From concept to engineering model

Registers are the active state of a Cortex-M core. Understanding register transitions is the shortest path to understanding function calls, exceptions, and scheduler behavior.

General registers carry data and addresses for arithmetic and memory instructions.

SP, LR, and PC define call/return and control-flow progression.

Status flags in xPSR influence conditional execution and branch outcomes.

Mathematical relationships worth memorizing

Instruction throughput estimate:

Where:

• $IPC$: instructions per cycle

Applied design scenario

Implementation sequence:

1. Trace one function in disassembly and log key register changes per instruction.
2. Correlate branch conditions with status flags instead of guessing flow.
3. Verify preserved/scratch register behavior across function boundaries.
4. Use debugger register windows as state snapshots, not as isolated numbers.

Mistakes to prevent before hardware or runtime tests

• Never assume register values survive calls unless ABI says so.
• Always account for exception stacking when debugging ISR paths.
• Treat LR and PC with context; raw values alone can mislead.
• Validate assumptions with actual instruction trace when possible.

Once registers feel predictable, most Cortex-M behavior stops feeling “magical” and becomes traceable state movement.