thanks it's work now

if you please explain The structure of the code

mov byte ptr num[ eax ], 0

First I have a question. Did your instructor tell you to use the API that comes with masm32 or do it yourself?

mov byte ptr num[eax],0

Memory access can be direct from memory

i didn't studied Assembly but I hear about assembly and I want to learn the assembly .I ask another forum how i can start with assembly they tell me to start with use API
I'm interested in system and virus protected
and I'm programmer c++,for that I want to learn Assembly

thanks for help me

Find a good book and start working the sample problems. You need to work your way up.
Having the ability to single step your code is best!
Since you have C++ how about starting with inline code!

I prefer an assembly file, but inline is much easier to work with initially!

There is an author I like that has two books about assembly language.
One is a more advanced book, "Vector Game Math Processors" which I really like. It came with a CD and is out of print but is still available from Amazon.
The other is very complete, is still in print "32/64-Bit 80x86 Assembly Language Architecture" and has downloadable code samples. It has mistakes as I understand the author was rushed but I still like the book.
The files download link...
http://www.jbpub.com/catalog/9781598220025/samples/
The samples are C code that has some inline but has external assembly files that it links with. The best of both worlds. Do your input/output in a higher level language such as C/C++ and call the assembly code directly to do the low level functions.

In your particular case, pick one, compile it and then run it. Set a breakpoint and then single-step following the code into the assembly function. You can use it as a testbench for learning your assembly. The single-stepping with disassembly view and register views turned on will allow you to see the registers and flags changing as the instructions are executed!