String Entry from keyboard--Could somebody help me?

I am asked to make a program in Assembly that I have to extract a sub-string from an initial string that would be entered from keyboard.
From the keyboard I also have to enter two numbers.
The first number signifies the sub-string start and the second number the length of it.
Before I extract and print to screen the sub-string, I have to check that the length of the sub-string is not Zero or it is not greater than the initial string. In case that it is Zero or greater a message has to be appeared on the screen.

Thanks for your help in advance!!!