Hi friend
First of all, You can use two type of process for reading switch.
One is Interrupt, for this you have to add some type of additional hardware to interrupt signal.
It is easy for program handling.
Second is polling, It is not required additional hardware, but it require constant attention in program flow.
I am using Polling system for reading switch in my all project.
Second thing is that, why do you use 8 switch in your project,
because your requirement is fulfill with only two switch.
I have done this type of project only with two switch
1) <- (digit selection move left side up to first digit and then rotate to forth digit)
2) ^ (digit increment up to 9 and then overflow to 0)
Hope this is help you
Shyam
INDIA