Question

Ok here’s my problem. I want to loop through a simple char buffer using
inline asm and VC++;

My agenda is to create a loop that reads information from memory as fast as physically possible

heres my code

char buffer[howmany];
memset(buffer,33,howmany);
char arr = 0;
__asm
{
MOV     eax,seg buffer ;operand size conflict
MOV     eds,eax
MOV ecx,[howmany]
MOV     ebx,ip
MOV     arr, ES::buffer[ecx] ;operand size conflict
                             ;inline assembler syntax error in 'third operand'
                             ;found 'bad token'
LOOP    ebx ;inline assembler syntax error in 'opcode'; found 'bad token'
                ; 'LOOP' : identifier is reserved word
}

I’m pretty new with assembly, but this seems right, but it doesn’t work? I’m not sure why. Thanks in advance.

Final Solution

__asm
{
    LEA         esi, buffer
    MOV      ecx,howmany
    buf_loop:   
    mov         eax, [esi]
    inc         esi
    dec         ecx
    jnz         buf_loop
}

Answer 1

First, there is no EDS, only DS. Even in 32-bit mode, the segment registers remain 16 bits.

Second, unless you’re working on an ancient system like a DOS extender, or something really unusual (a lot different from a typical desktop/server OS like Windows, Linux, OS/X, BSD, etc.) you shouldn’t modify any segment registers in any case. Most current systems use a “flat” memory model, where the OS sets up all¹ the segment registers with a base of 0 and a limit of the top of memory, so you never have any reason to modify any of them at all.

Unfortunately, while it’s easy to say your code is wrong, it’s a bit harder to say what would be right — you haven’t said enough about what you want to do. Right now, it looks like you’re copying from a buffer, but each time through the loop you’re overwriting the value you wrote in the last iteration, so you might as well just copy the last word and be done. For looping through the buffer to accomplish much, you’d want to copy it to a destination buffer of the same (or larger) size:

mov ecx, howmany
mov esi, offset FLAT:source
mov edi, offset FLAT:dest
rep movsd

As others have already pointed out, the operand for a loop instruction is a label, not a register. What they don’t seem to have pointed out is that with modern CPUs (anything newer than the original Pentium) you usually want to avoid using the LOOP instruction at all. Just for the sake of argument, however, a loop to do the move like above would look like:

    mov ecx, howmany
    mov esi, offset FLAT:source
    mov edi, offset FLAT:dest
move_loop:
    lodsd
    stosd
    loop move_loop

For a modern CPU, it’s usually better to use more, but simpler, instructions.

; same setup as above
move_loop:
    mov eax, [esi]
    mov [edi], eax
    inc esi
    inc edi
    dec ecx
    jnz move_loop

The other side of things is that in this case, it’s unlikely to matter — unless it all fits in cache, a block move like this will almost always be limited by the memory bandwidth — moves don’t get much faster, but to get the last little bit of improvement, you want to use SSE instructions/registers.

Edit: One last detail. VC++ (among others) won’t let you define a label inside an _asm block, so if you need a label, you do something like:

_asm {
    mov ecx, howmany
    mov esi, offset FLAT:source
    mov edi, offset FLAT:dest
}

move_loop:

_asm {
    lodsd
    stosd
    loop move_loop
}

¹Well, not all — FS and possibly GS won’t be this way, but CS, DS, ES and SS will be. You don’t want to change any of them anyway (in fact, attempting to do so will normally just get your program shut down).