From concept to engineering model

A task control block (TCB) is the scheduler’s source of truth. If TCB fields are incomplete or inconsistent, context switching becomes unreliable regardless of scheduler policy.

TCB must encode enough state to resume task execution safely and deterministically.

Context fields and assembly save/restore logic must match exactly.

Task lifecycle transitions should be encoded through explicit state machine paths.

Mathematical relationships worth memorizing

Context storage estimate (32-bit registers):

Where:

• $N_{saved}$: number of stored registers/words

Applied design scenario

Implementation sequence:

1. Define TCB fields from switch code requirements, not from generic templates.
2. Initialize task stack frame in a deterministic, inspectable layout.
3. Use explicit APIs for state changes (ready, running, blocked).
4. Add debug assertions for invariant checks on queue and state fields.

Mistakes to prevent before hardware or runtime tests

• Missing context fields often fail only under nested interrupts or rare switch paths.
• Stale state flags can produce phantom runnable tasks.
• Reusing TCB memory without full reset leaks old execution metadata.
• Queue corruption should be treated as critical scheduler fault.

If a TCB can be audited at any point to explain exact run eligibility, scheduler debugging becomes much faster.