Read-modify-write

讀-修改-寫（read-modify-write）是計算機科學中的一個原子操作（atomic operation，類似的還有test-and-set, fetch-and-add, compare-and-swap等），操作過程是讀一個內存位置（或IO端口），修改其值，再寫回原位置。^[1]^[2]

必須要先讀操作的一個原因是，系統架構往往只允許字（word）級的讀寫，必須先讀出那些不做修改的位元，保持不變再寫回。寫成C語言語句類似於：

*pRegister |= SOME_BIT;

Read-modify-write指令用於IO端口時，可能會產生出乎意料的結果，如無法給一個比特置位。這往往是因為寫操作並不影響到讀操作的源寄存器。^[3]

RAID也使用這一術語描述原子操作的read-modify-write序列。^[4] Such RAID levels include RAID 4, RAID 5 and RAID 6.

參見

可線性化（英語：Linearizability）
Read-erase-modify-write（英語：Read-erase-modify-write）

參考文獻

^ "Writing Lock-Free Code: A Corrected Queue". Dr. Dobb's Journal. （頁面存檔備份，存於網際網路檔案館） 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 Transactions on Programming Languages and Systems. January 1991, 13 (1): 124–149 [2007-05-20]. doi:10.1145/114005.102808. （原始內容存檔 (PDF)於2011-06-05）.
^ Massmind: "The read–modify–write problem". techref.massmind.org. [2017-11-22]. （原始內容存檔於2017-12-01）.
^ Basic RAID Organizations. umass.edu. [2013-10-04]. （原始內容存檔於2013-10-24）.

[1] "Writing Lock-Free Code: A Corrected Queue". Dr. Dobb's Journal. （頁面存檔備份，存於網際網路檔案館） 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 Transactions on Programming Languages and Systems. January 1991, 13 (1): 124–149 [2007-05-20]. doi:10.1145/114005.102808. （原始內容存檔 (PDF)於2011-06-05）.

[3] Massmind: "The read–modify–write problem". techref.massmind.org. [2017-11-22]. （原始內容存檔於2017-12-01）.

[4] Basic RAID Organizations. umass.edu. [2013-10-04]. （原始內容存檔於2013-10-24）.

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