.model small
.stack 1024

.data

myString byte "Hello World!",0    ; a null-terminated string

StringRam byte 256 dup (0)        ;256 null bytes

.code

mov ax,@data
mov ds,ax       ;load data segment into DS
mov es,ax       ;also load it into ES

mov si,offset myString
mov di,offset StringRam
mov cx,12       ;length of myString
cld             ;make MOVSB auto-increment rather than auto-decrement (I'm pretty sure DOS begins with
                ;the direction flag cleared but just to be safe)

rep movsb      ;copies 12 bytes from [ds:si] to [es:di]
mov al,0       ;create a null terminator
stosb          ;store at the end. (It's already there since we initialized StringRam to zeroes, but you may need to do this depending
               ;on what was previously stored in StringRam, if you've copied a string there already.

mov ax,4C00h
int 21h         ;quit program and return to DOS