Redundant DS segment override prefix inserted by nasm?

Question

I am working on a x86 disassembler for educational purposes. I have been using nasm for assembling different instructions.

Currently I am looking into the 16-bit addressing forms with the ModR/M byte. The "Intel 64 and IA-32 Architectures Software Developers Manual: Volume 2A Instruction Set Reference, A-M" (I think my version is outdated but this information should still hold true) states that "The default segment register is SS for the effective addresses containing a BP index, DS for other effective addresses".

So if I understand this correctly, the following instructions should do exactly the same:

mov [bx], eax
mov [ds:bx], eax

However, when I compile the above instructions using nasm and look at the bytes generated, I get the following:

67 89 07      mov [bx], eax
3e 67 89 07   mov [ds:bx], eax

I understand the meaning of all the bytes, but I do not understand why nasm would add a ds segment override prefix (3e) to the second instruction. It seems unnecessary to me since ds already is the default segment.

My question is: am I missing something here or is the ds segment override prefix not needed?

Answer 1

If you typed in mov [ds:bx], eax the assembler should assemble mov [ds:bx], eax. NASM is not (and should not!) an optimizing assembler which corrects your inputs. Maybe you exactly need space of four bytes for modifying the program code.