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
- 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.
^ Massmind: "The read–modify–write problem"
^ Basic RAID Organizations. umass.edu. [2013-10-04].
