help for c program in PIC16f877

Now I have a question that to saving data into pic16f877 and after a while i can use a push button then reload the data and show on lcd. I don't know how to program this part in c code, is there anyone can help me.

 

Use DATA EEPROM of 16f877. Its very simple. Follow instructions from datasheet n u'll get it.

 

Reply if u can't

 

Use the interrupt which is activated by a signal on a pin of the pic and then write in this interrupt the instructions you want. If you program in, as i do, read the C reference manual to see how you read and write to the EEPROM.

 

Anyone has any idea how to write the coding for an function say... "inverse()" in c programming which will only invert or toggle the 4 lower significant bits (a0~a3) and leave the 4 most significant bits (a4~a7) unchanged?

e.g.

porta = 0bx01010000;

and after the inverse function,

porta = inverse(porta);

so porta now is 0b01011111.

any one have any ideas? I've got a headache busting my brains out on this problem...

thanks in advance man...