O D I T S Z A P C OF: Overflow flag TF: Trap flag AF: Aux carry F F F F F F F F F DF: Direction flag SF: Sign flag PF: Parity flag * * * * * IF: Interrupt flag ZF: Zero flag CF: Carry flag
DEC m8 DEC m16 DEC m32 DEC r8 DEC r16 DEC r32
DEC AL DEC CX DEC EBX DEC BYTE [BP] ; Decrements the BYTE at [BP] DEC WORD [BX] ; Decrements the WORD at [BX] DEC DWORD [EDX] ; Decrements the DWORD at [EDX]
Remember that segment registers cannot be decremented with DEC. All register-half opcodes are 2 bytes in length, but all 16-bit register opcodes are 1 byte in length. If you can decrement an entire register of which only the lower half contains data, use the 16-bit opcode and save a byte.
As with all instructions that act on memory, memory data forms must be used with a data size specifier such as BYTE, WORD, and DWORD! NASM doesn't assume anything!
r8 = AL AH BL BH CL CH DL DH r16 = AX BX CX DX BP SP SI DI sr = CS DS SS ES FS GS r32 = EAX EBX ECX EDX EBP ESP ESI EDI m8 = 8-bit memory data m16 = 16-bit memory data m32 = 32-bit memory data i8 = 8-bit immediate data i16 = 16-bit immediate data i32 = 32-bit immediate data d8 = 8-bit signed displacement d16 = 16-bit signed displacement d32 = 32-bit unsigned displacement