4.6 String Instructions

String operations are one of the best ways to apply something to a range of memory locations. Typical string operations are copying from one range to another and filling a range of memory locations with a specified value. Some string operations are:

rep movsd -- copy one DWORD from one string to another

rep stosd -- set the DWORD to the value in EAX

The last letter "D" of MOVSD or STOSD signifies DWORD. WORDs and BYTEs can be specified by using "W" or "B," respectively.

To use a string operation, 5 steps should be taken:

1. Set the source segment and offset

2. Set the destination segment and offset

3. Specify the direction (usually forward) of processing

4. Specify the number of units (DWORDs, WORDs, BYTEs) to apply the operation to

5. Specify the operation

Use DS to point to the source segment:

        mov     ax, ScratchSeg   ; from a defined segment
        mov     ds, ax

Use ES to point to the destination segment:

        mov     ax, 0A000h       ; graphics segment
        mov     es, ax

Specify the direction:

        cld             ; set direction flag forward

Specify the source offset:

        mov     esi, ScratchPad  ; set the source offset

Specify the destination offset:

        xor     edi, edi         ; set to 0

Specify the number of times to repeat the operation:

        mov     ecx, 16000

Specify the operation:

        rep movsd