# DEC — Decrement by 1

## Description

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.

## Instruction

| Opcode        | Assembly  | Op/En | Modern Mode | Legacy Mode | Description                                                                  |
|---------------|-----------|-------|-------------|-------------|------------------------------------------------------------------------------|
| FE /1         | DEC r/m8  | M     | Valid       | Valid       | Decrement r/m byte by 1.                                                     |
| REX + FE /1   | DEC r/m8* | M     | Valid       | N.E.        | Decrement r/m byte by 1.                                                     |
| FF /1         | DEC r/m16 | M     | Valid       | Valid       | Decrement r/m word by 1.                                                     |
| FF /1         | DEC r/m32 | M     | Valid       | Valid       | Decrement r/m doubleword by 1.                                               |
| REX.W + FF /1 | DEC r/m64 | M     | Valid       | N.E.        | Decrement r/m quadword by 1.                                                 |
| 48+ rw**      | DEC r16   | O     | N.E.        | Valid       | Decrement word register by 1.                                                |
| 48+ rd        | DEC r32   | O     | N.E.        | Valid       | Decrement doubleword register by 1.                                          |

- 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.

- 40H through 47H are REX prefixes in 64-bit mode.

## Information

This instruction can be used with a LOCK prefix to allow the instruction to be executed atomically.

n 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.