Calibrated timer / clock

docs/vision.md

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+# Vision: a real-time microkernel
+
+danos is aiming to be a **real-time operating system built on a microkernel** —
+where drivers and services run isolated in user space for maximum stability, and
+scheduling gives real guarantees about timing. This page is the north star: the
+*why* that shapes every design decision below it. Read it before adding anything
+structural.
+
+## Microkernel
+
+The kernel stays **minimal** — only what genuinely must run in privileged mode:
+
+- scheduling,
+- inter-process communication (IPC),
+- memory management (address spaces, page tables),
+- low-level interrupt dispatch.
+
+Everything else — device drivers, filesystems, the network stack — runs as an
+**isolated user-space server**, each in its own address space with only the
+privileges it needs.
+
+The payoff is **stability through isolation**. A driver bug can't corrupt the
+kernel or another driver; a crashing service is contained and can be restarted,
+while the rest of the system keeps running. That's the opposite of a monolithic
+kernel, where a single driver fault can take everything down.
+
+The cost is that **IPC becomes the backbone**: whatever used to be a function call
+across a monolithic kernel is now a message between address spaces. In a
+microkernel, IPC performance essentially *is* system performance (the lesson of
+L4). So IPC must be fast, and it's a first-class concern, not an afterthought.
+Hardware interrupts, too, become IPC: the kernel turns an IRQ into a message to the
+driver task that owns that device.
+
+## Real-time
+
+danos schedules **preemptively, with guarantees about quanta** — the system must
+be able to promise that a task runs when it's supposed to, within bounded time.
+That imposes concrete requirements:
+
+- **Fixed-priority preemptive scheduling.** The highest-priority ready task always
+  runs; a higher-priority task that becomes ready preempts a lower one immediately.
+  Not round-robin (which is fair but not predictable).
+- **A calibrated, deterministic clock.** Guarantees measured in "quanta" are
+  meaningless on an arbitrary tick rate — real time requires a timer calibrated to
+  a known frequency.
+- **Bounded interrupt latency.** Interrupt-disabled sections must be short and
+  bounded, so a ready high-priority task is never delayed by an unbounded kernel
+  operation.
+- **Deterministic kernel operations.** Scheduling decisions should be O(1) (e.g. a
+  priority bitmap), not "walk a list of unknown length."
+- **Priority inheritance** (once there are locks/IPC), so a high-priority task
+  blocked on a resource held by a low-priority one can't be delayed indefinitely by
+  a middle-priority task — bounding priority inversion.
+
+A consequence worth stating early: the current [kernel heap](heap.md) is a
+first-fit free list, which has **unbounded allocation time** and can fragment — it
+is *not* real-time safe. It's fine for one-time kernel setup, but real-time paths
+must pre-allocate or use a bounded (fixed-size pool) allocator. Don't allocate on a
+hot real-time path.
+
+## What this means for the roadmap
+
+The vision reorders the obvious hobby-kernel path. Notably, **drivers are not
+built into the kernel** — so an in-kernel keyboard driver would be throwaway work.
+Input devices arrive later, as the *first user-space drivers*, once the machinery
+to isolate them exists. The trajectory:
+
+1. **Calibrated timer / clock** — a known-frequency, deterministic tick. The
+   foundation real-time quanta rest on. *(next)*
+2. **Real-time scheduler** — fixed-priority preemptive, kernel threads first:
+   context switch, task struct, priority run-queue, timer-driven preemption.
+3. **User mode + address-space isolation** — higher-half kernel, ring 3, per-process
+   page tables. The substrate for isolated servers.
+4. **IPC** — fast message passing between address spaces. The microkernel's heart.
+5. **User-space drivers** — interrupts delivered as IPC, plus MMIO/port-access
+   grants. The keyboard becomes the first one, validating the whole model.
+
+## Where we are
+
+The foundation is in place: UEFI boot, framebuffer + [serial](testing.md),
+[physical frames](frame-allocator.md), [paging](paging.md) with W^X, [exceptions
+and interrupts](interrupts.md), a [timer](device-interrupts.md), and a
+[heap](heap.md) — plus a [test harness](testing.md). The kernel boots and has its
+core services; the next milestones make it *schedule*, then *isolate*.