avr-libc-1.7.1/doc/api/sections.dox

/* Copyright (c) 2002,2005, Theodore Roth
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/* $Id: sections.dox 845 2005-09-06 18:17:43Z joerg_wunsch $ */

/** \page mem_sections Memory Sections

\remarks Need to list all the sections which are available to the avr.

\par Weak Bindings
FIXME: need to discuss the .weak directive.

The following describes the various sections available. 

\section sec_dot_text The .text Section

The .text section contains the actual machine instructions which make up
your program.  This section is further subdivided by the .initN and .finiN
sections dicussed below.

\note The \c avr-size program (part of binutils), coming from a Unix
background, doesn't account for the .data initialization space added to the
.text section, so in order to know how much flash the final program will
consume, one needs to add the values for both, .text and .data (but not .bss),
while the amount of pre-allocated SRAM is the sum of .data and .bss.

\section sec_dot_data The .data Section

This section contains static data which was defined in your code. Things like
the following would end up in .data:

\code
char err_str[] = "Your program has died a horrible death!";

struct point pt = { 1, 1 };
\endcode

It is possible to tell the linker the SRAM address of the beginning of the
.data section. This is accomplished by adding
<b><tt>-Wl,-Tdata,<em>addr</em></tt></b> to the \c avr-gcc command used to the
link your program. Not that <em><tt>addr</tt></em> must be offset by adding
0x800000 the to real SRAM address so that the linker knows that the address is
in the SRAM memory space. Thus, if you want the .data section to start at
0x1100, pass 0x801100 at the address to the linker.
[offset \ref harvard_arch "explained"]

\note
When using \c malloc() in the application (which could even happen inside
library calls), \ref malloc_extram "additional adjustments" are required.

\section sec_dot_bss The .bss Section

Uninitialized global or static variables end up in the .bss section.

\section sec_dot_eeprom The .eeprom Section

This is where eeprom variables are stored.

\section sec_dot_noinit The .noinit Section

This sections is a part of the .bss section. What makes the .noinit section
special is that variables which are defined as such:

\code
int foo __attribute__ ((section (".noinit")));
\endcode

will not be initialized to zero during startup as would normal .bss data.

Only uninitialized variables can be placed in the .noinit section. Thus, the
following code will cause \c avr-gcc to issue an error:

\code
int bar __attribute__ ((section (".noinit"))) = 0xaa;
\endcode

It is possible to tell the linker explicitly where to place the .noinit
section by adding <tt>-Wl,--section-start=.noinit=0x802000</tt> to the
<tt>avr-gcc</tt> command line at the linking stage. For example, suppose you
wish to place the .noinit section at SRAM address 0x2000:

\verbatim
	$ avr-gcc ... -Wl,--section-start=.noinit=0x802000 ...
\endverbatim

\anchor harvard_arch
\note Because of the Harvard architecture of the AVR devices, you must
manually add 0x800000 to the address you pass to the linker as the start of
the section.  Otherwise, the linker thinks you want to put the .noinit section
into the .text section instead of .data/.bss and will complain.

Alternatively, you can write your own linker script to automate this. [FIXME:
need an example or ref to dox for writing linker scripts.]

\section sec_dot_init The .initN Sections

These sections are used to define the startup code from reset up through
the start of main(). These all are subparts of the 
\ref sec_dot_text ".text section".

The purpose of these sections is to allow for more specific placement of code
within your program.

\note Sometimes, it is convenient to think of the .initN and .finiN sections as
functions, but in reality they are just symbolic names which tell the linker
where to stick a chunk of code which is \em not a function. Notice that the
examples for \ref asm_sections "asm" and \ref c_sections "C" can not be called
as functions and should not be jumped into.

The <b>.initN</b> sections are  executed in order from 0 to 9.

\par .init0:
Weakly bound to __init(). If user defines __init(), it will be jumped into
immediately after a reset.

\par .init1:
Unused. User definable.

\par .init2:
In C programs, weakly bound to initialize the stack, and to clear
__zero_reg__ (r1).

\par .init3:
Unused. User definable.

\par .init4:

For devices with > 64 KB of ROM, .init4 defines the code
which takes care of copying the contents of .data from the flash to
SRAM.
For all other devices, this code as well as the code to zero out the .bss
section is loaded from libgcc.a.

\par .init5:
Unused. User definable.

\par .init6:
Unused for C programs, but used for constructors in C++ programs.

\par .init7:
Unused. User definable.

\par .init8:
Unused. User definable.

\par .init9:
Jumps into main().

\section sec_dot_fini The .finiN Sections

These sections are used to define the exit code executed after return from
main() or a call to exit(). These all are subparts of the 
\ref sec_dot_text ".text section".

The <b>.finiN</b> sections are executed in descending order from 9 to 0.

\par .finit9:
Unused. User definable. This is effectively where _exit() starts.

\par .fini8:
Unused. User definable.

\par .fini7:
Unused. User definable.

\par .fini6:
Unused for C programs, but used for destructors in C++ programs.

\par .fini5:
Unused. User definable.

\par .fini4:
Unused. User definable.

\par .fini3:
Unused. User definable.

\par .fini2:
Unused. User definable.

\par .fini1:
Unused. User definable.

\par .fini0:
Goes into an infinite loop after program termination and completion of any
_exit() code (execution of code in the .fini9 -> .fini1 sections).

\section asm_sections Using Sections in Assembler Code

Example:

\code
#include <avr/io.h>

	.section .init1,"ax",@progbits
	ldi	  r0, 0xff
	out	  _SFR_IO_ADDR(PORTB), r0
	out	  _SFR_IO_ADDR(DDRB), r0
\endcode

\note The <b><tt>,"ax",\@progbits</tt></b> tells the assembler that the section
is allocatable ("a"), executable ("x") and contains data ("@progbits"). For
more detailed information on the .section directive, see the gas user manual.

<!-- I know, poorly named section. - TRoth -->
\section c_sections Using Sections in C Code

Example:

\code
#include <avr/io.h>

void my_init_portb (void) __attribute__ ((naked)) \
    __attribute__ ((section (".init3")));

void
my_init_portb (void)
{
	PORTB = 0xff;
	DDRB = 0xff;
}
\endcode

\note Section .init3 is used in this example, as this ensures the
inernal <tt>__zero_reg__</tt> has already been set up.  The code
generated by the compiler might blindly rely on <tt>__zero_reg__</tt>
being really 0.

*/