Why ARM Cortex M3 Bit Manipulation is Atomic?

The ARM Cortex-M3 processor implements atomic bit manipulation via exclusive access instructions that allow read-modify-write operations on single bits to execute atomically. This ensures thread safety for multithreaded programs accessing the same variables and prevents race conditions that could lead to corrupted data.

Overview of Atomic Operations

An atomic operation in computer science refers to a set of operations that can be combined to appear as one single operation to the rest of the system. Atomic operations have two key properties:

• Atomcity – The operations cannot be interrupted by any other process or thread. The set of operations must execute completely before any other operations can take place.
• Consistency – The system state must be consistent before and after the atomic operation. No intermediary states are observable.

Atomic operations are critical for implementing thread-safe code and preventing race conditions in concurrent programming. Race conditions occur when two or more threads try to access and manipulate the same data simultaneously. Without atomicity, the operations may interleave unpredictably and corrupt the data.

ARM Cortex-M3 Processor Architecture

The ARM Cortex-M3 is a 32-bit RISC processor optimized for embedded applications. It has a simplified 3-stage pipeline compared to more complex ARM application processors. Some key architectural features relevant to atomic bit manipulation include:

• Single-cycle 32-bit multiply
• Low-latency interrupt handling
• Bit-banding for atomic bit manipulation
• Exclusive access instructions for synchronization

The Cortex-M3 implements the ARMv7-M architecture. This includes Thumb-2 technology for efficient 16- and 32-bit instruction encoding and the Microcontroller Profile which eliminates features unnecessary for embedded applications.

Exclusive Access Instructions

The Cortex-M3 provides exclusive access instructions to facilitate atomic read-modify-write operations. These include:

• LDREX (load exclusive register) – Loads a word from memory into a register exclusively.
• STREX (store exclusive register) – Stores a word from a register back to memory exclusively.

Using LDREX and STREX together enables atomic RMW operations. The processor sets an exclusive access flag during LDREX. STREX will fail if any other access to the memory address occurred between LDREX and STREX, preventing modification of data changed by another thread.

Atomic Bit Manipulation

The Cortex-M3 implements bit-banding to provide atomic access to individual bits in memory mapped peripherals. Bit-banding maps each bit in a word to its own 32-bit address space. Writes to these bit-band addresses set or clear the corresponding bit atomically.

For example, consider a peripheral register at address 0x4000 0000. Bit 5 of this register is aliased at address 0x4220 0004. Writing a 1 or 0 to this address will set or clear bit 5 atomically. The processor handles the read-modify-write sequence internally.

Bit-band areas for peripherals are identified in the Memory Map section of the Cortex-M3 Devices Generic User Guide. The atomic bit manipulation capability simplifies thread-safe control of peripheral states.

Example Code for Atomic Bit Set

Here is an example code snippet to atomically set bit 5 of the peripheral register 0x4000 0000 using bit-band addressing:

#define BITBAND_PERI_BASE 0x42000000  // Peripheral bit-band region base
#define PERI_REG_ADDR 0x40000000      // Peripheral register address 

int main() {
  volatile unsigned int *bitband_alias; // Pointer to bit-band alias

  // Calculate bit-band alias address
  bitband_alias = BITBAND_PERI_BASE + ((PERI_REG_ADDR - PERI_BASE) * 32)  
                  + (5 * 4); 

  *bitband_alias = 1; // Atomic bit set  
}

This avoids the need for exclusive access instructions while ensuring the bit modification is atomic. The processor handles locking internally for the bit-band write.

Use Cases for Atomic Operations

Some common use cases that benefit from atomic operations on Cortex-M3 processors include:

• Thread synchronization – Atomic read-modify-write operations on shared data can coordinate thread execution.
• Resource protection – Atomic access prevents corruption from simultaneous requests.
• Device drivers – Atomic control of peripheral registers prevents race conditions.
• Embedded Linux – Kernel-level atomic operations require hardware support.

Interrupt handlers also require atomicity when modifying shared data used by main program flow. The Cortex-M3 exclusive instructions and bit-banding provide the necessary hardware support.

Considerations for Atomic Operations

There are some limitations to consider when using atomic operations on Cortex-M3:

• Atomicity is guaranteed only for single word or bit accesses. Multi-word sequences require more care.
• Spinlocks may be required to retry failed atomic operations.
• Interrupt handlers should use stack variables and save context to avoid corrupting state.
• Bit-banding trades memory size for atomicity. Each bit has a 4 byte address alias.

Despite these limitations, the Cortex-M3 exclusive instructions and bit-banding provide excellent support for atomic operations needed in most embedded applications.

Conclusion

Atomic bit manipulation is implemented on ARM Cortex-M3 processors through exclusive access instructions and bit-banding. LDREX and STREX allow atomic read-modify-write operations. Bit-banding provides atomic access to individual bits in memory mapped peripherals. Together, these features enable robust multithreaded programming by preventing race conditions and ensuring data consistency.