Bootloader Image Format

[中文]

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:

esptool --chip esp32 image-info ./build/bootloader/bootloader.bin

The resultant output will resemble the following:

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

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

  • secure_version: the secure version used by the bootloader anti-rollback feature, see CONFIG_BOOTLOADER_ANTI_ROLLBACK_ENABLE.

  • version: bootloader version, see CONFIG_BOOTLOADER_PROJECT_VER

  • idf_ver: ESP-IDF version. [1]

  • date and time: compile date and time

  • reserved2: reserved for the future IDF use

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

    or

    PRIV_REQUIRES esp_bootloader_format

Functions

const esp_bootloader_desc_t *esp_bootloader_get_description(void)

Return esp_bootloader_desc structure.

Intended for use by the bootloader.

Returns:

Pointer to esp_bootloader_desc structure.

Structures

struct esp_bootloader_desc_t

Bootloader description structure.

Public Members

uint8_t magic_byte

Magic byte ESP_BOOTLOADER_DESC_MAGIC_BYTE

uint8_t reserved[2]

reserved for IDF

uint8_t secure_version

The version used by bootloader anti-rollback feature

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

Macros

ESP_BOOTLOADER_DESC_MAGIC_BYTE

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

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