nextInstruction problem

; multi-segment executable file template.

data segment
    ; add your data here!
    pkey db "press any key...$"
    vnesiString db "vnesi string:$"
    string1 db ?
    string2 db ?
    string1Size db 0
    string2Size db 0
    stringSize db 0
    nextInstruction dw 0
ends

stack segment
    dw   128  dup(0)
ends

code segment

transform proc
pop dx ; next instruction is popped
mov nextInstruction, dx
pop ax   ; size of the string
mov stringSize, al
pop bx ; effective address of the string
mov ch, 0d
mov cl, stringSize
dec cx ; not to read the last '$'

tr:
cmp [bx], 65d
jle smallLetter
cmp [bx], 90
jge smallLetter
add [bx], 32d
smallLetter:
inc bx
loop tr

mov dx, nextInstruction
push dx
ret
transform endp

;;;;
;;;;
;;;;

enterString proc
pop dx ; this is next instruction
mov nextInstruction, dx
mov ax, nextInstruction
pop bx ; this is effective address of the string
mov cx, 50d

inputS:
mov ah, 1h
int 21h
cmp al, 36d
je stringEntered
mov ah, 0d
mov [bx], ax
inc bx
inc stringSize
loop inputS

stringEntered:
mov ah, 0d
mov [bx], al
mov ax, dx
mov dx, nextInstruction
push dx ;return next instruction
ret
enterString endp

start:
; set segment registers:
    mov ax, data
    mov ds, ax
    mov es, ax

    ; add your code here

    lea dx, vnesiString
    mov ah, 9
    int 21h


    mov bx, OFFSET string1
    push bx
    call enterString

    mov al, stringSize
    mov string1Size, al
    mov stringSize, 0d
    lea dx, vnesiString
    mov ah, 9
    int 21h

    mov bx, OFFSET string2
    push bx
    call enterString
    mov al, stringSize
    mov string2Size, al
    mov stringSize, 0d
    ; here we have entered 2 strings
    mov al, string1Size
    mov dl, string2Size



    mov bx, OFFSET string1
    mov ah, 0d
    mov al, string1Size
    push bx
    push ax
    call transform

    ; test
    mov dl, 10d
    mov ah, 2h
    int 21h

    lea dx, string1
    mov ah, 9
    int 21h




    mov ax, 4c00h ; exit to operating system.
    int 21h
ends

end start ; set entry point and stop the assembler.