# Card plan — FC03

## Goal

Make scheduler policy observable without teaching delays, queues, semaphores
or C++ prematurely. A student must be able to distinguish priority selection
from time slicing and show the difference in Hazard3/GDB.

## Task

```text
Task01 · Controlled scheduler experiment
├── A1 CONTEXT      timer -> tick ISR -> scheduler -> tasks
├── A2 STRUCTURE    contexts and bounded evidence records
├── A4 APPLICATION  four-task topology and priority contract
├── A5 FLOW         deterministic E01-E12 replay
├── A6 STATE        Ready/Running/Deleted and priority transition
├── A7 RUNTIME      CSRs, ISR stack, TCB/stack and committed traces
└── Exercise        bounded observation with time slicing disabled
```

A3 is not repeated because callback dispatch was established in FC02. A8 is
not used because the scheduler policy is a kernel configuration rather than an
application design pattern.

## Runtime contract

1. `high_probe` is Ready at priority 0.
2. `peer_a` and `peer_b` are CPU-bound at priority 2.
3. `verifier` is Ready at priority 1 and cannot run while peers remain Ready.
4. The committed task-change trace must contain A-B-A.
5. Peer A records `before_raise` and calls `vTaskPrioritySet(high, 3)`.
6. High records `HIGH` and self-deletes before the API call returns.
7. Peer A records `after_raise`, requests peer termination and both peers
   self-delete.
8. The verifier checks every invariant and exits with PASS.

The exact ordering claim applies to the three-marker projection only. Other
legal scheduling activity may occur before peer A is selected again.

## Exercise

Disable `configUSE_TIME_SLICING` while keeping both CPU-bound peers at the same
priority. This is an expected-noncompletion experiment: observe for a bounded
number of ticks that one peer may starve the other, capture the debugger state,
then restore the configuration. The exercise does not redefine the card's PASS
contract.

The series-level rationale and all FC03 acceptance gates are maintained in:

`meta/workspace-info/plans/informatyka/freertos-c-fc03-a1-a8.md`.