| Opcode | Instruction | Op/En | 64-Bit Mode | Compat/Leg Mode | Description | | ------------- | ------------ | ----- | ----------- | --------------- | ----------------------- | | FE /1 | DEC _r/m8_ | M | Valid | Valid | Decrement _r/m8_ by 1. | | REX + FE /1 | DEC _r/m8_\* | M | Valid | N.E. | Decrement _r/m8_ by 1. | | FF /1 | DEC _r/m16_ | M | Valid | Valid | Decrement _r/m16_ by 1. | | FF /1 | DEC _r/m32_ | M | Valid | Valid | Decrement _r/m32_ by 1. | | REX.W + FF /1 | DEC _r/m64_ | M | Valid | N.E. | Decrement _r/m64_ by 1. | | 48+rw | DEC _r16_ | O | N.E. | Valid | Decrement _r16_ by 1. | | 48+rd | DEC _r32_ | O | N.E. | Valid | Decrement _r32_ by 1. | > * In64-bit mode, r/m8 cannot been coded to access the following byte registers if a REX prefix is used: AH, BH, CH, DH.