Read-modify-write
Encyclopedia
In computer science
Computer science
Computer science or computing science is the study of the theoretical foundations of information and computation and of practical techniques for their implementation and application in computer systems...

, read-modify-write is a class of atomic operations
Linearizability
In concurrent programming, an operation is atomic, linearizable, indivisible or uninterruptible if it appears to the rest of the system to occur instantaneously. Atomicity is a guarantee of isolation from concurrent processes...

 such as test-and-set
Test-and-set
In computer science, the test-and-set instruction is an instruction used to write to a memory location and return its old value as a single atomic operation. If multiple processes may access the same memory, and if a process is currently performing a test-and-set, no other process may begin...

, fetch-and-add
Fetch-and-add
In computer science, the fetch-and-add CPU instruction is a special instruction that atomically modifies the contents of a memory location. It is used to implement mutual exclusion and concurrent algorithms in multiprocessor systems, a generalization of semaphores.In uniprocessor systems, it is...

, and compare-and-swap
Compare-and-swap
In computer science, the compare-and-swap CPU instruction is a special instruction that atomically compares the contents of a memory location to a given value and, only if they are the same, modifies the contents of that memory location to a given new value...

 which both read a memory location and write a new value into it simultaneously, either with a completely new value or some function of the previous value. These operations prevent race conditions in multi-threaded applications. Typically they are used to implement mutexes or semaphores
Semaphore (programming)
In computer science, a semaphore is a variable or abstract data type that provides a simple but useful abstraction for controlling access by multiple processes to a common resource in a parallel programming environment....

. These atomic operations are also heavily used in non-blocking synchronization
Non-blocking synchronization
In computer science, a non-blocking algorithm ensures that threads competing for a shared resource do not have their execution indefinitely postponed by mutual exclusion...

.

Maurice Herlihy
Maurice Herlihy
Maurice Herlihy is a computer scientist active in the field of multiprocessor synchronization. Herlihy has contributed to the design of concurrent algorithms, and in particular to the exposition and quantification of the properties and uses of hardware synchronization operations...

 (1991) ranks atomic operations by "consensus number", as follows:
  • ∞: memory-to-memory move and swap, augmented queue, compare-and-swap
    Compare-and-swap
    In computer science, the compare-and-swap CPU instruction is a special instruction that atomically compares the contents of a memory location to a given value and, only if they are the same, modifies the contents of that memory location to a given new value...

    , fetch-and-cons, sticky byte, Load-Link/Store-Conditional
    Load-Link/Store-Conditional
    In computer science, load-link and store-conditional are a pair of instructions that together implement a lock-free atomic read-modify-write operation....

  • 2n-2: n-register assignment
  • 2: test-and-set
    Test-and-set
    In computer science, the test-and-set instruction is an instruction used to write to a memory location and return its old value as a single atomic operation. If multiple processes may access the same memory, and if a process is currently performing a test-and-set, no other process may begin...

    , swap, fetch-and-add
    Fetch-and-add
    In computer science, the fetch-and-add CPU instruction is a special instruction that atomically modifies the contents of a memory location. It is used to implement mutual exclusion and concurrent algorithms in multiprocessor systems, a generalization of semaphores.In uniprocessor systems, it is...

    , queue, stack
  • 1: atomic read and atomic write


It is impossible to implement an operation that requires a given consensus number with only operations with a lower consensus number, no matter how many of such operations one uses.
Read-modify-write instructions often produce unexpected results when used on I/O
I/O
I/O may refer to:* Input/output, a system of communication for information processing systems* Input-output model, an economic model of flow prediction between sectors...

 devices, as a write operation may not affect the same internal register
Hardware register
In digital electronics, especially computing, a hardware register stores bits of information, in a way that all the bits can be written to or read out simultaneously.The hardware registers inside a central processing unit are called processor registers....

that would be accessed in a read operation.
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