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Docs: By-example Software tasks

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book/en/src/by-example/software_tasks.md
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# Software tasks & spawn
Software tasks, as hardware tasks, are run as interrupt handlers where all software tasks at the
same priority shares a "free" interrupt handler to run from, called a dispatcher. These free
interrupts are interrupt vectors not used by hardware tasks.
Software tasks are tasks which are not directly assigned to a specific interrupt vector.
To declare tasks in the framework the `#[task]` attribute is used on a function.
By default these tasks are referred to as software tasks as they do not have a direct coupling to
an interrupt handler. Software tasks can be spawned (started) using the `task_name::spawn()` static
method which will directly run the task given that there are no higher priority tasks running.
They run as interrupt handlers where all software tasks at the
same priority level shares a "free" interrupt handler acting as a dispatcher.
Thus, what differentiates software and hardware tasks are the dispatcher versus
bound interrupt vector.
To indicate to the framework which interrupts are free for use to dispatch software tasks with the
`#[app]` attribute has a `dispatchers = [FreeInterrupt1, FreeInterrupt2, ...]` argument. You need
to provide as many dispatchers as there are priority levels used by software tasks, as an
dispatcher is assigned per interrupt level. The framework will also give a compile error if there
are not enough dispatchers provided.
These free interrupts used as dispatchers are interrupt vectors not used by hardware tasks.
This is exemplified in the following:
The `#[task]` attribute used on a function declare it as a software tasks.
The static method `task_name::spawn()` spawn (start) a software task and
given that there are no higher priority tasks running the task will start executing directly.
A list of “free” and usable interrupts allows the framework to dispatch software tasks.
This list of dispatchers, `dispatchers = [FreeInterrupt1, FreeInterrupt2, ...]` is an
argument to the `#[app]` attribute.
Each interrupt vector acting as dispatcher gets assigned to one priority level meaning that
the list of dispatchers need to cover all priority levels used by software tasks.
Example: The `dispatchers =` argument needs to have at least 3 entries for an application using
three different priorities for software tasks.
The framework will give a compilation error if there are not enough dispatchers provided.
See the following example:
``` rust
{{#include ../../../../examples/spawn.rs}}