assembler calc

.386
.model flat,stdcall
option   casemap:none
    include \masm32\include\masm32.inc
    include \masm32\include\gdi32.inc
    include \masm32\include\user32.inc
    include \masm32\include\kernel32.inc
    include \masm32\include\shell32.inc
    include \masm32\macros\macros.asm
    include \masm32\include\msvcrt.inc

    includelib \masm32\lib\masm32.lib
    includelib \masm32\lib\gdi32.lib
    includelib \masm32\lib\user32.lib
    includelib \masm32\lib\kernel32.lib
    includelib \masm32\lib\shell32.lib
    includelib  \masm32\lib\msvcrt.lib
    P proto :DWORD, :DWORD
.DATA
    Hello db "Type your command!: ",0
    InvalidOp db "This is invalid..",0Ah,0
    FirstOp db "Type first operand: ",0
    SecOp db "Type second operand: ",0
    Answer db "%d %c %d = %d",0Ah,0
    AnswerF db "%d %c %d = %d,%d",0Ah,0
    Oper byte 0
    Plus db "+",0
    Op db 8 dup (0)
    Numa db 8 dup (0)
    N_a dword ?
    Numb db 8 dup (0)
    N_b dword ?
.CODE
START:
    lea eax,Hello
    push eax
    call StdOut
    push 8
    lea eax,Op
    push eax
    call StdIn
    xor eax,eax
    lea eax,Op
    cmp byte ptr[eax],'+'
    je ADDICTION
    cmp byte ptr[eax],'-'
    je SUBSTRACTION
    cmp byte ptr[eax],'/'
    je DIVISION
    cmp byte ptr[eax],'*'
    je MULTIPLY
    jmp INVALID
CONTINUE:
    cls
    lea eax, FirstOp
    push eax
    call StdOut
    xor eax,eax
    push 8
    lea eax,Numa
    push eax
    call StdIn
    lea eax,Numa
    push eax
    call atol
    mov N_a,eax
    cls
    push 8
    lea eax,SecOp
    push eax
    call StdOut
    push 8
    lea eax,Numb
    push eax
    call StdIn
    lea eax,Numb
    push eax
    call atol
    mov N_b,eax
    cmp Oper,1
    je AD
    cmp Oper,2
    je SU
    cmp Oper,3
    je DV
    jne MU
    jmp INVALID
AD:
    mov eax,N_a
    add eax,N_b
    push eax
    push N_b
    push '+'
    push N_a
    lea eax,Answer
    push eax
    call crt_printf
    jmp START
SU:
    mov eax,N_a
    sub eax,N_b
    push eax
    push N_b
    push '-'
    push N_a
    lea eax, Answer
    push eax
    call crt_printf
    jmp START
DV:
    mov eax,N_a
    push edx
    xor edx,edx
    div N_b
    mov ebx,edx
    pop edx
    push ebx
    push eax
    push N_b
    push '/'
    push N_a
    lea eax, AnswerF
    push eax
    call crt_printf
    jmp START
MU:
    mov eax,N_a
    mul N_b
    push eax
    push N_b
    push '*'
    push N_a
    lea eax, Answer
    push eax
    call crt_printf
    jmp START
ADDICTION:
    mov Oper,1
    jmp CONTINUE
SUBSTRACTION:
    mov Oper,2
    jmp CONTINUE
DIVISION:
    mov Oper,3
    jmp CONTINUE
MULTIPLY:
    mov Oper,4
    jmp CONTINUE
INVALID:
    push offset InvalidOp
    call StdOut
    jmp START
EXIT:
    invoke  ExitProcess, 0
END  START