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POHow to prevent implicit linker script changing the LMA of a section
 

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  1. POHow to prevent implicit linker script changing the LMA of a section
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    <p>I'm using a modular build system for software than runs on an ARM embedded target as well as on a normal X86 (linux) machine. I'm compiling with a GNU toolchain, thus linking using <code>ld</code>.</p> <p>One of the modules makes use of a linkerscript trick to assemble an array of 'registered' objects. The objects are created with a macro like this:</p> <pre><code>#define RegObject(name, arg1, arg2, etc) \ static TRegObject name \ __attribute__((section ("regobj_table"), used)) = \ { arg1, arg2, etc } </code></pre> <p>The module also adds an <a href="http://sourceware.org/binutils/docs/ld/Implicit-Linker-Scripts.html" rel="nofollow">implicit linker script</a> to the link step, which looks like this:</p> <pre><code>SECTIONS { .data : ALIGN(4) { regobj_table_start = .; KEEP(*(regobj_table)) regobj_table_end = .; } } </code></pre> <p>The <code>regobj_table_start</code> and <code>regobj_table_end</code> symbols are used by the code to find the registered objects. This solution works just fine for native (Linux) compiled targets.</p> <p>This doesn't work for the ARM target however. The reason is that I have a custom default linker script for the target (it's a tiny microcontroller, running without an OS), which defines a load memory address for the <code>.data</code> section. This is because the section is stored in Flash memory, but once the microcontroller boots it's copied to RAM. The relevant part of the linker script looks like this:</p> <pre><code>MEMORY { ROM (rx) : ORIGIN = 0x00000000, LENGTH = 512k RAM (rwx) : ORIGIN = 0x40000000, LENGTH = 32k } SECTIONS { /* ... other stuff ... */ .data : { _data = .; *(.data) *(.data.*) } &gt;RAM AT&gt;ROM /* ... even more stuff ... */ } </code></pre> <p>This sets the VMA of the <code>.data</code> section to somewhere in the 0x4000000 range, and the LMA in the 0x00000000 range.</p> <p><strong>The problem is</strong> that when the implicit linker script is added to the command-line of <code>ld</code>, it just forgets about the LMA and it becomes equal to the VMA again. I'm desperately looking for a method to tell <code>ld</code> to not touch the LMA when loading the implicit linker script.</p>
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    1. COJust a wild guess: would adding another implicit script (after the one already being used) for the ARM build do the trick: `SECTIONS { .data { } AT >ROM }`. Alternatively, if the ARM build can use a different implicit script in the first place, just add the `AT>ROM` in the appropriate place in that script.
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      1. USMichael Burr
    2. COThat doesn't work because the ROM memory region isn't available outside the script where it's defined. I already tried that.
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    3. COHave you considered embedding your implicit script into the explicit script for ARM (thus you'd have - unfortunately - two scripts - one for ARM, and the implicit short one for x86) - this really is the simpliest solution... Or maybe use a custom section for the objects?
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      1. POHow to prevent implicit linker script changing the LMA of a section
      1. USFreddie Chopin
 

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