DEC subtracts 1 from its operand. It does not affect the
carry flag: to affect the carry flag, use SUB something,1 (see
SUB). DEC affects all the other flags according to the result.
This instruction can be used with a LOCK prefix to allow atomic
execution.
See also INC (INC).
Every form of dec the assembler accepts, from NASM's own instruction table.
The second column is the processor that introduced the form; a badge marks a form that needs
an extension, that is lockable, or that long mode dropped.
| Form | Since | Notes |
|---|---|---|
| dec reg16 | 8086 | nolong |
| dec reg32 | 386 | nolong |
| dec r/m8 | 8086 | lock |
| dec r/m16 | 8086 | lock |
| dec r/m32 | 386 | lock |
| dec r/m64 | x86-64 | locklong |
| dec [reg8], r/m8 | x86-64 | apxlocklong |
| dec [reg16], r/m16 | x86-64 | apxlocklong |
| dec [reg32], r/m32 | x86-64 | apxlocklong |
| dec [reg64], r/m64 | x86-64 | apxlocklong |
