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STOSQ

Stores the value contained in the RAX register into the memory location pointed to by RDI, then increments or decrements RDI by 8 bytes according to the current state of the Direction Flag (DF).

The table after the description covers what the source and destinations can be.

DO NOT support LOCK

This instruction is available in 64-bit mode and compatibility mode. In 32-bit mode, it is not available; the corresponding instruction is STOSD.

The operation depends on the Direction Flag (DF). If DF = 0, RDI is incremented; if DF = 1, RDI is decremented. To avoid unintended memory corruption or buffer overflows, the programmer SHALL ensure the DF is explicitly set using CLD or STD before execution.

The instruction is typically used with the REP prefix to fill a block of memory. When using REP, the RCX register SHALL contain the count of qword elements to be stored.