# Advanced 4: A Minimal Filesystem — LittleFS

## Sample code

- Directory: `docs/os-on-arduino/code/adv4_fs`

```bash
uv run pio run -d adv4_fs -t upload
uv run pio device monitor -b 115200
```

Output starts when you send the single character `S`.

## Source code

The sketch for this chapter. Click a file name to expand. The code is mirrored automatically from the manuscript repository, so what you see here is always current.

:::{dropdown} `src/main.cpp` — 97 lines
:icon: code

[Open on GitHub](https://github.com/iory/learning-os-from-arduino/blob/main/docs/os-on-arduino/code/adv4_fs/src/main.cpp)

```{literalinclude} ../../../../code/adv4_fs/src/main.cpp
:language: cpp
:linenos:
```

:::

:::{dropdown} `include/fs.h` — 18 lines
:icon: code

[Open on GitHub](https://github.com/iory/learning-os-from-arduino/blob/main/docs/os-on-arduino/code/adv4_fs/include/fs.h)

```{literalinclude} ../../../../code/adv4_fs/include/fs.h
:language: cpp
:linenos:
```

:::

:::{dropdown} `include/fs_flash_hal.h` — 19 lines
:icon: code

[Open on GitHub](https://github.com/iory/learning-os-from-arduino/blob/main/docs/os-on-arduino/code/adv4_fs/include/fs_flash_hal.h)

```{literalinclude} ../../../../code/adv4_fs/include/fs_flash_hal.h
:language: cpp
:linenos:
```

:::

:::{dropdown} `src/fs.cpp` — 70 lines
:icon: code

[Open on GitHub](https://github.com/iory/learning-os-from-arduino/blob/main/docs/os-on-arduino/code/adv4_fs/src/fs.cpp)

```{literalinclude} ../../../../code/adv4_fs/src/fs.cpp
:language: cpp
:linenos:
```

:::

:::{dropdown} `src/fs_flash_hal.cpp` — 53 lines
:icon: code

[Open on GitHub](https://github.com/iory/learning-os-from-arduino/blob/main/docs/os-on-arduino/code/adv4_fs/src/fs_flash_hal.cpp)

```{literalinclude} ../../../../code/adv4_fs/src/fs_flash_hal.cpp
:language: cpp
:linenos:
```

:::


## On the board

```{figure} ../_static/adv4_bootcount.gif
:name: fig-adv4-bootcount
:width: 100%

Recorded while pressing RESET six times. `boot count` goes 9, 10, 11 … with each press. A RAM variable would go back to 0, so this shows it is stored in Data Flash.
```

## Pitfalls

(Collected from reader questions — ask via
[GitHub Issues](https://github.com/iory/build-your-own-arduino-rtos/issues).)