org 100h
jmp start
string1 db 'Hello '
string2 db 'World'
string3 db 'XXXXXXXXXXX'
start:
cld
lea si, string1
lea di, string3
mov cx, 6
rep movsb
lea si, string2
mov cx, 5
rep movsb
end
would append World to Hello and put the result in string3 for example.