This only applies to NON-FOTA firmware versions. The Espressif SDK family does not natively support a 16 MB flash mapping profile for application code execution. Instead, it handles a 16 MB chip (like the one on the WEMOS D1 Mini Pro) by addressing it as a 4 MB (32 Mbit) layout profile. The remaining 12 MB is invisible to the linker script and can only be accessed manually using raw sector reading/writing APIs (spi_flash_read).

In order to flash ESP8266 the GPIO0 should be connected to GND. This is D3 on Wemos D1 Mini Pro.

In a non-OTA layout, your build outputs two primary binary files:

1. eagle.flash.bin (Flashed at 0x00000): Contains code initialized straight into internal RAM—specifically IRAM (code required to execute with no flash latency, interrupt handlers) and DRAM (initialized global/static variables).
2. eagle.irom0text.bin (Flashed at offset 0x10000 or 0x20000): Contains main application code and SDK functions tagged with ICACHE_FLASH_ATTR. It remains in flash memory and is read dynamically via the Xtensa instruction cache (irom0).

In a traditional Non-OTA setup the flash is mapped using Flash Map 4 (or 6 depending on the exact SDK version configuration). In Espressif SDK nomenclature, Flash Maps correspond to numerical IDs (selected via FLASH_SIZE or map configuration during compile time).

Layout Structure:

• 0x00000 – 0x0B000 (~44 KB): eagle.flash.bin (IRAM + Data).
• 0x10000 or 0x20000 – 0x7B000 (~364 KB to 428 KB): eagle.irom0text.bin (Flash-cached application code).
• 0x7B000 – 0x80000 (Last 20 KB): System Parameter Area (RF calibration, Wi-Fi configuration saved by the SDK: esp_init_data_default.bin and blank.bin).

Layout Structure:

• 0x00000 – 0x0B000 (~44 KB): eagle.flash.bin (IRAM + Data).
• 0x20000 – 0xFC000 (~880 KB): eagle.irom0text.bin (Flash-cached application code).
• 0x100000 – 0x3FB000 (~3 MB): Unmapped / Free user space (available for SPIFFS/LittleFS or user storage).
• 0x3FC000 – 0x400000 (Last 16 KB): System Parameter Area (esp_init_data_default.bin & blank.bin) positioned at the end of the 4 MB physical flash boundary.

Because it is targeted as a 4 MB envelope, the default critical system parameters are pushed to the very end of the first 4 MB segment, not the physical end of the 16 MB chip.

Your application compilation must use the correct linker script. Look inside your sdk directory (ld/) and ensure you target:

eagle.app.v6.ld

This forces the compiler to build the code assuming a single application image (Non-OTA) rather than splitting it into user1.bin and user2.bin layout profiles typical of an OTA environment.

When flashing an esp-open-sdk firmware onto the WEMOS D1 Mini Pro, you must be careful with the -fs (flash size) parameter. If you set -fs 16MB, the Espressif system configuration files will be expected at the absolute end of the 16MB space (0xFFC000), which will cause a boot loop (rf_cal_size error) because the SDK logic is expecting them at the 4MB mark.

To successfully flash a Non-OTA open-sdk project on this board, force a 4MB size parameter during upload:

esptool.py --port /dev/ttyUSB0 --baud 460800 write_flash \
  -fm dio -fs 4MB \
  0x00000 eagle.flash.bin \
  0x10000 eagle.irom0text.bin \
  0x3FC000 esp_init_data_default.bin \
  0x3FF000 blank.bin

Note: If you explicitly want to map out the hidden 12 MB for custom partition spaces or filesystems, you must flash esp_init_data_default.bin at 0xFFC000 and blank.bin at 0xFFF000, but you must manually ensure your custom open-sdk Makefile overrides the default FLASH_SIZE constants to prevent SDK crashes.

# Full erase first
esptool.py --port /dev/ttyUSB0 --baud 115200 erase_flash

# Flash esp-open-sdk/sdk/bin/esp_init_data_default.bin at 0x3fc000
esptool.py --port /dev/ttyUSB0 --baud 115200 write_flash 
               --flash_mode dio --flash_size 32m 
               0x00000 firmware/0x00000.bin 
               0x10000 firmware/0x10000.bin 
               0x3fc000 /home/oscar/esp8266/esp-open-sdk/sdk/bin/esp_init_data_default.bin

esptool.py --port /dev/ttyUSB0 --baud 115200 write_flash 
		--flash_mode qio --flash_size 32m 
                0x00000 firmware/0x00000.bin 
                0x10000 firmware/0x10000.bin

sudo python esptool/esptool.py erase_flash
sudo python esptool/esptool.py -p /dev/ttyUSB0 -b 460800 write_flash -fm qio 0x00000 0x00000.bin 

Finally got the Wemos D1 mini pro with 16 MByte flash.

esptool.py erase_flash
esptool.py -b 460800 write_flash -fm qio 0x00000 bin/0x00000.bin 0x10000 bin/0x10000.bin

And finally it was resolved with dev version of NodeMcu prepared in https://nodemcu-build.com/

My best result is 17 modules in the BIN. Many was unsuccessful I reported result in GitHub (with modules) - https://github.com/nodemcu/nodemcu-firmware/pull/1529

Also their own fixed esptool used - https://github.com/nodemcu/nodemcu-firmware/tree/dev/tools

• https://github.com/nodemcu/nodemcu-firmware/tree/dev
• http://nodemcu.readthedocs.io/en/dev/en/
• https://github.com/nodemcu/nodemcu-firmware/pull/1529
• https://nodemcu-build.com/stats.php
• http://tiestvangool.ghost.io/2016/08/13/flash-nodemcu-esp8266/
• http://www.letscontrolit.com/wiki/index.php/ESPEasy#Stable