;es:di = string to search
;ds:si = string to search for
;cx = length of string to search
;dx = length of string to search for
;for a backwards search, set di and si to the end bytes and set df.
;call instr
;di = offset found at or -1 if not found
;destroys cx, dx, oh,al

instr:      lodsb            ; al = op1[0]; op1++
            sub dx, 1        ; len2--
            jc .found        ; if len2 was 0 then quit now
            sub cx, dx       ; len1 -= len2
            jc .notfound     ; if len1 flipped goto notfound
.loop:        jcxz .notfound   ; if len1==0 jmp notfound
               repne scasb     ; op1 = instr(op1[], op2[0])+1
               jne .notfound    ; if not instr(op1[], op2[0]) goto notfound
                push cx           
                push si          
                push di
                mov cx, dx
                repe cmpsb
                pop di
                pop si          
                pop cx          
                jne .loop       ; if op1[] != op2[] goto instrhere
            dec di
.found:     clc
            ret
.notfound:  mov di, -1
            stc
            ret