xolatilization/xanual/dec.md

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.