DEC
Decrement by 1
| Instruction | Description |
|---|---|
DEC r/m8 | Decrement r/m8 by 1 |
DEC r/m16 | Decrement r/m16 by 1 |
DEC r/m32 | Decrement r/m32 by 1 |
DEC r/m64 | Decrement r/m64 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.)
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 0x48 through 0x4F 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.
Operation
DEST: operand
DEST = DEST - 1;
Flags Affected
The CF flag is not affected. The OF, SF, ZF, AF, and PF flags are set according to the result.