Linker Scripts

mohitmishra786/low-level-dev-skills/skills/embedded/linker-scripts

作者 mohitmishra786bdc58472fa9f無授權條款253 個星標收錄於 2026年10月9日更新於 2026年10月9日儲存庫3 個月前更新

Linker script skill for embedded bare-metal targets. Use when writing or modifying GNU ld linker scripts, placing code and data in specific flash or RAM regions, understanding VMA vs LMA, configuring startup .bss/.data initialization, using MEMORY and SECTIONS commands, or debugging linker errors about regions. Activates on queries about linker scripts, MEMORY command, SECTIONS command, .bss init, VMA vs LMA, weak symbols, or placing functions in specific memory regions.

僅含說明Software Development
AI 產生的概覽

指導為嵌入式裸機目標撰寫與修改 GNU ld 連結腳本，涵蓋記憶體區域、VMA/LMA 與啟動初始化。

功能
此技能為撰寫與修改嵌入式裸機韌體的 GNU ld 連結腳本提供指引與參考資料。它說明 MEMORY 與 SECTIONS 指令、VMA 與 LMA 的配置方式、啟動階段的 .bss/.data 初始化、將程式碼與資料放到指定的 flash 或 RAM 區域，以及 KEEP、ALIGN、PROVIDE、FILL 等指令。它也列出常見的連結錯誤及其原因與修正方式，並附上一份關於連結腳本結構的參考文件。
適用情境
在為 MCU 撰寫或修改連結腳本、把函式或資料放到指定的 flash 或 RAM 區域、理解 VMA 與 LMA 的差異，或設定啟動時的 .bss/.data 初始化時使用。它也適合用來診斷區域溢位或缺少連結腳本符號等連結錯誤。
執行需求
不隨附腳本，僅為說明與參考內容。需要熟悉 GNU ld 以及 arm-none-eabi 等嵌入式工具鏈，以便檢視區段大小與位址。

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

ld
/* 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).

ld
/* 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():

c
// startup.c or startup.s equivalent in Cextern uint32_t _sdata, _edata, _sidata; // _sidata = LMA of .dataextern 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):

ld
.data : AT(_etext){    _sdata = .;    *(.data)    _edata = .;} > RAM
_sidata = LOADADDR(.data);    /* LMA of .data for startup code */

4. Placing code in specific regions

ld
/* 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 */
c
// 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

ld
/* 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

ld
/* In linker script — provide weak default ISR */PROVIDE(NMI_Handler        = Default_Handler);PROVIDE(HardFault_Handler  = Default_Handler);PROVIDE(SysTick_Handler    = Default_Handler);
c
// 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 namevoid SysTick_Handler(void) {    tick_count++;}

7. Common linker errors

ErrorCauseFix
region 'FLASH' overflowedBinary too large for flashEnable LTO, -Os, remove unused code; --gc-sections
region 'RAM' overflowedToo much RAM usedReduce stack size, use static buffers, check .bss size
undefined reference to '_estack'Missing linker script symbolDefine _estack in linker script
no rule to process file.ld extension not recognizedPass with -T script.ld
cannot find linker scriptWrong pathUse -L dir -T name.ld
Data section .data at wrong addressLMA not setAdd AT(_etext) after .data section definition
bash
# Analyze section sizesarm-none-eabi-size firmware.elfarm-none-eabi-size -A firmware.elf    # verbose per-section
# Show all sections and their addressesarm-none-eabi-objdump -h firmware.elf
# Check if .data LMA is in flash rangearm-none-eabi-readelf -S firmware.elf | grep -A2 "\.data"

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

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

來源與署名

來源：mohitmishra786/low-level-dev-skills位於skills/embedded/linker-scripts提交bdc5847

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