mohitmishra786/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 */

LMA of .data is automatically placed after .text if you use AT(_etext).

3. Startup .bss / .data initialization

The C runtime must copy .data from flash to RAM and zero .bss before main():

// startup.c or startup.s equivalent in C
extern uint32_t _sdata, _edata, _sidata; // _sidata = LMA of .data
extern uint32_t _sbss, _ebss;

void Reset_Handler(void) {
    // Copy .data from flash to RAM
    uint32_t *src = &_sidata;
    uint32_t *dst = &_sdata;
    while (dst < &_edata) *dst++ = *src++;

    // Zero-initialize .bss
    dst = &_sbss;
    while (dst < &_ebss) *dst++ = 0;

    // Call C++ constructors
    // (call __libc_init_array() if using newlib)

    main();
    for (;;);  // should never return
}

Linker script provides _sidata (LMA of .data):

.data : AT(_etext)
{
    _sdata = .;
    *(.data)
    _edata = .;
} > RAM

_sidata = LOADADDR(.data);    /* LMA of .data for startup code */

4. Placing code in specific regions

/* Place time-critical code in RAM for faster execution */
MEMORY
{
    FLASH (rx) : ORIGIN = 0x08000000, LENGTH = 512K
    RAM   (rwx): ORIGIN = 0x20000000, LENGTH = 128K
    CCM   (rwx): ORIGIN = 0x10000000, LENGTH = 64K   /* Cortex-M4 CCM */
}

.fast_code : AT(_etext)
{
    _sfast = .;
    *(.fast_code)   /* sections marked __attribute__((section(".fast_code"))) */
    _efast = .;
} > CCM              /* runs from CCM RAM */

// Mark a function to go in fast_code section
__attribute__((section(".fast_code")))
void critical_isr_handler(void) {
    // runs from CCM RAM
}

5. KEEP, ALIGN, PROVIDE

/* KEEP — prevent garbage collection of section */
KEEP(*(.isr_vector))        /* linker gc won't remove interrupt table */
KEEP(*(.init))
KEEP(*(.fini))

/* ALIGN — advance location counter to alignment boundary */
. = ALIGN(8);               /* align to 8 bytes */

/* PROVIDE — define symbol only if not already defined (weak default) */
PROVIDE(_stack_size = 0x400);  /* default 1KB stack; override in code */

/* Symbols for stack */
.stack :
{
    . = ALIGN(8);
    . += _stack_size;
    _stack_top = .;
} > RAM

/* FILL — fill unused bytes */
.text :
{
    *(.text)
    . = ALIGN(4);
    FILL(0xFF)             /* fill flash gaps with 0xFF (erased state) */
} > FLASH

6. Weak symbols

/* In linker script — provide weak default ISR */
PROVIDE(NMI_Handler        = Default_Handler);
PROVIDE(HardFault_Handler  = Default_Handler);
PROVIDE(SysTick_Handler    = Default_Handler);

// In C — weak default handler
__attribute__((weak)) void Default_Handler(void) {
    for (;;);  // spin — override this in application
}

// Override by defining a non-weak symbol with the same name
void SysTick_Handler(void) {
    tick_count++;
}

7. Common linker errors

| Error | Cause | Fix | |-------|-------|-----| | region 'FLASH' overflowed | Binary too large for flash | Enable LTO, -Os, remove unused code; --gc-sections | | region 'RAM' overflowed | Too much RAM used | Reduce stack size, use static buffers, check .bss size | | undefined reference to '_estack' | Missing linker script symbol | Define _estack in linker script | | no rule to process file | .ld extension not recognized | Pass with -T script.ld | | cannot find linker script | Wrong path | Use -L dir -T name.ld | | Data section .data at wrong address | LMA not set | Add AT(_etext) after .data section definition |

# Analyze section sizes
arm-none-eabi-size firmware.elf
arm-none-eabi-size -A firmware.elf    # verbose per-section

# Show all sections and their addresses
arm-none-eabi-objdump -h firmware.elf

# Check if .data LMA is in flash range
arm-none-eabi-readelf -S firmware.elf | grep -A2 "\.data"

For linker script anatomy details, see references/linker-script-anatomy.md.

Related skills

• Use skills/embedded/openocd-jtag for flashing to addresses defined in linker script
• Use skills/embedded/freertos for FreeRTOS heap placement in specific RAM regions
• Use skills/binaries/linkers-lto for linker LTO and symbol flags
• Use skills/binaries/elf-inspection to inspect section sizes and addresses