| 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.  |