DEC - Decrement by 1

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.

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.

Instruction Operand Encoding

Op/En

Operand 1

Operand 2

Operand 3

Operand 4

M

ModRM:r/m (r, w)

N/A

N/A

N/A

O

opcode + rd (r, w)

N/A

N/A

N/A

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 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.

Operation 

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.

Protected Mode Exceptions

#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.

Real-Address Mode Exceptions

#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.

Virtual-8086 Mode Exceptions

#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.

Compatibility Mode Exceptions

Same exceptions as in protected mode.

64-Bit Mode Exceptions

#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.