on disk and executed in RAM.  Things
are a bit different for embedded targets: the target will execute the
program each time it is rebooted or switched on.   The application
program is stored in non-volatile memory such as ROM, PROM, EEPROM,
or Flash. On the other hand, data processing occurs in RAM.

This leads us to the structure of an embedded program.  In rough terms,
an embedded program is made of sections.  It is the responsibility of
the application programmer to place these sections in the appropriate
place in target memory.  To make this clearer, if using the COFF
object file format on the Motorola m68k family of microprocessors,
the following sections will be present:

- *code (``.text``) section*:
  is the program's code and it should not be modified.
  This section may be placed in ROM.

- *non-initialized data (``.bss``) section*:
  holds uninitialized variables of the program. It can stay in RAM.

- *initialized data (``.data``) section*:
  holds the initialized program data which may be modified during the
  program's life.  This means they have to be in RAM.
  On the other hand, these variables must be set to predefined values, and
  those predefined values have to be stored in ROM.

*NOTE:* Many programs and support libraries unknowingly assume that the``.bss`` section and, possibly, the application heap are initialized
to zero at program start.  This is not required by the ISO/ANSI C Standard
but is such a common requirement that most BSPs do this.

That brings us up to the notion of the image of an executable: it consists
of the set of the sections that together constitute the application.

Image of an Executable
======================

As a program executable has many sections (note that the user can define
their own, and that compilers define theirs without any notice), one has to
specify the placement of each section as well as the type of memory
(RAM or ROM) the sections will be placed into.
For instance, a program compiled for a Personal Computer will see all the
sections to go to RAM, while a program destined to be embedded will see
some of his sections going into the ROM.

The connection between a section and where that section is loaded into
memory is made at link time.  One has to let the linker know where
the different sections are to be placed once they are in memory.

The following example shows a simple layout of program sections.  With
some object formats, there are many more sections but the basic
layout is conceptually similar.
.. code:: c

    +-----------------+-------------+
    |     .text       |  RAM or ROM |
    +-----------------+-------------+
    |     .data       |  RAM        |
    +-----------------+-------------+
    |     .bss        |  RAM        |
    +-----------------+-------------+

Example Linker Command Script
=============================

The GNU linker has a command language to specify the image format.  This
command language can be quite complicated but most of what is required
can be learned by careful examination of a well-documented example.
The following is a heavily commented version of the linker script
used with the the ``gen68340`` BSP  This file can be found at
$BSP340_ROOT/startup/linkcmds.
.. code:: c

    /*
    *  Specify that the output is to be coff-m68k regardless of what the
    *  native object format is.
    \*/
    OUTPUT_FORMAT(coff-m68k)
    /*
    *  Set the amount of RAM on the target board.
    *
    *  NOTE: The default may be overridden by passing an argument to ld.
    \*/
    RamSize = DEFINED(RamSize) ? RamSize : 4M;
    /*
    *  Set the amount of RAM to be used for the application heap.  Objects
    *  allocated using malloc() come from this area.  Having a tight heap
    *  size is somewhat difficult and multiple attempts to squeeze it may
    *  be needed reducing memory usage is important.  If all objects are
    *  allocated from the heap at system initialization time, this eases
    *  the sizing of the application heap.
    *
    *  NOTE 1: The default may be overridden by passing an argument to ld.
    *
    *  NOTE 2: The TCP/IP stack requires additional memory in the Heap.
    *
    *  NOTE 3: The GNAT/RTEMS run-time requires additional memory in
    *  the Heap.
    \*/
    HeapSize = DEFINED(HeapSize) ? HeapSize : 0x10000;
    /*
    *  Set the size of the starting stack used during BSP initialization
    *  until first task switch.  After that point, task stacks allocated
    *  by RTEMS are used.
    *
    *  NOTE: The default may be overridden by passing an argument to ld.
    \*/
    StackSize = DEFINED(StackSize) ? StackSize : 0x1000;
    /*
    *  Starting addresses and length of RAM and ROM.
    *
    *  The addresses must be valid addresses on the board.  The
    *  Chip Selects should be initialized such that the code addresses
    *  are valid.
    \*/
    MEMORY {
    ram : ORIGIN = 0x10000000, LENGTH = 4M
    rom : ORIGIN = 0x01000000, LENGTH = 4M
    }
    /*
    *  This is for the network driver.  See the Networking documentation
    *  for more details.
    \*/
    ETHERNET_ADDRESS =
    DEFINED(ETHERNET_ADDRESS) ? ETHERNET_ADDRESS : 0xDEAD12;
    /*
    *  The following defines the order in which the sections should go.
    *  It also defines a number of variables which can be used by the
    *  application program.
    *
    *  NOTE: Each variable appears with 1 or 2 leading underscores to
    *        ensure that the variable is accessible from C code with a
    *        single underscore.  Some object formats automatically add
    *        a leading underscore to all C global symbols.
    \*/
    SECTIONS {
    /*
    *  Make the RomBase variable available to the application.
    \*/
    _RamSize = RamSize;
    __RamSize = RamSize;
    /*
    *  Boot PROM  - Set the RomBase variable to the start of the ROM.
    \*/
    rom : {
    _RomBase = .;
    __RomBase = .;
    } >rom
    /*
    * Dynamic RAM - set the RamBase variable to the start of the RAM.
    \*/
    ram : {
    _RamBase = .;
    __RamBase = .;
    } >ram
    /*
    *  Text (code) goes into ROM
    \*/
    .text : {
    /*
    *  Create a symbol for each object (.o).
    \*/
    CREATE_OBJECT_SYMBOLS
    /*
    *  Put all the object files code sections here.
    \*/
    \*(.text)
    . = ALIGN (16);      /*  go to a 16-byte boundary \*/
    /*
    *  C++ constructors and destructors
    *
    *  NOTE:  See the CROSSGCC mailing-list FAQ for
    *         more details about the "\[......]".
    \*/
    __CTOR_LIST__ = .;
    \[......]
    __DTOR_END__ = .;
    /*
    *  Declares where the .text section ends.
    \*/
    etext = .;
    _etext = .;
    } >rom
    /*
    *  Exception Handler Frame section
    \*/
    .eh_fram : {
    . = ALIGN (16);
    \*(.eh_fram)
    } >ram
    /*
    *  GCC Exception section
    \*/
    .gcc_exc : {
    . = ALIGN (16);
    \*(.gcc_exc)
    } >ram
    /*
    *  Special variable to let application get to the dual-ported
    *  memory.
    \*/
    dpram : {
    m340 = .;
    _m340 = .;
    . += (8 * 1024);
    } >ram
    /*
    *  Initialized Data section goes in RAM
    \*/
    .data : {
    copy_start = .;
    \*(.data)
    . = ALIGN (16);
    _edata = .;
    copy_end = .;
    } >ram
    /*
    *  Uninitialized Data section goes in ROM
    \*/
    .bss : {
    /*
    *  M68K specific: Reserve some room for the Vector Table
    *  (256 vectors of 4 bytes).
    \*/
    M68Kvec = .;
    _M68Kvec = .;
    . += (256 * 4);
    /*
    *  Start of memory to zero out at initialization time.
    \*/
    clear_start = .;
    /*
    *  Put all the object files uninitialized data sections
    *  here.
    \*/
    \*(.bss)
    \*(COMMON)
    . = ALIGN (16);
    _end = .;
    /*
    *  Start of the Application Heap
    \*/
    _HeapStart = .;
    __HeapStart = .;
    . += HeapSize;
    /*
    *  The Starting Stack goes after the Application Heap.
    *  M68K stack grows down so start at high address.
    \*/
    . += StackSize;
    . = ALIGN (16);
    stack_init = .;
    clear_end = .;
    /*
    *  The RTEMS Executive Workspace goes here.  RTEMS
    *  allocates tasks, stacks, semaphores, etc. from this
    *  memory.
    \*/
    _WorkspaceBase = .;
    __WorkspaceBase = .;
    } >ram
    }

Initialized Data
================

Now there's a problem with the initialized data: the ``.data`` section
has to be in RAM as this data may be modified during the program execution.
But how will the values be initialized at boot time?

One approach is to place the entire program image in RAM and reload
the image in its entirety each time the program is run.  This is fine
for use in a debug environment where a high-speed connection is available
between the development host computer and the target.  But even in this
environment, it is cumbersome.

The solution is to place a copy of the initialized data in a separate
area of memory and copy it into the proper location each time the
program is started.  It is common practice to place a copy of the initialized ``.data`` section at the end of the code (``.text``) section
in ROM when building a PROM image. The GNU tool ``objcopy``
can be used for this purpose.

The following figure illustrates the steps a linked program goes through
to become a downloadable image.


+--------------+-----+--------------------+--------------------------+
| .data    RAM |     | .data          RAM |                          |
+--------------+     +--------------------+                          |
| .bss     RAM |     | .bss           RAM |                          | 
+--------------+     +--------------------+-----+--------------------+
| .text    ROM |     | .text          ROM |     |     .text          |
+--------------+-----+---------+----------+-----+--------------------+
| copy of .data  ROM |         | copy of .data  |                    |
+--------------------+---------+----------------+--------------------+
|  Step 1            |Step 2                       Step 3            |
+--------------------+--------------------------+--------------------+

In Step 1, the program is linked together using the BSP linker script.

In Step 2, a copy is made of the ``.data`` section and placed
after the ``.text`` section so it can be placed in PROM.  This step
is done after the linking time.  There is an example
of doing this in the file $RTEMS_ROOT/make/custom/gen68340.cfg:
.. code:: c

    # make a PROM image using objcopy
    m68k-rtems-objcopy \\
    --adjust-section-vma .data= \\
    \`m68k-rtems-objdump --section-headers \\
    $(basename $@).exe \\
    | awk '\[...]` \\
    $(basename $@).exe

NOTE: The address of the "copy of ``.data`` section" is
created by extracting the last address in the ``.text``
section with an ``awk`` script.  The details of how
this is done are not relevant.

Step 3 shows the final executable image as it logically appears in
the target's non-volatile program memory.  The board initialization
code will copy the ""copy of ``.data`` section" (which are stored in
ROM) to their reserved location in RAM.

.. COMMENT: COPYRIGHT (c) 1988-2011.

.. COMMENT: On-Line Applications Research Corporation (OAR).

.. COMMENT: All rights reserved.