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.

`ds` is the default segment in that case, so indeed there is no difference except for the extra prefix byte. Also, under a typical OS, the segments `cs`, `ds`, `es` and `ss` all reference the same memory. — Jester, May 05 '15 at 10:59
@Jester just wondering, what is not typical OS with segments in different memory. — ST3, May 05 '15 at 11:14
For example, a real-mode one such as DOS (if you can still call that an OS). Also, the segments `fs` and/or `gs` are used for special purposes such as thread-local storage, and those are normally placed at an offset. — Jester, May 05 '15 at 11:18
The only mainstream OSes to use segments nontrivially were MS-DOS and Windows 3.x. But then, those were 16-bit, so 32-bit registers eax and such were not typically used. — Seva Alekseyev, May 05 '15 at 17:55
Possible duplicate of [What does "DS:\[40207A\]" mean in assembly?](https://stackoverflow.com/questions/3819699/what-does-ds40207a-mean-in-assembly) — user202729, Aug 06 '18 at 09:38

score 6 · Accepted Answer · answered May 05 '15 at 11:00

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.

Great thanks. I thought something similar, but good to have confirmation. — ST3, May 05 '15 at 11:03

score 4 · Answer 2 · answered May 05 '15 at 10:56

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

Meaning of "ds:" in assembly language

2 Answers2