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Migrating from ARMLINK to LD script with overlays

Alexey Tsvetkov

I have a app.sct ARMLINK script file with overlay section:

OVERLAY_A ImageLimit(RAM_DATA_ON) OVERLAY
{
    *(.app.overlay_a)
}

I'm trying to achive the same result with gcc linker script as follows:

OVERLAY SIZEOF(RAM_DATA_ON) : 
{
    OVERLAY_A 
    {
            *(.app.overlay_a)
    }
}

ARMLINK automatically defines these symbols for overlay section:

Image$$OVERLAY_A$$ZI$$Length

Image$$OVERLAY_A$$RW$$Length

Image$$OVERLAY_A$$RO$$Length

Image$$OVERLAY_A$$RW$$Base

Image$$OVERLAY_A$$RO$$Base

Load$$OVERLAY_A$$RW$$Base

Load$$OVERLAY_A$$RO$$Base

which are actively used by application.

How to correctly define these in ld script file overlay? How do I get RO, RW, ZI parts from .app.overlay_a input section?

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  • 0

    Hi Alexey,

    So, unlike armlink, GNU ld does not automatically create these linker-defined symbols around sections. Instead, those need to be defined specifically in the linker script by doing something like:


    _my_data_start_marker_ = . ;
    .data:
    {
    *(.data*)
    }
    _my_data_end_marker_ = . ;


    which would give you the equivalent of:

    Image$$something$$RW$$Base
    Image$$something$$RW$$Limit

    and then the equivalent of Image$$something$$RW$$Length would be `_my_data_end_marker_ - _my_data_start_marker_`

    Also note that GNU uses different naming for:
    RO: .text
    RW: .data
    ZI: .bss

    As such, it looks like you'll have to break down the `*(.app.overlay_a)` into 3: one for the .text, one for .bss and one for .data and create symbols around them as `symbol = .`.

    Hope this helps!

    (I think these might be created as `*(.app.overlay_a.data) , *(.app.overlay_a.text) , *(.app.overlay_a.bss)` ... I don't have a linker script and build to check this at hand rn and it may depend on how you've created `.app.overlay_a`, so you might have to fiddle with that until it works)

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  • 0

    Hi Alexey,

    So, unlike armlink, GNU ld does not automatically create these linker-defined symbols around sections. Instead, those need to be defined specifically in the linker script by doing something like:


    _my_data_start_marker_ = . ;
    .data:
    {
    *(.data*)
    }
    _my_data_end_marker_ = . ;


    which would give you the equivalent of:

    Image$$something$$RW$$Base
    Image$$something$$RW$$Limit

    and then the equivalent of Image$$something$$RW$$Length would be `_my_data_end_marker_ - _my_data_start_marker_`

    Also note that GNU uses different naming for:
    RO: .text
    RW: .data
    ZI: .bss

    As such, it looks like you'll have to break down the `*(.app.overlay_a)` into 3: one for the .text, one for .bss and one for .data and create symbols around them as `symbol = .`.

    Hope this helps!

    (I think these might be created as `*(.app.overlay_a.data) , *(.app.overlay_a.text) , *(.app.overlay_a.bss)` ... I don't have a linker script and build to check this at hand rn and it may depend on how you've created `.app.overlay_a`, so you might have to fiddle with that until it works)

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