NASM assembler - unwanted 66 in generated machine code

Question

• os:windows 10
• cpu:intel core i5-5300U
• architecture:x64

I just started to learn assembly language. I used the online compiler, but today I downloaded NASM. I tried to assemble this simple code that multiplies 10 by 15:

mov eax, 0xa
mov edx, 0xf
imul eax, edx
ret

An attempt to execute the machine code generated by NASM has failed so I opened this code in hex editor and compared it with code generated by online assembler that I can successfully execute.

NASM:

66 b8 0a 00 00 00 66 ba 0f 00 00 00 66 0f af c2 c3

online editor:

b8 0a 00 00 00 ba 0f 00 00 00 0f af c2 c3

The only differences between these two codes are the unwanted 66's in the code generated by NASM. Can anyone help me?

Answer 1

It is as Jester suggested. I do not included BITS 64 declaration. Now when I included BITS 64 everything works