linker script symbols syntax and other errors

[Nick Clifton]

Hi Klaus,

> SECTIONS
> {
>  .text :
>  {
>    ./startup.o (.text)         /* Startup code */
>     *(.text)
>    *(.glue_7t) *(.glue_7)
>  } >IntCodeRAM =0
>  .ROM :
>  {
>      *(.linkToRam)
>  }
> 
> 1->    start_of_ROM   = .ROM;
> 2->  /* end_of_ROM     = .ROM + sizeof (.ROM) - 1;
> }
> ----
> The line 1 give a
> test2.ld:134: undefined symbol `.ROM' referenced in expression

This is correct.  .ROM is not a symbol defined in the symbol table of the 
executable, but rather a symbol in the SECTION directive's namespace.  You can 
fix this by assigning the value of "." to start_of_ROM at the start of the .ROM 
section, like this:

    } >IntCodeRAM =0

    . = .;                 /* <=== NEW */
    start_of_ROM = .;      /* <=== NEW */

    .ROM :
    {

Note, for an explanation of the ". = .;" line in the above snippet see the 
"Location Counter" section of the linker manual.


> :test2.ld:50: syntax error

Also correct and for the same reason.  You can fix this one as well, like this:

     *(linkToRam);
   }
   end_of_ROM = . - 1;

Assuming that you want end_of_ROM to be the address of the last byte in the 
.ROM section and not the address of the first byte after the end of the .ROM 
section.

> what is the meaning of 
> a PROVIDE statement? I read the manual 2 times, but it only confuses me. 

OK, so basically what the PROVIDE statement does is to create a symbol and 
assign it a value, but only if the symbol has not appeared in any of the input 
files.  So for example, suppose that you have this in your linker script:

   PROVIDE (foo = 1);

Then if none of the input files define a symbol called "foo", the linker will 
create one and assign it the value of 1.  If an input file does define a symbol 
called "foo" then the linker will use that version instead and ignore the value 
defined in the PROVIDE statement.

Essentially the PROVIDE statement allows the linker script to define a default 
value for a symbol which can be overridden by any of the input files.

Cheers
   Nick