# Chapter 3: Implementing Context Switching

## Sample code

- Directory: `docs/os-on-arduino/code/03_context_switch`

```bash
uv run pio run -d 03_context_switch -t upload
uv run pio device monitor -b 115200
```

## 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` — 176 lines
:icon: code

[Open on GitHub](https://github.com/iory/learning-os-from-arduino/blob/main/docs/os-on-arduino/code/03_context_switch/src/main.cpp)

```{literalinclude} ../../../../code/03_context_switch/src/main.cpp
:language: cpp
:linenos:
```

:::

:::{dropdown} `include/os_kernel.h` — 66 lines
:icon: code

[Open on GitHub](https://github.com/iory/learning-os-from-arduino/blob/main/docs/os-on-arduino/code/03_context_switch/include/os_kernel.h)

```{literalinclude} ../../../../code/03_context_switch/include/os_kernel.h
:language: cpp
:linenos:
```

:::

:::{dropdown} `src/os_kernel.cpp` — 200 lines
:icon: code

[Open on GitHub](https://github.com/iory/learning-os-from-arduino/blob/main/docs/os-on-arduino/code/03_context_switch/src/os_kernel.cpp)

```{literalinclude} ../../../../code/03_context_switch/src/os_kernel.cpp
:language: cpp
:linenos:
```

:::


## On the board

```{figure} ../_static/ch03_yield.gif
:name: fig-ch03-yield
:width: 100%

`Task1` (registered as LED1) → `Task2` (LED2) → `Task3` (Serial) keep handing over to each other. They go round about 8 times a second, so the output scrolls quickly.
```

```{figure} ../_static/ch03_timing.svg
:name: fig-ch03-timing
:width: 100%

When each of the three tasks has the CPU and when the LEDs are lit, over two rounds measured
on the board. The tasks hand over with `os_yield()` and one round takes about 117 ms. The L LED
is lit for only about 22 ms of that and the left half of the matrix for about 12 ms, which is
why a video or a simulator GIF does not show them as blinking. (Measured by recording
`micros()` timestamps inside each task; the extra measuring code is not part of the book's sample.)
```

## Pitfalls

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