linker-scripts

Linker Scripts

Purpose

Guide agents through writing and modifying GNU ld linker scripts for embedded targets: MEMORY and SECTIONS commands, VMA vs LMA for code relocation, startup .bss/.data initialization, placing sections in specific regions, and using PROVIDE/KEEP/ALIGN directives.

Triggers

• "How do I write a linker script for my MCU?"
• "How do I place a function in a specific flash/RAM region?"
• "What's the difference between VMA and LMA in a linker script?"
• "How does .bss and .data initialization work at startup?"
• "Linker error: region 'FLASH' overflowed"
• "How do I use weak symbols in a linker script?"

Workflow

1. Linker script anatomy

/* Minimal Cortex-M linker script */

ENTRY(Reset_Handler)            /* entry point symbol */

MEMORY
{
    FLASH (rx)  : ORIGIN = 0x08000000, LENGTH = 512K
    RAM   (rwx) : ORIGIN = 0x20000000, LENGTH = 128K
}

SECTIONS
{
    .text :                     /* code section */
    {
        KEEP(*(.isr_vector))    /* interrupt vector must be first */
        *(.text)
        *(.text.*)
        *(.rodata)
        *(.rodata.*)
        . = ALIGN(4);
        _etext = .;             /* end of flash content */
    } > FLASH

    .data : AT(_etext)          /* VMA = RAM, LMA = FLASH */
    {
        _sdata = .;
        *(.data)
        *(.data.*)
        . = ALIGN(4);
        _edata = .;
    } > RAM

    .bss :
    {
        _sbss = .;
        *(.bss)
        *(.bss.*)
        *(COMMON)
        . = ALIGN(4);
        _ebss = .;
    } > RAM

    /* Stack at top of RAM */
    _estack = ORIGIN(RAM) + LENGTH(RAM);
}

2. VMA vs LMA

• VMA (Virtual Memory Address): where the section runs at runtime
• LMA (Load Memory Address): where the section is stored in the image (flash)

For .data: stored in flash (LMA), copied to RAM at startup (VMA).

/* AT() sets LMA explicitly */
.data : AT(ADDR(.text) + SIZEOF(.text))
{
    _sdata = .;
    *(.data)
    _edata = .;
} > RAM          /* VMA goes in RAM */