# ESP8266 Non-FOTA Flash Layout: WEMOS D1 Mini Pro (16MB)
----
## Framework: esp-open-sdk (NON-OTA Configuration)
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`).
### Flash execution
In order to flash ESP8266 the GPIO0 should be connected to GND. This is D3 on Wemos D1 Mini Pro.
### Non-OTA Binary Structure
In a non-OTA layout, your build outputs two primary binary files:
  - **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).
  - **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).

### Non-OTA Flash Maps: Map 4 vs. Map 6

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).

| Attribute | **Flash Map 4** | **Flash Map 6** |
| --- | --- | --- |
|Target Hardware|ESP-01 modules or older legacy ESP8266 boards with 512 KB flash chips|ESP-12E/F, NodeMCU, WeMos D1 Mini, and most modern ESP8266 development boards containing 4 MB flash|
| **Physical Flash Size** | 512 KB (4 Mbit) | 4 MB (32 Mbit) |
| **Max Code Space (`irom0`)** | ~364 KB – 428 KB | ~880 KB (Bounded by 1 MB MMU cache limit) |
| **Storage Beyond 1 MB** | None | ~3 MB available for SPIFFS/LittleFS |

#### 1. Flash Map 4 (512 KB Flash / 4 Mbit)
**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`).

#### 2. Flash Map 6 (4 MB Flash / 32 Mbit)
**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.

---

### 2. Crucial `esp-open-sdk` Compilation & Flashing Quirks

#### The Linker Script (`.ld`) Constraint

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

```text

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.

#### The `esptool.py` Flash Size Trick

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:

```bash
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.

==== ESP-OPEN-SDK Firmwares =====

==== First initial flash ====
<code>
# 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
</code>         
==== Subsequential flash ====
<code>
esptool.py --port /dev/ttyUSB0 --baud 115200 write_flash 
		--flash_mode qio --flash_size 32m 
                0x00000 firmware/0x00000.bin 
                0x10000 firmware/0x10000.bin
</code>
===== EASY ESP Firmware =====
<code>
sudo python esptool/esptool.py erase_flash
sudo python esptool/esptool.py -p /dev/ttyUSB0 -b 460800 write_flash -fm qio 0x00000 0x00000.bin 
</code>
===== NODEMCU Firmware =====
Finally got the Wemos D1 mini pro with 16 MByte flash.
<code>
esptool.py erase_flash
esptool.py -b 460800 write_flash -fm qio 0x00000 bin/0x00000.bin 0x10000 bin/0x10000.bin
</code>
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

===== LINKS ======
  * 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