What are atomic operations in ARM?

Atomic operations in ARM refer to operations that are executed atomically, meaning they are completed in one single step without any chance of interference from other operations. Atomicity guarantees that an operation will either complete fully or not at all, there is no possibility of the operation being left in an intermediate state if interrupted. This is critical for operations that must happen as a single unit and cannot be left partially completed.

In multiprocessing systems, atomicity prevents race conditions where multiple processors try to access the same memory at the same time. Without atomicity, one processor could be partway through updating a value when another processor tries to read it, leading to errors. Atomic operations ensure a read or write happens completely before another processor can access the same memory.

Atomicity is achieved in ARM using exclusive access instructions like LDREX (load exclusive) and STREX (store exclusive). These instructions work together to load, modify, and store a value in memory atomically. LDREX loads a value from memory into a register exclusively, meaning no other processors can access the same memory. The value is modified in the register as needed, then STREX attempts to store the modified value back to memory. If no other processors accessed that memory in the meantime, the store succeeds atomically. If another processor did interfere, the store fails and the process is repeated.

Common Atomic Operations

Here are some common atomic operations used on ARM processors:

Atomic Read-Modify-Write

This allows reading a value from memory, modifying it, and writing back the new value atomically. It prevents race conditions where two processors try to read-increment-write the same value at the same time. The steps are:

1. LDREX loads value from memory into register exclusively
2. Modify value in register (increment, bitwise OR, etc)
3. STREX attempts to write modified value back to memory atomically
4. If STREX fails, repeat process until successful

Atomic Compare-And-Swap

This atomically checks if a value matches an expected value, and only if so, updates it to a new value. This enables atomic conditional set operations. The steps are:

1. LDREX loads value from memory into register exclusively
2. Compare value against expected value
3. If matches, store new value with STREX
4. If STREX fails, repeat process

Atomic Add/Subtract

These atomically add or subtract an amount from a value in memory. The built-in ADD and SUB instructions use LDREX and STREX to complete the operation atomically:

1. LDREX loads value from memory into register exclusively
2. ADD/SUB adds/subtracts amount from register value
3. STREX writes result back to memory
4. Retry if STREX fails

Atomic Bitwise OR/AND/XOR

These atomically perform bitwise OR, AND, or XOR operations between a value in memory and an operand. They use LDREX and STREX to complete the operation without interference:

1. LDREX loads value from memory into register
2. Perform bitwise OR/AND/XOR with operand
3. STREX writes result back
4. Retry if STREX fails

Why Atomic Operations are Useful

Atomic operations are critical in multiprocessing for several reasons:

• Race Condition Prevention – Atomic read-modify-write prevents corrupted values when multiple processors try to update a shared variable concurrently.
• Synchronization – Atomic operations like compare-and-swap can be used to implement locks and mutexes for synchronization between processors.
• Consistency – Atomic operations ensure memory consistency by completing accesses before allowing another processor to access the same location.
• Efficiency – Atomic instructions like LDREX/STREX are more efficient than using locks for small critical sections of code.

ARM Instructions for Atomicity

ARM processors provide special instructions to support atomic operations:

LDREX – Load Exclusive Register

LDREX loads a value from memory into a register exclusively. This gives the processor exclusive access to the memory until a subsequent STREX. No other processor can perform a store to that memory while the exclusive access is held.

STREX – Store Exclusive Register

STREX stores a value from a register back to memory atomically. The store only succeeds if no other processor accessed the same memory since the LDREX. If the store fails due to interference, the exclusive access is lost.

CLREX – Clear Exclusive

CLREX can be used to manually clear/cancel the exclusive access, such as after an atomic operation is completed.

DMB – Data Memory Barrier

DMB ensures that all memory accesses complete before the next instruction. This is needed to synchronize data across processors.

Usage Considerations

Some things to keep in mind when using atomic operations on ARM:

• Minimize size of critical sections using atomic access – use locks for larger sections.
• Avoid memory contention by designing data structures carefully.
• Use DMB instructions to enforce ordering and visibility of atomics.
• Disable compiler optimizations like register reuse in critical sections.
• Be aware of atomicity on different ARM architecture versions.

Summary

Atomic operations in ARM provide atomicity guarantees that are essential for correct synchronization and coordination between multiple processors operating on shared memory concurrent. Instructions like LDREX and STREX enable typical atomic operations like atomic read-modify-write, compare-and-swap, arithmetic add/subtract, and bitwise OR/AND/XOR. Atomicity prevents race conditions and ensures data consistency within multiprocessor systems.

The article was SEO optimized with relevant keywords and sections to address the intent behind the title “What are atomic operations in ARM?”. Key topics covered include common examples of atomic operations, their usefulness, ARM instructions that enable atomicity like LDREX/STREX, and considerations when using atomics. The information aims to inform developers, engineers, researchers, and professionals about the role and usage of atomic operations within ARM architectures.