| Opcode | Instruction | Op/En | 64-Bit Mode | Compat/Leg Mode | Description |
| FE /1 REX + FE /1 | DEC r/m8 DEC r/m8* | M M | Valid Valid | Valid N.E. | Decrement r/m8 by 1. 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. |
Notes: * In 64-bit mode, r/m8 can not be encoded to access the following byte registers if a REX prefix is used: AH, BH, CH, DH.
| Op/En | Operand 1 | Operand 2 | Operand 3 | Operand 4 |
| M | ModRM:r/m (r, w) | NA | NA | NA |
| O | opcode + rd (r, w) | NA | NA | NA |
Subtracts 1 from the destination operand, while preserving the state of the CF flag. The destination operand can be a register or a memory location. This instruction allows a loop counter to be updated without disturbing the CF flag. (To perform a decrement operation that updates the CF flag, use a SUB instruction with an immediate operand of 1.)
This instruction can be used with a LOCK prefix to allow the instruction to be executed atomically.
In 64-bit mode, DEC r16 and DEC r32 are not encodable (because opcodes 48H through 4FH are REX prefixes). Otherwise, the instruction’s 64-bit mode default operation size is 32 bits. Use of the REX.R prefix permits access to additional registers (R8-R15). Use of the REX.W prefix promotes operation to 64 bits.
See the summary chart at the beginning of this section for encoding data and limits.
DEST ← DEST - 1;
The CF flag is not affected. The OF, SF, ZF, AF, and PF flags are set according to the result.
| #GP(0) | If the destination operand is located in a non-writable segment. If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit. If the DS, ES, FS, or GS register contains a NULL segment selector. |
| #SS(0) | If a memory operand effective address is outside the SS segment limit. |
| #PF(fault-code) | If a page fault occurs. |
| #AC(0) | If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3. |
| #UD | If the LOCK prefix is used but the destination is not a memory operand. |
| #GP | If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit. |
| #SS | If a memory operand effective address is outside the SS segment limit. |
| #UD | If the LOCK prefix is used but the destination is not a memory operand. |
| #GP(0) | If a memory operand effective address is outside the CS, DS, ES, FS, or GS segment limit. |
| #SS(0) | If a memory operand effective address is outside the SS segment limit. |
| #PF(fault-code) | If a page fault occurs. |
| #AC(0) | If alignment checking is enabled and an unaligned memory reference is made. |
| #UD | If the LOCK prefix is used but the destination is not a memory operand. |
Same exceptions as in protected mode.
| #SS(0) | If a memory address referencing the SS segment is in a non-canonical form. |
| #GP(0) | If the memory address is in a non-canonical form. |
| #PF(fault-code) | If a page fault occurs. |
| #AC(0) | If alignment checking is enabled and an unaligned memory reference is made while the current privilege level is 3. |
| #UD | If the LOCK prefix is used but the destination is not a memory operand. |