| Opcode | Instruction | Op/En | 64-Bit Mode | Compat/Leg Mode | Description | | ---------- | ----------- | ----- | ----------- | --------------- | ------------------------------ | | 99 | CWD | ZO | Valid | Valid | DX:AX := sign-extend of AX. | | 99 | CDQ | ZO | Valid | Valid | EDX:EAX := sign-extend of EAX. | | REX.W + 99 | CQO | ZO | Valid | N.E. | RDX:RAX:= sign-extend of RAX. |