# Configuration Flexibility and Modularization

> **The criterion.** Engineers do not want to pay a resource penalty for features they don't use: the kernel
> should compile out unused components, and expose a clean, human-readable way to customize ticks, hooks, and
> priority levels.

## How Revera answers it

Revera is **minimal-safe by default** and scales up by compile-time knobs. Every optional feature is gated by
a `TK_CONFIG_*` macro that removes both its code *and* its per-thread struct fields when off — so a build pays
only for what it uses. There is no runtime feature registry: the build flag alone decides. Configuration is
plain, documented **header + command-line** (no code generation required), and ready-made **profiles** move
you along the size/safety ladder in one flag. Asking for hardware a target can't provide fails **loud** at
compile time — never a silent, broken build.

## Feature · how to turn it on · how to use it · where it works

| Feature | Turn on with | Use these APIs | Where |
|---|---|---|---|
| Compile out unused features | each `TK_CONFIG_*` (0/1) | see [`kernel/tk_config.h`](../../kernel/tk_config.h) — one documented block per knob | all |
| Ready-made profiles | `PROFILE=profiles/tk_profile_<nano\|dev\|hardened\|full>.h` | — | all |
| Static object pools (sized) | `TK_CONFIG_MAX_THREADS` / `_SEMAPHORES` / `_MUTEXES` / `_CONDS` | — | all |
| Kernel policy | setup | `tk_kernel_set_sched_policy` / `set_preempt_mode` / `set_idle_hook` | all |
| Fail-loud capability gates | — | compile-time `#error` on an impossible config (e.g. unpriv on a non-MPU arch) | all |

*Every knob — values, default, requirements, effect — lives in one place:
[`kernel/tk_config.h`](../../kernel/tk_config.h). See also the [API reference](../reference/api.md).*

## Limits & roadmap

- Configuration is **header + `-D` + profiles** — there is no graphical / Kconfig-style configurator (a
  future codegen path exists in the project's history: the `tkgen` YAML→C generator, not in 0.1.0).
- The chosen **profile is part of the ABI** (it changes `sizeof(struct tk_tcb)`), so build directories are
  profile-tagged.

## Learn more

- **How it works:** [composability](../architecture/composability.md) · [kernel tiers](../architecture/kernel-tiers.md).
- **The knobs:** [`kernel/tk_config.h`](../../kernel/tk_config.h) · [profiles/](../../profiles/).
- **Full API:** [API reference](../reference/api.md).