Read-modify-write

读-修改-写（read-modify-write）是计算机科学中的一个原子操作（atomic operation，类似的还有test-and-set, fetch-and-add, compare-and-swap等），操作过程是读一个内存位置（或IO端口），修改其值，再写回原位置。^[1]^[2]

必须要先读操作的一个原因是，系统架构往往只允许字（word）级的读写，必须先读出那些不做修改的比特，保持不变再写回。写成C语言语句类似于：

*pRegister |= SOME_BIT;

Read-modify-write指令用于IO端口时，可能会产生出乎意料的结果，如无法给一个比特置位。这往往是因为写操作并不影响到读操作的源寄存器。^[3]

RAID也使用这一术语描述原子操作的read-modify-write序列。^[4] Such RAID levels include RAID 4, RAID 5 and RAID 6.

参见

可线性化（英语：Linearizability）
Read-erase-modify-write（英语：Read-erase-modify-write）

参考文献

^ "Writing Lock-Free Code: A Corrected Queue" by Herb Sutter: "Compare-and-swap (CAS) is ... widely available ... However, some systems instead provide the equivalently powerful load-linked/store-conditional (LL/SC) instead."
^ Herlihy, Maurice. Wait-free synchronization (PDF). ACM Trans. Program. Lang. Syst. January 1991, 13 (1): 124–149 [2007-05-20]. doi:10.1145/114005.102808. （原始内容存档 (PDF)于2011-06-05）.
^ Massmind: "The read–modify–write problem"
^ Basic RAID Organizations. umass.edu. [2013-10-04].

[1] "Writing Lock-Free Code: A Corrected Queue" by Herb Sutter: "Compare-and-swap (CAS) is ... widely available ... However, some systems instead provide the equivalently powerful load-linked/store-conditional (LL/SC) instead."

[2] Herlihy, Maurice. Wait-free synchronization (PDF). ACM Trans. Program. Lang. Syst. January 1991, 13 (1): 124–149 [2007-05-20]. doi:10.1145/114005.102808. （原始内容存档 (PDF)于2011-06-05）.

[3] Massmind: "The read–modify–write problem"

[4] Basic RAID Organizations. umass.edu. [2013-10-04].

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