Using String Commands for Data Transfer

Many programs spend quite a lot of time on operations that involve copying and moving large amounts of data. For these purposes, the Intel processors have a number of commands that allow you to speed up the operations over multibyte arrays and strings. There are a number of processor instructions, called string commands, which have been developed for these tasks .

The group of string commands includes commands such as movs , lods , cmps , scas . When these commands are used without the repetition prefix ( rep ), they will hinder the application performance. Since these commands are complex, they require a fairly large number of processor cycles, and therefore cannot be optimized by the processor well enough. A good alternative to such instructions is the program code using the ordinary commands.

But if you use the string commands with the repetition prefix ( rep ), you can achieve high performance levels in copying and moving the data. In this case, the most efficient commands are those containing a mnemonic indication of the size of the operands (such as lodsb , lodsw , lodsd , movsb , movsw , etc.). For instance, if you need to copy 100 double words from the src memory buffer to the dst buffer, you can use the following sequence of commands:

 . . . mov  ECX, 100  lea  ESI, src  lea  EDI, dst  cld  rep  movsd  . . . 

The following sequence of commands may be a good alternative to the code fragment considered above:

 . . .   mov  ECX, 100    lea  ESI, src    lea  EDI, dst  next:    mov  EAX, [ESI]    mov  [EDI], EAX    add  ESI, 4    add  EDI, 4    dec  ECX    jnz  next    . . . 

If you proceed even further, unrolling the loop as to copy two double words simultaneously in one iteration, you will obtain a much more efficient code than the variant using the rep movsd command:

 . . .    mov  ECX, 100    lea  ESI, src    lea  EDI, dst  next:    mov  EAX, [ESI]    mov  [EDI], EAX    mov  EAX, [ESI+4]    mov  [EDI+4], EAX    add  ESI, 8    add  EDI, 8    sub  ECX, 2    jnz  next    . . .