how to cute a buff?

Well actually at run-time the formal parameters look like this

procedure copy(var:string; index, length:integer; var result: string);

This conversion from function to procedure is required because a string obviously doesn't fit in a CPU register (the same happens when you don't return simple types).

Well after the conversion this is one way

Code:

proc copy, var, index, length, result
  push    esi edi

  mov     esi, [var]
  mov     edi, [result]
  add     esi, [index]
  mov     ecx, [length]
  rep     movsb

  pop     edi esi
  ret
endp

Please note that the proc I written is unsafe since it doesn't checks if index is beyond the last character nor checks if length is too large. So assuming that the string are ASCIIZ (null terminated like in C) here another proc

Code:

proc copy, var, index, length, result
  push    esi edi

  mov     esi, [var]
  mov     edi, [result]
  mov     ecx, [index]
  test    ecx, ecx
  jz      .done

.checkZero: 
  lodsb
  test    al, al
  jz      .exit
  dec     ecx
  jnz     .checkZero

.done: 
  mov     ecx, [length]
  test    ecx, ecx
  jz      .exit

.copyChars:
  lodsb
  stosb
  test    al, al
  jz      .return
  dec     ecx
  jnz     .copyChars

.exit: 
  mov     byte[edi], 0

.return: 
  pop     edi esi
  ret
endp

[edit]Actually the Pascal-like procedure declaration of my ASM codes is:
procedure copy(const var: string; index, length: integer; var result: string); {string type actually is ASCIIZ in my code}[/edit]