Bootloader Image Format

:link_to_translation:zh_CN:[中文]

The bootloader image consists of the same structures as the application image, see :ref:Application Image Structures <app-image-structures>. The only difference is in the :ref:image-format-bootloader-description structure.

To get information about the bootloader image, please run the following command:

.. code-block::

esptool --chip {IDF_TARGET_PATH_NAME} image-info ./build/bootloader/bootloader.bin

The resultant output will resemble the following:

.. code-block::

esptool v5.0.2
Image size: 26352 bytes

ESP32 Image Header
==================
Image version: 1
Entry point: 0x40080644
Segments: 3
Flash size: 2MB
Flash freq: 40m
Flash mode: DIO

ESP32 Extended Image Header
===========================
WP pin: 0xee (disabled)
Flash pins drive settings: clk_drv: 0x0, q_drv: 0x0, d_drv: 0x0, cs0_drv: 0x0, hd_drv: 0x0, wp_drv: 0x0
Chip ID: 0 (ESP32)
Minimal chip revision: v0.0, (legacy min_rev = 0)
Maximal chip revision: v3.99

Segments Information
====================
Segment   Length   Load addr   File offs  Memory types
-------  -------  ----------  ----------  ------------
    0  0x018e8  0x3fff0030  0x00000018  BYTE_ACCESSIBLE, DRAM, DIRAM_DRAM
    1  0x03e58  0x40078000  0x00001908  CACHE_APP
    2  0x00f5c  0x40080400  0x00005768  IRAM

ESP32 Image Footer
==================
Checksum: 0x6b (valid)
Validation hash: 09fdc81d436a927b5018e19073a787cd37ffce655f505ad92675edd784419034 (valid)

Bootloader Information
======================
Bootloader version: 1
ESP-IDF: v6.0-dev-1620-g15d7e41a848-dirt
Compile time: Aug  8 2025 16:22:1

.. _image-format-bootloader-description:

Bootloader Description

The DRAM0 segment of the bootloader binary starts with the :cpp:type:esp_bootloader_desc_t structure which carries specific fields describing the bootloader. This structure is located at a fixed offset = sizeof(:cpp:type:esp_image_header_t) + sizeof(:cpp:type:esp_image_segment_header_t).

magic_byte: the magic byte for the esp_bootloader_desc structure
reserved: reserved for the future IDF use
secure_version: the secure version used by the bootloader anti-rollback feature, see :ref:CONFIG_BOOTLOADER_ANTI_ROLLBACK_ENABLE.
version: bootloader version, see :ref:CONFIG_BOOTLOADER_PROJECT_VER
idf_ver: ESP-IDF version. [#f1]_
date and time: compile date and time
reserved2: reserved for the future IDF use

.. [#f1] The maximum length is 32 characters, including null-termination character.

To get the :cpp:type:esp_bootloader_desc_t structure from the running bootloader, use :cpp:func:esp_bootloader_get_description.

To get the :cpp:type:esp_bootloader_desc_t structure from a running application, use :cpp:func:esp_ota_get_bootloader_description.

API Reference

.. include-build-file:: inc/esp_bootloader_desc.inc