# 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 esp32s2 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

[1]

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

  > 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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