What is this piece of code supposed to do?

if (P0^0==d4) <-the compiler complains about this line. What value does d4 have? You've only allocated the variable, first time through it hasn't got a value so how can you test for equality? Equal to what??
d1==d1; <- what does this do? Should it not be d1 = d1? And besides, what would that do? Copy the same value to itself.


Also you're adding front (sbit) to a variable declared as a char. What result are you expecting?

If you had some useful comments in your code, we might be able to divine what you want to do, otherwise we're just guessing.