I vaguely remember that it had trouble remembering more than one prefix past an interrupt -- so if you wanted to combine REP with a segment override prefix you should put the REP first, then the segment override prefix, then the string instruction... and then some conditional jump stuff that jumped back to the REP if CX != 0.

So basically:

    repeat: rep
            cs:
            scasb
            jcxz   done
            jmp    short repeat
    done: