[U-Boot] How/Where does "_start" get assigned a value ?

[Gerhard Sittig]

On Wed, Oct 09, 2013 at 09:07 -0700, djoker wrote:
> 
> Hi Everyone,

Why do you "re-send" your request without giving readers time to
respond (first message at 17:00 and second on 09:00 the very next
morning)?

This unnecessarily splits the same subject across several
threads, hinders discussion and prevents easy lookup in archives
(and reduces your score and thus the willingness to help such a
"demanding" person) ...

> I have a armv7 board and am looking at the "_start" symbol address, using
> the following command:
> *nm u-boot | grep -w _start*
> 
> It returned the following:
> *67000020 T _start*
> 
> I couldn't help notice that the the _start value is very "close" in vlaue to
> the value of CONFIG_SYS_TEXT_BASE defined in my board file:
> *#define CONFIG_SYS_TEXT_BASE 0x67000004*

As with any ARM based CPU, I'd expect the "executable" to start
with a vector table, before the actual body of the start()
routine follows.

The "reset vector" only has a few bytes in the vector table, and
should consist of a branch to the actual initialization code.

So I'd rather be baffled if the _start symbol would reside at the
very start of the binary, since it then would clobber all the
vectors.

> But, I have searched through the source code, but not found where _start
> gets assigned a value.. Could someone please help me understand this ?

The linker (sometimes referred to as the loader) is the entity to
assign addresses to symbols.  So check the linker command line
and control files when in doubt.

Optionally tell the compiler to "save temp files", and/or check
whether the resulting *.o files or executables match what you'd
expect from looking at the C/assembly source.


virtually yours
Gerhard Sittig
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