Decrement by One

Encoding

EncodingOperand
mModRM.r/m[rw]
oopcode(0..2)

Description

The DEC instruction decrements the operand by one. It is equivalent to a SUB instruction with an operand of 1, but without touching the EFLAGS.CF flag.

The opcode forms (bytes 0x48..0x4F) were repurposed in Long Mode for the REX prefix. As such, they are only encodable outside of Long Mode. If they are encountered in Long Mode, they will be mistakenly interpretted as REX prefixes for the following byte.

This instruction can be used with the LOCK prefix to allow atomic exectution.

Operation

public void DEC(ref U8 arg)
{
    arg--;
}
public void DEC(ref U16 arg)
{
    arg--;
}
public void DEC(ref U32 arg)
{
    arg--;
}
public void DEC(ref U64 arg)
{
    arg--;
}

Flags Affected

CF (carry flag)
Unmodified.
PF (parity flag)
Set according to the result.
AF (auxiliary flag)
Set according to the result.
ZF (zero flag)
Set according to the result.
SF (sign flag)
Set according to the result.
OF (overflow flag)
Set according to the result.

Exceptions

Real-Address Mode

#UD
  • If the LOCK prefix is used, but the destination is not a memory operand.
#SS(0)
  • If a memory operand using the SS segment has an effective address that is outside the SS segment's limit.
#GP(0)
  • If a memory operand (using a segment other than SS) has an effective address that is outside the segment's limit.

Virtual-8086 Mode

#UD
  • If the LOCK prefix is used, but the destination is not a memory operand.
#SS(0)
  • If a memory operand using the SS segment has an effective address that is outside the SS segment's limit.
#GP(0)
  • If a memory operand (using a segment other than SS) has an effective address that is outside the segment's limit.
#PF(fc)
  • If a page fault occurs.
#AC(0)
  • If alignment checking is enabled while the current privilege level is 3 and an unaligned memory access is made.

Protected Mode

#UD
  • If the LOCK prefix is used, but the destination is not a memory operand.
#SS(0)
  • If a memory operand using the SS segment has an effective address that is outside the SS segment's limit.
#GP(0)
  • If the destination is located in a non-writable segment.
  • If a memory operand uses a segment containing a NULL selector.
  • If a memory operand (using a segment other than SS) has an effective address that is outside the segment's limit.
#PF(fc)
  • If a page fault occurs.
#AC(0)
  • If alignment checking is enabled while the current privilege level is 3 and an unaligned memory access is made.

Compatibility Mode

#UD
  • If the LOCK prefix is used, but the destination is not a memory operand.
#SS(0)
  • If a memory operand using the SS segment has an effective address that is outside the SS segment's limit.
#GP(0)
  • If the destination is located in a non-writable segment.
  • If a memory operand uses a segment containing a NULL selector.
  • If a memory operand (using a segment other than SS) has an effective address that is outside the segment's limit.
#PF(fc)
  • If a page fault occurs.
#AC(0)
  • If alignment checking is enabled while the current privilege level is 3 and an unaligned memory access is made.

Long Mode

#UD
  • If the LOCK prefix is used, but the destination is not a memory operand.
#SS(0)
  • If a memory operand using the SS segment is in non-canonical form.
  • If a memory operand using the SS segment has an effective address that is outside the SS segment's limit.
#GP(0)
  • If a memory operand (using a segment other than SS) is in non-canonical form.
  • If the destination is located in a non-writable segment.
  • If a memory operand uses a segment containing a NULL selector.
  • If a memory operand (using a segment other than SS) has an effective address that is outside the segment's limit.
#PF(fc)
  • If a page fault occurs.
#AC(0)
  • If alignment checking is enabled while the current privilege level is 3 and an unaligned memory access is made.