Firmware Image Format

This is technical documentation for the firmware image format used by the ROM bootloader. These are the images created by esptool.py elf2image.

The firmware file consists of a header, an extended header, a variable number of data segments and a footer. Multi-byte fields are little-endian.

File Header

The image header is 8 bytes long:

Byte

Description

0

Magic number (always 0xE9)

1

Number of segments

2

SPI Flash Mode (0 = QIO, 1 = QOUT, 2 = DIO, 3 = DOUT)

3

High four bits - Flash size (0 = 1MB, 1 = 2MB, 2 = 4MB, 3 = 8MB, 4 = 16MB)

Low four bits - Flash frequency (0 = 40MHz, 1 = 26MHz, 2 = 20MHz, 0xf = 80MHz)

4-7

Entry point address

esptool.py overrides the 2nd and 3rd (start from 0) bytes according to the SPI flash info provided through command line option, but only if there is no SHA256 digest appended after the image. Therefore, if you would like to change SPI flash info during flashing, i.e. with the esptool.py write_flash command, then generate images without SHA256 digests. This can be achieved by running esptool.py elf2image` with the --dont-append-digest argument.

Extended File Header

The 16-byte long extended header comes right after the image header, individual segments come right after it:

Byte

Description

0

WP pin when SPI pins set via efuse (read by ROM bootloader)

1-3

Drive settings for the SPI flash pins (read by ROM bootloader)

4-5

Chip ID (which ESP device is this image for)

6

Minimal chip revision supported by the image (deprecated, use the following field)

7-8

Minimal chip revision supported by the image (in format: major * 100 + minor)

9-10

Maximal chip revision supported by the image (in format: major * 100 + minor)

11-14

Reserved bytes in additional header space, currently unused

15

Hash appended (If 1, SHA256 digest is appended after the checksum)

Segment

Byte

Description

0-3

Memory offset

4-7

Segment size

8…n

Data

Analyzing a Binary Image

To analyze a binary image and get a complete summary of its headers and segments, use the image_info command with the --version 2 option.