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Update target to support bootloader.

  1. Update linker script.
  2. Add required metadata to targets.json.
  3. Implement mbed_start_application.
  4. Implement flash HAL API.
  5. Verify changes with tests.

Linker script updates

When building a bootloader application or an application that uses a bootloader, the Arm Mbed OS build system automatically defines values for the start of application flash, MBED_APP_START, and size of application flash, MBED_APP_SIZE, when preprocessing the linker script. When updating a target to support this functionality, linker scripts must place all flash data in a location starting at MBED_APP_START and must limit the size of that data to MBED_APP_SIZE. This change must occur for the linker scripts of all toolchains - GCC Arm (.ld) and Arm (.sct). You can find examples of this for the k64f, stm32f429, odin-w2.

Use these 2 defines in place of flash start and size for a target:

  • MBED_APP_START - defines an address where an application space starts.
  • MBED_APP_SIZE - the size of the application.

Note: When an application does not use any of the bootloader functionality, then MBED_APP_START and MBED_APP_SIZE are not defined. For this reason, the linker script must define default values that match flash start and flash size..

An example of how a target could define MBED_APP_START and MBED_APP_SIZE in the linker script file:

#if !defined(MBED_APP_START)
  #define MBED_APP_START 0

#if !defined(MBED_APP_SIZE)
  #define MBED_APP_SIZE 0x100000

Be careful with these defines because they move the application flash sections. Therefore, you should move any sections within flash sectors accordingly.

Note: The VTOR must be relative to the region in which it is placed. To confirm, search for NVIC_FLASH_VECTOR_ADDRESS and SCB->VTOR, and ensure the flash address is not hardcoded.

Problematic declaration of flash VTOR address:

#define NVIC_RAM_VECTOR_ADDRESS   (0x20000000)
#define NVIC_FLASH_VECTOR_ADDRESS (0x00000000)

Bootloader-ready declaration of flash VTOR address:

#define NVIC_RAM_VECTOR_ADDRESS   (0x20000000)
#if defined(__ICCARM__)
    #pragma section=".intvec"
    #define NVIC_FLASH_VECTOR_ADDRESS   ((uint32_t)__section_begin(".intvec"))
#elif defined(__CC_ARM)
    extern uint32_t Load$$LR$$LR_IROM1$$Base[];
    #define NVIC_FLASH_VECTOR_ADDRESS   ((uint32_t)Load$$LR$$LR_IROM1$$Base)
#elif defined(__GNUC__)
    extern uint32_t vectors[];
    #define NVIC_FLASH_VECTOR_ADDRESS   ((uint32_t)vectors)
    #error "Flash vector address not set for this toolchain"

targets.json metadata

The managed and unmanaged bootloader builds require some target metadata from CMSIS Packs. Add a "device_name" attribute to your target as Adding and configuring targets describes.

Start application

The mbed_start_application implementation exists only for Cortex-M3, Cortex-M4 and Cortex-M7. You can find it in the Arm Mbed_application code file. If mbed_start_application does not support your target, you must implement this function in the target HAL.

Flash HAL

For a bootloader to perform updates, you must implement the flash API. This consists of implementing the function in flash_api.h and adding the correct fields to targets.json.

There are two options to implement flash HAL:

Option 1: CMSIS flash algorithm routines

These are quick to implement. They use CMSIS device packs and scripts to generate binary blobs. Because these flash algorithms do not have well-specified behavior, they might disable cache, reconfigure clocks and other actions you may not expect. Therefore, proper testing is required. First, make sure CMSIS device packs support your device. Run a script in mbed-os to generate flash blobs. Check the flash blobs into the target's HAL. Arm provides an example of how to do this.

To enable a CMSIS flash algorithm common layer, a target should define FLASH_CMSIS_ALGO. This macro enables the wrapper between CMSIS flash algorithm functions from the flash blobs and flash HAL.

    "extra_labels": [FLASH_CMSIS_ALGO]

The CMSIS algorithm common layer provides a trampoline, which uses a flash algorithm blob. It invokes CMSIS FLASH API, which the CMSIS-Pack Algorithm Functions page defines.

Option 2: Your own HAL driver

If CMSIS packs do not support a target, you can implement flash HAL by writing your own HAL driver.

Functions to implement:

int32_t flash_init(flash_t *obj);
int32_t flash_free(flash_t *obj);
int32_t flash_erase_sector(flash_t *obj, uint32_t address);
int32_t flash_program_page(flash_t *obj, uint32_t address, const uint8_t *data, uint32_t size);
uint32_t flash_get_sector_size(const flash_t *obj, uint32_t address);
uint32_t flash_get_page_size(const flash_t *obj);
uint32_t flash_get_start_address(const flash_t *obj);
uint32_t flash_get_size(const flash_t *obj);

To enable flash HAL, define FLASH in targets.json file inside device_has:

   "device_has": ["FLASH"]

Finally, to indicate that your device fully supports bootloaders, set the field bootloader_supported to true for the target in the targets.json file:

"bootloader_supported": true


The following tests for the FlashIAP class and flash HAL are located in the mbed-os/TESTS folder.

  • Flash IAP unit tests: tests-mbed_drivers-flashiap.
  • Flash HAL unit tests: tests-mbed_hal-flash.

They test all flash API functionality. To run the tests, use these commands:

  • Flash IAP: mbed test -m TARGET_NAME -n tests-mbed_drivers-flashiap.
  • Flash HAL: mbed test -m TARGET_NAME -n tests-mbed_hal-flash.


  • For targets with VTOR, a target might have a VTOR address defined to a hardcoded address as mentioned in the Linker script updates section.

  • Using Flash IAP might introduce latency as it might disable interrupts for longer periods of time.

  • Program and erase functions might operate on different sized blocks - page size might not equal to a sector size. The function erase erases a sector, the program function programs a page. Use accessor methods to get the values for a sector or a page.

  • Sectors might have different sizes within a device.

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