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The memory order is said to be strong or sequentially consistent when either the order of operations cannot change or when such changes have no visible effect on any thread. [1] [4] Conversely, the memory order is called weak or relaxed when one thread cannot predict the order of operations arising from another thread.
The final revision of the proposed memory model, C++ n2429, [6] was accepted into the C++ draft standard at the October 2007 meeting in Kona. [7] The memory model was then included in the next C++ and C standards, C++11 and C11. [8] [9] The Rust programming language inherited most of C/C++'s memory model. [10]
Program order guarantees that each process issues a memory request ordered by its program and write atomicity defines that memory requests are serviced based on the order of a single FIFO queue. In relaxing program order, any or all the ordering of operation pairs, write-after-write, read-after-write, or read/write-after-read, can be relaxed.
However, on the 80386, with its paged memory management unit it is possible to protect individual memory pages against writing. [4] [5] Memory models are not limited to 16-bit programs. It is possible to use segmentation in 32-bit protected mode as well (resulting in 48-bit pointers) and there exist C language compilers which support that. [6]
However, when the memory is shared with multiple devices, such as other CPUs in a multiprocessor system, or memory-mapped peripherals, out-of-order access may affect program behavior. For example, a second CPU may see memory changes made by the first CPU in a sequence that differs from program order.
This can be used to model stack-based memory allocation (contrast with linear types which can be used to model heap-based memory allocation). [ 1 ] : 30–31 Without the exchange property, an object may only be used when at the top of the modelled stack, after which it is popped off, resulting in every variable being used exactly once in the ...
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The Causal Precedence relation in shared memory is related to the happened-before relation in message-based communication. [3] Thus, a system provides causal consistency if this following condition holds: write operations that are related by potential causality are seen by each process of the system in their causal precedence order.