The control register inside the Cortex-M processor core serves a critical role in configuring and controlling the operation of the processor. This register allows software to enable/disable certain processor features, control power modes, set stack pointers, handle exceptions, and more. Having fine-grained control over the processor’s operation is essential for building high-performance yet energy-efficient embedded systems.
Overview of the Cortex-M Processor Architecture
The Cortex-M processor from ARM is a 32-bit RISC processor architecture commonly used in embedded and IoT applications. Its optimized design provides high performance and power efficiency. The Cortex-M processor contains multiple components like:
- Processor core – executes instructions
- Nested Vectored Interrupt Controller (NVIC) – manages interrupts
- System control block (SCB) – configures system and handles exceptions
- Memory Protection Unit (MPU) – provides memory access control
- Bus matrix – connects components to physical memory
At the heart of the Cortex-M processor is an execution pipeline that fetches, decodes, and executes ARM Thumb instructions. Surrounding this simple 3-stage pipeline are the other components that enable the flexible, low-power operation desired in embedded systems.
The System Control Block (SCB)
A key component of the Cortex-M processor is the System Control Block (SCB). This implements functionality like power management, stack pointers, exception handling, and configuration through special registers including the control register.
The SCB contains registers like:
- System control register
- Configuration and control register
- Priority mask registers
- Exception return addresses
- Interrupt control and state registers
- Processor core register
These registers allow privileged software to customize the environment and operation of the Cortex-M system. The control register is one such important SCB register.
Overview of the Control Register
The control register is a 32-bit writeable CPU register present in the System Control Block of Cortex-M processors. It allows enabling key capabilities of the processor. The control register consists of multiple fields that control specific functions when set to 1 or disabled when set to 0. It provides an essential configuration interface for the processor.
The main functions managed through the control register are:
- Enable Stack alignment checking for Exceptions
- Enable Divide by 0 Fault
- Enable User Mode access to Floating Point registers
- Enable Lazy context save of Floating Point state
- Enable Branch target exception
- Enable Instruction cache
- Enable Data cache
- Control processor MMU
- Manage power modes
- Set process stack pointer
- Set exception stack pointer
Setting these fields appropriately allows tailoring the processor to the application needs. The specific bit assignments differ slightly between the Cortex-M variants like Cortex-M3, Cortex-M4, Cortex-M7 etc. But the overall functionality remains similar.
Key Functions of the Control Register
1. Managing Processor Stack Pointers
The control register configures the stack pointers used by the processor – the Main Stack Pointer (MSP) and Process Stack Pointer (PSP). These point to memory regions used as stacks during normal operation and exception handling. The control register controls:
- The current stack pointer – MSP or PSP
- The memory address loaded into MSP on reset
- The memory address loaded into PSP on reset
Setting these stack pointers appropriately allows flexibility in stack allocation across privilege levels and power modes.
2. Handling Exceptions
The processor relies on the control register during exception handling. Key configurations include:
- The base address of the exception stack frame
- Enabling fault exceptions like divide-by-zero
- Enabling stack alignment checking on exception entry
This allows tailoring exception handling and resilience to faults as needed by the application.
3. Power Management
The control register manages entry into low power modes:
- Idle mode – CPU sleeps until next interrupt
- Sleep mode – System sleeps until reset or wakeup event
- Deep sleep mode – Lowest power mode
This allows optimizing for low power operation in resource constrained devices.
4. Caches and MMU Control
The control register enables and disables options like:
- Instruction cache
- Data cache
- Memory Protection Unit (MPU)
- Branch target exception
This provides software control over memory architectures for performance or reliability.
5. Floating Point Configuration
For variants like Cortex-M4F with floating point, the control register manages:
- Enabling access to floating point registers in user mode
- Enabling lazy context saving of floating point state
This allows optimizing floating point management for user applications.
Using the Control Register
The control register is updated by writing to it using the MSR instruction in privileged modes. A read can also be performed using the MRS instruction. The register is typically configured during processor initialization to tailor it to the system.
Values like stack pointers, exception addresses, and power mode settings are written before kicking off main application execution. Other fields like cache enabling may be set dynamically. Updates during run-time allow adapting to changing system requirements.
Reading the control register allows system utilities to validate configuration during operation. It provides a crucial insight into the processor’s current state.
Conclusion
The Cortex-M control register is at the heart of configuring the processor’s operation. It enables critical capabilities and system-level tuning required in embedded applications. Proper configuration via this register helps build efficient and robust ARM-based systems leveraging the Cortex-M processor family’s strengths.
