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STOSW

Stores a word value from the ax register into the memory location pointed to by the destination index register. The destination index is then incremented or decremented by 2 bytes, depending on the value of the direction flag (DF).

The table below covers what the source and destinations can be.

DO NOT support LOCK

This instruction is available in 64-bit mode, but it specifically operates on a word (2 bytes). In x86-64, the destination index register is determined by the address size; for STOSW, the destination is typically rdi.

When using the REP prefix, the instruction SHALL repeat until rcx reaches 0. If rcx is initially 0, the instruction SHALL NOT execute the store operation.

The destination memory address MUST be aligned to a 2-byte boundary to avoid performance penalties or exceptions in specific alignment-checking environments. If the direction flag (DF) is set, the destination index is decremented; otherwise, it is incremented. Failure to correctly set the DF before a REP STOSW operation SHALL result in memory corruption of the unintended address range.