AVR32795: Using the GNU Linker Scripts on
AVR UC3 Devices
Features
• Basic GNU linker script concepts
• 32-bit AVR® UC3™ GNU linker scripts
• Controlling the location of functions and variables in the flash
1 Introduction
32-bit
Microcontrollers
Application Note
This document highlights the main purpose of the GNU linker script, which is to
control the location of code and variables in the executable.
Figure 1-1. AVR GNU toolchain build steps.
Rev. 32158A-AVR-01/11
2 Memory map
The Atmel® AVR UC3 microcontroller architecture has a 32-bit memory space and
separate memory types (program and data) connected with distinct buses. Such a
memory architecture allows the processor to access both program and data
memories at the same time. Each memory type has its own address space.
Figure 2-1. Example of the Atmel AT32UC3A3256 memory map.
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3 Basic linker script concepts
3.1 Sections
The linker combines input files (object file format) into a single output file
(executable).
Each object file has, among other things, a list of sections. We refer to a section in an
input file as an input section. Similarly, a section in the output file is an output section.
Each section in an object file has a name and a size. Most sections also have an
associated block of data (the section contents).
3.2 Section properties
A section may be marked as loadable, which means that its contents should be
loaded into memory when the executable is run.
A section with no contents may be allocatable, which means that an area in memory
should be set aside, but nothing in particular should be loaded there (and, in some
cases, this memory must be zeroed out).
A section which is neither loadable nor allocatable typically contains some sort of
debugging information.
3.3 VMA and LMA
Every loadable or allocatable output section has two addresses. The first is the VMA,
or virtual memory address. This is the address the section will have when the output
file is run. The second is the LMA, or load memory address. This is the address at
which the section will be loaded. In most cases the two addresses will be the same.
An example of when the LMA and VMA might be different is when a data section is
loaded into ROM, and then copied into RAM when the program starts up (a technique
often used to initialize global variables in a ROM-based system). In this case, the
ROM address would be the LMA and the RAM address would be the VMA.
3.4 Symbols
Every object file also has a list of symbols, known as the symbol table. A symbol may
be defined or undefined. Each symbol has a name, and each defined symbol has an
address, among other information.
The compilation of a C or C++ program into an object file will generate a defined
symbol for every defined function and global or static variable. Every undefined
function or global variable which is referenced in the input file will become an
undefined symbol.
3.5 Well-known sections
.text:
usually contains the code, and is usually loaded to a non-volatile memory,
such as the internal flash.
.data:
initialized data; usually contains initialized variables.
.bss:
usually contains non-initialized data.
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NOTE
The section, properties, VMA, and LMA are available in an object file or output file by
using the avr32-objdump program with the -h option. For example:
Idx Name
0 .reset
Size
VMA
LMA
File off
Algn
00002004
80000000
80000000
00001000
2**2
CONTENTS, ALLOC, LOAD, READONLY, CODE
1 .got
00000000
0000001c
80003634
0000501c
2**2
CONTENTS, ALLOC, LOAD, RELOC, DATA
2 .init
0000001a
80002004
80002004
00003004
2**2
CONTENTS, ALLOC, LOAD, READONLY, CODE
3 .text
00001074
80002020
80002020
00003020
2**2
CONTENTS, ALLOC, LOAD, READONLY, CODE
4 .exception
00000200
80003200
80003200
00004200
2**9
CONTENTS, ALLOC, LOAD, READONLY, CODE
5 .fini
00000018
80003400
80003400
00004400
2**2
CONTENTS, ALLOC, LOAD, READONLY, CODE
6 .rodata
00000208
80003418
80003418
00004418
2**2
CONTENTS, ALLOC, LOAD, READONLY, DATA
7 .dalign
00000004
00000004
00000004
00000000
2**0
00000008
80003620
00005008
2**2
00005010
2**2
00005018
2**2
0000501c
2**2
ALLOC
8 .ctors
00000008
CONTENTS, ALLOC, LOAD, DATA
9 .dtors
00000008
00000010
80003628
CONTENTS, ALLOC, LOAD, DATA
10 .jcr
00000004
00000018
80003630
CONTENTS, ALLOC, LOAD, DATA
11 .data
00000820
0000001c
80003634
CONTENTS, ALLOC, LOAD, DATA
12 .bss
0000015c
00000854
00000854
00000000
2**2
000009b0
000009b0
00000000
2**0
ALLOC
13 .heap
0000e650
ALLOC
NOTE
4
The symbols are available in an object or output file by using the avr32-nm program,
or by using the avr32-objdump program with the -t option.
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4 Default versus specific linker script
The linker always uses a linker script. If none is explicitly supplied, the linker will use a
default script that is compiled into the linker executable. The AVR GNU toolchain
default linker scripts are under the directory:
C:/Program Files/Atmel/AVR Tools/AVR(32) Toolchain/avr32/lib/ldscripts/
Other linker scripts can be supplied by using the –T command line option (or the long
form: –script=<file>). When this is done, the linker script specified will replace the
default linker script.
Extensions of the GNU toolchain linker scripts and their meanings:
.x:
default linker script, for “regular” executables.
.xbn:
default linker script used when the –N option is specified; mix text and data on
the same page; don't align data.
.xn:
default linker script used when the –n option is specified; mix text and data on
the same page.
.xr:
default linker script used when the
relocation.
–r
option is specified; link without
.xu:
default linker script used when the
relocation, create constructors.
–Ur
option is specified; link without
.xw:
linker script to use for writable .rodata section.
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5 GNU linker script walkthrough
/* … */
: Comments
OUTPUT_FORMAT("elf32-avr32", "elf32-avr32", "elf32-avr32"):
The OUTPUT_FORMAT command names the BFD format to use for the output file. In this
case, this is strictly equivalent to OUTPUT_FORMAT("elf32-avr32")
OUTPUT_FORMAT(default, big, little):
is usable with three arguments to use different formats based on the
and -EL command line options.
OUTPUT_FORMAT
–EB
This permits the linker script to set the output format based on the desired
endianness.
OUTPUT_ARCH(avr32:uc):
Specify a particular output machine architecture.
ENTRY(_start):
Set the first instruction to execute in a program (called the
entry point).
5.1 MEMORY command
Describes the location and size of blocks of memory in the target.
MEMORY
Name used in the linker script to refer to that memory region
{
FLASH (rxai!w) : ORIGIN = 0x80000000, LENGTH = 256K
INTRAM (wxa!ri) : ORIGIN = 0x00000004, LENGTH = 0x0000FFFC
USERPAGE : ORIGIN = 0x80800000, LENGTH = 0x00000200
Optional list of attributes
}
• r: read-only section
• w: r/w section
• x: executable section
• a: allocatable section
• i: initialized section
• !: invert the sense of the following attributes
The linker will use the region attributes to select the memory region for the output
section that it creates (if not explicitly mentioned later in the script).
Once a memory region is defined, the linker script can direct the linker to place
specific output sections into that memory region.
5.2 PHDRS command
The ELF object file format uses program headers, aka segments. The program
headers describe how the program should be loaded into the target memory.
PHDRS
{
This program header describes a segment to be loaded from the file
FLASH PT_LOAD;
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INTRAM_ALIGN PT_NULL;
INTRAM_AT_FLASH PT_LOAD;
INTRAM PT_NULL;
Indicates an unused (for loading) program header
USERPAGE PT_LOAD;
}
To place an output section in a particular segment, use the :phdr output section
attribute.
When the executable is programmed to target, only the loadable segments should be
programmed.
NOTE
There are types other than PT_LOAD and PT_NULL (refer to the GCC linker scripts
documentation for details).
5.3 SECTIONS command
Tells the linker how to map input sections into output sections, and how to place the
output sections in memory.
SECTIONS
{
sections-command
sections-command
...
}
Each SECTIONS command may be one of the following:
•
•
•
•
an ENTRY command
a symbol assignment
an output section description
an overlay description
5.3.1 Symbol assignment
A value can be assigned to a symbol. This will define the symbol as a global symbol.
Example 1:
/* Use a default stack size if stack size was not defined. */
__stack_size__ = DEFINED(__stack_size__) ? __stack_size__ : 4K;
Example 2:
PROVIDE (__executable_start = 0x80000000);
. = 0x80000000;
Indicates an unused (for loading) program header
NOTE
The special symbol “.” is the location counter.
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NOTE
If the address of an output section is not specified, the address is set from the current
value of the location counter. The location counter is then incremented by the size of
the output section. At the start of the SECTIONS command, it equals zero by default.
5.3.2 Output section description
Most programs consist only of code, initialized data, and uninitialized data. These will
be in the .text, .data, and .bss sections, respectively. For most programs, these are
also the only sections that appear in the input files.
5.3.2.1 Sections
SECTIONS
Set the value of the location counter
{
. = 0x80000000;
.text : { *(.text) }
. = 0x00000000;
.data : { *(.data) }
List the names of the input sections that should be
placed into this output section: “all .text input
sections in all input files”
.bss : { *(.bss) }
}
Section name
The first line inside the SECTIONS command of the above example sets the value of the
special symbol “.”, which is the location counter. If any address of an output section is
specified in some other way, the address is set from the current value of the location
counter. The location counter is then incremented by the size of the output section. At
the start of the SECTIONS command, the location counter has the value 0.
The second line defines an output section, .text. Within the curly braces after the
output section name, it lists the names of the input sections that should be placed into
this output section.
The “*” is a wildcard, which matches any file name. The expression
all .text input sections in all input files.
*(.text)
means
Because the location counter is 0x80000000 when the output section .text is
defined, the linker will set the address of the .text section in the output file to be
0x80000000.
The remaining lines define the .data and .bss sections in the output file. The linker
will place the .data output section at address 0x00000000.
After the linker places the .data output section, the value of the location counter will
be 0x00000000 plus the size of the .data output section.
The effect is that the linker will place the
.data output section in memory.
.bss
output section immediately after the
The linker will ensure that each output section has the required alignment, by
increasing the location counter if necessary.
In this example, the specified addresses for the .text and .data sections will probably
satisfy any alignment constraints, but the linker may have to create a small gap
between the .data and .bss sections.
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Figure 5-1. Memory mapping.
5.3.2.2 Text
.text
The program may use external libraries (for example,
gcc libraries, newlib) that use specific input sections
:
{
*(.text .stub .text.* .gnu.linkonce.t.*)
KEEP (*(.text.*personality*))
/* .gnu.warning sections are handled specially by elf32.em.*/
*(.gnu.warning)
} >FLASH
AT>FLASH
:FLASH
=0xd703d703
Set the fill pattern for an entire section
Assign a section to a previously defined segment (cf PHDRS
command)
Specify a memory region for the .text section LMA
Assign a section to a previously defined region of memory (cf. MEMORY
command)
NOTE
When link-time garbage collection is in use (-gc-sections), it is often useful to mark
sections that should not be eliminated. This is accomplished by surrounding an input
section's wildcard entry with KEEP().
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6 Examples
6.1 Controlling the location of functions and variables in the flash
6.1.1 Process flow
A. Define a custom section, .flash_nvram, located in flash:
A.1. Handle a default or pre-defined size for this section:
/* Use a default flash NVRAM size if flash NVRAM size was not defined. */
__flash_nvram_size__ = DEFINED(__flash_nvram_size__)?__flash_nvram_size__:4K;
A.2. Describe the output section, .flash_nvram:
.flash_nvram
ORIGIN(FLASH) + LENGTH(FLASH) - __flash_nvram_size__ :
{
*(.flash_nvram)
} >FLASH
VMA of this output section
AT>FLASH
:FLASH_NVRAM
Include all input sections .flash_nvram
Assign this section to a program segment (cf PHDRS{ })
Specify the memory region FLASH for the section’s LMA
Assign this section to the FLASH region of memory (cf. MEMORY{ })
B. Locate a variable in a custom section, .flash_nvram:
Extensions to the C language family:
o
Specifying attributes of variables
__attribute__((__section__(".flash_nvram"))) static int flash_nvram_data;
C. Locate a function in a custom section, .flash_nvram:
Extensions to the C language family:
o
Declaring attributes of functions
__attribute__ ((__section__(".flash_nvram"))) void Func(void) {…}
NOTE
10
Placing a variable or a function at a specific address has to be done through the linker
script (place the custom section at the specific address).
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6.1.2 Related examples in the software framework
• drivers/flashc/flash_example/: Controlling the location of a variable in flash
• drivers/cpu/mpu/example/:
Controlling the location of a function in flash
6.2 Controlling the location of functions in internal RAM
6.2.1 Process flow
A. Define a custom output section, .ram_fcts, with VMA in RAM and LMA in flash:
.ram_fcts
:
{
Include all input sections .ram_fcts
*(.ram_fcts.*)
} >INTRAM
AT>FLASH
:FLASH
Assign a section to a previously defined segment
(cf PHDRS command)
Specify a memory region for the .text section LMA
Assign this section’s VMA to the INTRAM region of memory
(defined in MEMORY{ })
B. Locate a function in a custom section, .ram_fcts:
Extensions to the C language family:
o
Declaring attributes of functions
__attribute__ ((__section__(".ram_fcts"))) void Func(void) {…}
C. The startup routine is responsible for copying the .ram_fcts LMA (in flash) to the
.ram_fcts VMA (in INTRAM), as is done for the .data section.
6.2.2 Related examples
Examples of a startup routine implementation can be found in the AVR Software
Framework
under
utils/startup_files/gcc/
in
ASF
v1
and
under
utils/startup/startup_uc3.S in ASF v2.
An example using the same method can be found here:
AVR32749 Application note: Software Workaround Implementation for the Erratum
Flash Read-after-Write for AT32UC3A0512 / AT32UC3A1512 Revision E, H and I.
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6.3 Controlling the location of variables and the heap in external SDRAM
6.3.1 Process flow
A. Linker script customization
A.1. Adding the external SDRAM memory (MEMORY{})
MEMORY
{
FLASH (rxai!w) : ORIGIN = 0x80000000, LENGTH = 0x00040000
INTRAM (wxa!ri) : ORIGIN = 0x00000004, LENGTH = 0x0000FFFC
ERAM0 (wxa!ri) : ORIGIN = 0xFF000000, LENGTH = 0x00008000
ERAM1 (wxa!ri) : ORIGIN = 0xFF008000, LENGTH = 0x00008000
SDRAM (wxa!ri) : ORIGIN = 0xD0000000, LENGTH = 0x02000000
USERPAGE : ORIGIN = 0x80800000, LENGTH = 0x00000200
}
A.2. Adding the external SDRAM segments (PHDRS{})
PHDRS
{
FLASH PT_LOAD;
INTRAM_ALIGN PT_NULL;
INTRAM_AT_FLASH PT_LOAD;
INTRAM PT_NULL;
SDRAM_AT_FLASH PT_LOAD;
SDRAM PT_NULL;
USERPAGE PT_LOAD;
}
A.3. Defining two output sections for variables placed in external SDRAM
(.data_sdram, .bss_sdram)
. = ORIGIN(SDRAM);
.data_sdram
{
Set the location counter to SDRAM start address
ORIGIN(SDRAM) :
Set the VMA of .data sdram
PROVIDE(_data_sdram = .);
*(.data_sdram)
Will be used by the startup routine for data init
. = ALIGN(8);
PROVIDE(_edata_sdram = .);
} >SDRAM AT>FLASH :SDRAM_AT_FLASH
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PROVIDE(_data_sdram_lma = ABSOLUTE(LOADADDR(.data_sdram)));
.bss_sdram :
Will be used by the startup routine for data init
{
PROVIDE(__bss_sdram_start = .);
Will be used by the startup routine for zero init
*(.bss_sdram)
PROVIDE(_bss_sdram_end = .);
} >SDRAM AT>SDRAM :SDRAM
Assign a section to a previously defined segment
(cf PHDRS{})
Specify the memory region SDRAM for the section’s LMA
Assign this section’s VMA to the region of memory SDRAM
(defined in MEMORY{ })
NOTE
Because the program header SDRAM was defined as PT_NULL, the .bss_sdram section
won’t be loaded to target (which is ok because the .bss section is supposed to hold
uninitialized data). And so the AT>SDRAM is unnecessary and ignored. This section will
just be zeroed out in the startup routine.
NOTE
Definitions:
_data_sdram:
VMA address of the start of the .data_sdram section
_data_sdram:
VMA address of the end of the .data_sdram section
_data_sdram_lma:
LMA start address of the .data_sdram section
>SDRAM AT>FLASH :SDRAM_AT_FLASH
>SDRAM:
VMA of .data_sdram in external SDRAM
AT>FLASH:
LMA of .data_sdram (in flash)
:SDRAM_AT_FLASH:
specified as loadable in PHDRS{} (for the data init value)
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A.4. Specifying the size and location of the heap to external SDRAM
.heap :
VMA address symbol used by the malloc gnu lib
{
__heap_start__ = .;
Force the .heap section to be of size
heap size
*(.heap)
. = __heap_size__;
VMA address symbol used by the malloc gnu lib
__heap_end__ = .;
} >SDRAM AT>SDRAM :SDRAM
Assign a section to a previously defined segment
(cf PHDRS{})
Specify the memory region SDRAM for the section’s LMA
Assign this section’s VMA to the region of memory SDRAM
(defined in MEMORY{ })
NOTE
Because the program header SDRAM was defined as PT_NULL, the .heap section won’t
be loaded to target. And so the AT>SDRAM is unnecessary and ignored.
B. Initialization of the SDRAM controller:
When: before the first SDRAM access, which is performed during the startup
process (for .data_sdram and .bss_sdram sections initialization)
How:
using the startup customization API (_init_startup()), which is called
by the startup routine before doing the SDRAM access
C. Startup routine customization
C.1. Call the startup customization function:
call _init_startup: this is when the SDRAM controller must be initialized
C.2.
.data_sdram
and .bss_sdram sections initialization:
Load initialized external SDRAM data having a global lifetime from the
LMA section using the symbols previously defined in the linker
script (_data_sdram and _edata_sdram (the VMA addresses), _data_sdram_lma
(the LMA start address))
.data_sdram
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Clear uninitialized external SDRAM data having a global lifetime in the
section using the symbols previously defined in the linker script
(_bss_sdram_start and _bss_sdram_end (VMA addresses))
.bss_sdram
D. Using dynamic allocation: use malloc() and free() “as usual”
E. Assigning an initialized variable to external SDRAM:
__attribute__((__section__(".data_sdram")))
static int AllGoodChildrenGoToHeaven[7] = { 1,2,3,4,5,6,7 };
F. Assigning a non-initialized variable to external SDRAM:
__attribute__((__section__(".bss_sdram")))
static int HelloGoodbye;
6.3.2 Details (documentation and source code)
AVR32733 application note: Placing data and the heap in external SDRAM.
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7 Specific linker script examples
• The AVR Software Framework provides generic linker scripts under
utils/linker_scripts/ or specific linker scripts for some examples; these scripts have
a .lds extension (the extension doesn’t matter).
• drivers/flashc/flash_example/: Controlling the location of variables in flash
• drivers/cpu/mpu/example/:
Controlling the location of a function in flash
• Application Note AVR32733: Placing data and the heap in external SDRAM
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8 Frequently asked questions
Q: How can I use my own linker script instead of the default one inside a 32-bit
AVR project?
A: By default, avr32-gcc uses the default linker script from the AVR GNU toolchain.
To use your own linker script in a 32-bit Atmel AVR32 Studio® project, use the
following procedure:
• Copy your linker script in the root of your project (use the import
command, or simply copy/paste)
• Open the Project Properties view (Properties item from the contextual
menu)
• Select the Tool Settings tab
• Select the AVR32/GNU C Linker -> Miscellaneous item
• Add -T../my_linker_script.lds to the linker flags command line
Your project is now ready to link with your own linker script.
Q: How can I declare a variable at a specific location of the flash memory?
A: To do so with GCC, a specific section must be created at link time.
Here is an example of how to place a string variable at address 0x80010000:
1. Declare the variable, specifying location in the .testloc section:
const char string[] __attribute__ ((section (".testloc"))) = "String at
fixed address";
2. Create the section in the linker options:
• Open the Project Properties: highlight the project name and press
Alt+Enter
• Select the C/C++ Build / Settings category
• Select the Tool Settings tab
• Expand the AVR32/GNU Linker and highlight the miscellaneous item
• In the Linker Flags field, add the -Wl,-section-start=.testloc=0x80010000
option
Refer also to the screen shot below.
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Q: When compiling the ARV Software Framework examples, I got the following
kinds of warnings:
ld: uc3a0512-ctrlpanel.elf: warning: allocated section ‘.dalign’ not in segment
ld: uc3a0512-ctrlpanel.elf: warning: allocated section ‘.bss’ not in segment
ld: uc3a0512-ctrlpanel.elf: warning: allocated section ‘.heap’ not in segment
ld: uc3a0512-ctrlpanel.elf: warning: allocated section ‘.stack’ not in segment
What are these warnings?
A: These are normal warnings, and you don't need to care about them. Here is an
explanation of these warnings:
When using the default linker scripts provided with avr32-gcc, the ELF LOAD
program headers are generated automatically from the output sections, including
BSS and the stack which are only allocated areas.
In its current revision, avr32program programs allocate LOAD program headers
that do not have to be filled with data from the ELF file, which wastes time. This
avr32program behavior will be changed in a future release, but until this is
achieved, the linker scripts provided with AVR Software Framework are modified
to place the allocated-only output sections in NULL ELF program headers, which
are ignored by avr32program, explaining the warnings when linking.
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Q: How can I create a 32-bit AVR32 Studio project from an existing standalone
project that has its own makefile and linker script?
A: Here is the step-by-step procedure to import your existing project into 32-bit
AVR32 Studio and reuse the makefile and linker scripts of the project:
1. Create an empty project:
• Open the New wizard selection: menu File -> New -> Other
• Expand the C folder and highlight the AVR32 C project (Make) item
• Click Next to open the New Project wizard
• Select the Target MCU from the list, enter a project name, and click Finish
2. Add the existing source code and makefile:
• Open the New wizard selection again: menu File -> New -> Other
• Expand the General folder and highlight the Folder item
• Click Next to open the New Folder wizard
• Click on Advanced>>, and check the Link to folder in the file system box
• Browse to the location of your existing stand-alone project, and click Finish
3. Create a make target:
• In the Project Explorer view, browse to the folder that contains the makefile
• Right-click on the makefile file, and select the Create make target item
• Enter a name in the Target Name: field (for example, Build), and click
Create
NOTE
Other targets could be added by repeating the above steps.
NOTE
The make target creations depend on the keywords defined in the makefile:
4. Build the make target:
• In the Project Explorer view, right-click on the project name, and select the
Build make target item
• Select the make target to build, and click Finish
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9 References
The official GNU ld linker documentation:
http://sourceware.org/binutils/docs-2.19/ld/index.html
http://sourceware.org/binutils/docs-2.19/ld/Scripts.html#Scripts
Using the GNU Compiler Collection:
http://gcc.gnu.org/onlinedocs/: online HTML or PDF document
10 Support
Atmel has several support channels available:
•
•
•
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Email:
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http://support.atmel.no/
[email protected]
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11 Table of Contents
Features ............................................................................................... 1
1 Introduction ...................................................................................... 1
2 Memory map..................................................................................... 2
3 Basic linker script concepts ........................................................... 3
3.1 Sections............................................................................................................... 3
3.2 Section properties ............................................................................................... 3
3.3 VMA and LMA ..................................................................................................... 3
3.4 Symbols............................................................................................................... 3
3.5 Well-known sections............................................................................................ 3
4 Default versus specific linker script............................................... 5
5 GNU linker script walkthrough ....................................................... 6
5.1 MEMORY command ........................................................................................... 6
5.2 PHDRS command ............................................................................................... 6
5.3 SECTIONS command ......................................................................................... 7
5.3.1 Symbol assignment ................................................................................................... 7
5.3.2 Output section description ......................................................................................... 8
6 Examples ........................................................................................ 10
6.1 Controlling the location of functions and variables in the flash ......................... 10
6.1.1 Process flow ............................................................................................................ 10
6.1.2 Related examples in the software framework.......................................................... 11
6.2 Controlling the location of functions in internal RAM......................................... 11
6.2.1 Process flow ............................................................................................................ 11
6.2.2 Related examples.................................................................................................... 11
6.3 Controlling the location of variables and the heap in external SDRAM ............ 12
6.3.1 Process flow ............................................................................................................ 12
6.3.2 Details (documentation and source code) ............................................................... 15
7 Specific linker script examples..................................................... 16
8 Frequently asked questions.......................................................... 17
9 References...................................................................................... 20
10 Support ......................................................................................... 20
11 Table of Contents......................................................................... 21
21
32158A-AVR-01/11
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