Bootloader Image Format

The bootloader image consists of the same structures as the application image, see Application Image Structures. The only difference is in the Bootloader Description structure.

To get information about the bootloader image, please run the following command: --chip esp32s3 image_info build/bootloader/bootloader.bin --version 2
File size: 26576 (bytes)

ESP32 image header
Image version: 1
Entry point: 0x40080658
Segments: 4
Flash size: 2MB
Flash freq: 40m
Flash mode: DIO

ESP32 extended image header
WP pin: 0xee
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
Minimal chip revision: v0.0, (legacy min_rev = 0)
Maximal chip revision: v3.99

Segments information
Segment   Length   Load addr   File offs  Memory types
-------  -------  ----------  ----------  ------------
    1  0x01bb0  0x3fff0030  0x00000018  BYTE_ACCESSIBLE, DRAM, DIRAM_DRAM
    2  0x03c90  0x40078000  0x00001bd0  CACHE_APP
    3  0x00004  0x40080400  0x00005868  IRAM
    4  0x00f2c  0x40080404  0x00005874  IRAM

ESP32 image footer
Checksum: 0x65 (valid)
Validation hash: 6f31a7f8512f26f6bce7c3b270f93bf6cf1ee4602c322998ca8ce27433527e92 (valid)

Bootloader information
Bootloader version: 1
ESP-IDF: v5.1-dev-4304-gcb51a3b-dirty
Compile time: Mar 30 2023 19:14:17

Bootloader Description

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

  • magic_byte - the magic byte for the esp_bootloader_desc structure.

  • reserved - reserved for the future IDF use.

  • version - bootloader version, see CONFIG_BOOTLOADER_PROJECT_VER

  • idf_ver - ESP-IDF version. *

  • date and time - compile date and time.

  • reserved2 - reserved for the future IDF use.

* - The maximum length is 32 characters, including null-termination character.

To get the esp_bootloader_desc_t structure from the running bootloader, use esp_bootloader_get_description().

To get the esp_bootloader_desc_t structure from a running application, use esp_ota_get_bootloader_description().

API Reference

Header File

  • components/esp_bootloader_format/include/esp_bootloader_desc.h

  • This header file can be included with:

    #include "esp_bootloader_desc.h"
  • This header file is a part of the API provided by the esp_bootloader_format component. To declare that your component depends on esp_bootloader_format, add the following to your CMakeLists.txt:

    REQUIRES esp_bootloader_format


    PRIV_REQUIRES esp_bootloader_format


const esp_bootloader_desc_t *esp_bootloader_get_description(void)

Return esp_bootloader_desc structure.

Intended for use by the bootloader.


Pointer to esp_bootloader_desc structure.


struct esp_bootloader_desc_t

Bootloader description structure.

Public Members

uint8_t magic_byte


uint8_t reserved[3]

reserved for IDF

uint32_t version

Bootloader version

char idf_ver[32]

Version IDF

char date_time[24]

Compile date and time

uint8_t reserved2[16]

reserved for IDF



The magic byte for the esp_bootloader_desc structure that is in DRAM.