Cortex M4 Context Switching

Context switching refers to the process of storing and restoring the state of a CPU so that execution can be resumed from the same point at a later time. This allows multiple processes to share a single CPU resource by multitasking between them. Context switching is an essential part of any multi-tasking operating system.

The Cortex-M4 is an extremely popular 32-bit ARM processor used in a wide range of embedded and IoT applications. As a high-performance microcontroller with DSP capabilities, efficient context switching is critical for the Cortex-M4 to handle multiple real-time tasks in embedded systems.

Cortex M4 Architecture for Context Switching

The Cortex-M4 architecture has several features that support low-latency context switching between threads or tasks:

• Banked register file – The M4 has 16 general purpose 32-bit registers, split into two banks. This allows threads to have dedicated registers without the need to save/restore on context switch.
• PendSV and SVC exceptions – Hardware interrupts for handling context switching and OS related service calls.
• In-built SYSTICK timer – Generates periodic interrupts for time slicing between threads.
• Memory Protection Unit – To enforce thread isolation and prevent illegal memory access.
• ATOMIC instructions – For safe manipulation of shared data without interruption.

The banked register file is a key enabler, as each thread can be assigned its own dedicated physical registers. This avoids register saving/restoring during context switch. The PendSV handler is triggered by software to initiate a context switch. This stores thread contexts in stack memory and updates the thread execution state.

Context Switching Sequence

Here are the typical steps involved in context switching between two threads on Cortex M4:

1. Thread 1 is currently executing using its dedicated register bank.
2. Thread 1 core state is saved e.g. PC, PSR, LR to its stack frame.
3. Critical section handling if required using PRIMASK or BASEPRI registers to prevent interruption.
4. Issue PENDSV software interrupt to trigger context switch.
5. PENDSV interrupt stores remaining Thread 1 context e.g. other registers, stack pointer.
6. PENDSV loads Thread 2 context from stack frame to registers and stack pointer.
7. Resume Thread 2 execution using its restored context.

The key point is that the banked registers avoid needing to save/restore all registers on each context switch. Only the core execution state and active stack needs to be swapped between threads. This optimization makes Cortex-M4 context switching highly efficient with nanosecond latencies.

Context Switch Time Optimization

While the Cortex-M4 architecture enables fast context switching, the actual latency is still dependent on several software factors:

• Number of registers saved/restored – Minimize this to only essential core registers.
• Stack frame design – Optimize for size and position to reduce memory access times.
• Critical section handling – Use PRIMASK/BASEPRI instead of disabling all interrupts.
• PendSV priority – Set PENDSV priority high enough to preempt lower priority threads.
• Interrupt latency – Reduce interrupt handler overheads and avoid nesting.

Saving only core registers to the stack rather than all context can reduce switching time. Positioning stacks in fast on-chip RAM also improves memory access speed during save/restore. Using the PRIMASK or BASEPRI registers to prevent higher priority threads preempting critical sections also avoids incurring full context switch overhead when disabling all interrupts.

Tuning the above parameters requires finding the optimal balance between minimal context switching overhead and retaining enough context and flexibility for practical RTOS usage.

Context Switch Latency Measurement

Measuring context switch latency accurately requires careful timing using hardware timers and interrupts. A typical approach is:

1. Start Timer 1 and trigger Thread 1 to perform a task, e.g. toggling a GPIO
2. When Thread 1 completes its task, it triggers Timer 2 and performs a context switch to Thread 2
3. Thread 2 handles the Timer 2 interrupt, stopping the timer interval
4. The elapsed time between Timer 1 and Timer 2 gives the context switch latency

Repeating this measurement over many iterations allows an accurate average latency to be calculated. The latency goal is usually in the order of microseconds for real-time embedded applications.

Debug and profiling tools like SEGGER SystemView can also measure context switch times and identify hotspots in an RTOS environment. This helps optimize the switching behavior during development.

Role of DSP/FPU Registers

The Cortex-M4 FPU provides an additional 32 x 64-bit floating point registers. These are banked similar to the core registers, allowing dedicated FPU resources per thread without save/restore overhead.

DSP instructions also use the core registers for increased performance. This means the DSP/FPU register states must also be preserved during context switching through additional stacking.

For threads performing intensive DSP or math operations, the additional FPU/DSP context can impact switching times. Techniques like lazy stacking may be used, where FPU registers are only saved when actually modified.

RTOS Context Switching

Commercial RTOS kernels like FreeRTOS, ThreadX, and Micrium uC/OS provide the mechanisms for efficient context switching on Cortex-M4. This includes:

• APIs for thread creation, synchronization, messaging
• Scheduler policy e.g. preemptive, cooperative, time sliced
• Prioritized thread execution based on readiness and criticality
• Interrupt handling with configurable priority levels
• Efficient PendSV and context switch handlers
• Kernel services optimized for low overhead

RTOS thread APIs allow defining separate stacks, priorities and other attributes. The kernel scheduler then manages context switching based on thread state, synchronization policies, priority preemption and identified ready-to-run threads.

Choosing an RTOS strategy involves tradeoffs between thread priorities, latency, resource usage and throughput. The optimal approach depends on the specific application requirements.

Bare Metal Context Switching

Some deeply embedded Cortex-M4 applications require hand crafted context switching without an RTOS. This “bare metal” approach has benefits like:

• No RTOS licensing cost overhead
• Avoids RTOS memory and CPU resource overheads
• More optimization and control over context switching behavior
• Better real-time predictability for time critical tasks

However, programming bare-metal context switching requires significant time and expertise. Challenges include:

• Manual scheduler and prioritization logic
• Synchronization mechanisms for shared resources
• Stack and memory management for each thread
• No assistance for race conditions, deadlocks etc.
• Re-implementation of RTOS services like messaging

Therefore, bare metal context switching is usually reserved for specialized cases where an RTOS is unsuitable or the expertise exists to develop an optimized custom scheduler.

Context Switching Pitfalls

Some common pitfalls to avoid with Cortex-M4 context switching include:

• Unnecessary stacking of inactive registers
• Context corruption due to interrupts during switch
• Priority inversion blocking high priority threads
• Insufficient stack allocation causing overflow
• Starvation of low priority threads
• Uncontrolled growth of stack usage over time

Tracking stack usage, avoiding extended interrupt handlers, prioritizing threads by criticality, and using an RTOS to manage concurrency helps avoid these issues.

Context switching is complex, so starting with an efficient vendor RTOS provides a robust foundation before attempting custom optimizations.

Summary

Efficient context switching enables effective multitasking on the Cortex-M4 MCU. The banked registers and PendSV architecture minimize overhead by avoiding full register saving on each switch. RTOS kernels or custom bare metal schedulers leverage these features to enable low-latency context switching suitable for real-time embedded applications.

Careful design considering parameters like stack usage, interrupt handling, critical sections and thread priorities is needed to optimize switching times. Benchmarking context switch latency helps tune performance during development.

On the Cortex-M4, context switching is measured in microseconds or even nanoseconds – enabling advanced responsive and deterministic behavior in embedded systems.