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SCAS

Scans a value in the al, ax, eax, or rax register against a value in the memory location pointed to by rdi. The instruction compares the two values and updates the CPU flags. After the comparison, the rdi register is incremented or decremented by the size of the operand, depending on the Direction Flag (DF).

The following table covers what the source and destinations can be:

DO NOT support LOCK

The instruction is available in 32-bit and 64-bit modes. In 64-bit mode, if a 32-bit operand size is used, rdi is still updated based on the 32-bit size, but the upper 32 bits of the register are preserved.

The rdi register MUST be correctly initialized to the starting address of the memory array before execution. If the Direction Flag (DF) is set to 1, rdi will be decremented; if DF is 0, rdi will be incremented. Failure to clear or set DF according to the intended traversal direction SHALL result in an out-of-bounds memory access.

When used with the REP prefix, the instruction continues until the Zero Flag (ZF) is cleared or the rcx register reaches zero. If rcx is initially zero, the instruction SHALL NOT execute the comparison and the loop will terminate immediately.