Meaning of "ds:" in assembly language

Question

I'm just a beginner in assembly language and have probably silly question. What is difference between those two lines?

3e 8b 06 mov eax,DWORD PTR ds:[esi]
8b 06    mov eax,DWORD PTR [esi]

In my opinion it does the same, I also tried this program:

int main()
{
    __asm
    {
        mov esi, ebx
        mov eax,DWORD PTR [esi]
        mov ebx,DWORD PTR ds:[esi]
    }

    return 0;
}

And it confirmed my guess, so asking for you guys, if there is any difference. Why would we need two instructions that does the same but have different length opcodes.

Answer 1

The 3E byte in the first instruction is a DS segment override prefix (see "2.1.1 Instruction Prefixes" in Intel's Software Developer's Manual).

Group 2
— Segment override prefixes:
• 3EH—DS segment override prefix (use with any branch instruction is reserved)

In this case it's redundant because ds is the default segment for most memory accesses. Also, if you have a flat 32-bit memory space you typically never explicitly specify the segment register since they're set up to point to the same memory anyway.

Answer 2

1) DS = DATA-segment

2) Two different opcodes because two different "adressing modes". 3) These are two different types of x86 er based so called "adressing modes". A very basic stuff in asm.

a) https://cs.nyu.edu/courses/fall10/V22.0201-002/addressing_modes.pdf b) http://www.ic.unicamp.br/~celio/mc404s2-03/addr_modes/intel_addr.html