newbies questions about ARM ASM

iostrym wrote:
> Hi everyone, I would like to know the difference between
>
> #define STACK_ADR 0x1000
>
> and
>
> .equ  EBI_BASE,0xFFE00000
>
> I know the fisrt one is used in C and the second is ASM but the two one
> are in a same file because GCC can compile ASM and C. So I would like to
> know if these two instructions are identical.

They are not identical. But both methods can be used for certain task
like defining a constant value for say LDR reg, #const. Be aware that
the internal processing is very different.

.equ sets the value of an assembler symbol

#define creates a C-preprocessor macro. The C-preprocessor is used if
the assembler file has the .S (captial S) extension.

Read the fine manuals:
http://www.gnu.org/software/binutils/manual/gas-2.9.1/html_node/as_84.html#SEC86
http://developer.apple.com/documentation/DeveloperTools/gcc-4.0.1/cpp/Macros.html#Macros

Martin Thomas

armand iostrym wrote:
> Also, I know that .data and .text are used to specify in wich section
> the following statement will be but where exactly is specified the
> adresses of the data section and the text section? is it in a .S file or
> is it specified to GCC with another file with the needed parameters ?

This is the task of the linker command script. Examine some example
projects. You will see a reference to <linker command script>.ld in the
makefile and in the project directories.

Overview:
http://www.embedded.com/2000/0002/0002feat2.htm

Reference:
http://www.gnu.org/software/binutils/manual/ld-2.9.1/html_chapter/ld_3.html

> I would like to know if there is a difference between
>
>         ldr  r12, PtInitRemap  /* get the real jump address (after remap)
> */
>   mov  pc, r12      /* jump and break the pipeline */
>
> InitRemap:
>   bl  init_sections    /* copy fastram+data sections */
>   b   start
>
> PtInitRemap:
>   .long  InitRemap         /* address where to jump after REMAP */
>
> and
> [...]

Yes, there is a difference. And the reason for the first approach is
described well in the comments which are already there.

REMAP is hard to understand for a newbie like me too.

We have to learn that our code is running from 0x0 but in ROM first,
then we do a REMAP and some of our code should run e.g. from 0x01000000
in ROM and some essential code (exception handlers like reset...) is
running from 0x0 in RAM until power off...

OK. First time when ROM-code runs, the code has to do init_sections to
get data... from ROM into RAM.

But when the same code (which is part of the reset code) is used after
REMAP, the init_sections should not be called!

How can you do this? We have to change the code flow. Therefore when
this code is called in RAM (e.g. when a RESET occurs) PtInitRemap has
another content than the first time. Who changes the content? E.g.
init_sections can change the content.

The essential of the indirect access to the jump address in the first
case is, that you are able to modify the jump address easily because it
is data value (access with a pointer). Otherwise you would have to
produce self-modifying code by patching adguments of code instructions.

Stefan