most avx-512 instructions may indicate one of 8 opmask registers (k0–k7). for instructions which use a mask register as an opmask, register `k0` is special: a hardcoded constant used to indicate unmasked operations. for other operations, such as those that write to an opmask register or perform arithmetic or logical operations, `k0` is a functioning, valid register. in most instructions, the opmask is used to control which values are written to the destination. a flag controls the opmask behavior, which can either be "zero", which zeros everything not selected by the mask, or "merge", which leaves everything not selected untouched. the merge behavior is identical to the blend instructions.