The deal:

If several push buttons are connected say, to PORTA, what is the best way to check that there is only one button pressed?

Here is an assembler suggestion (a snippet from Scott Dattalo):

     MOVF  PORTA,W
     ANDLW BUTTON_MASK
     SKPNZ 
     return       ; No buttons are pressed

     MOVWF temp   ; Clear one bit by using x &(x-1)
     ADDLW 0xff   ; W=PORTA-1
     ANDWF temp,W ; W=PORTA&(PORTA-1)
     SKPZ 
     return       ; More than one button is pressed 

     ; temp holds the one and only button press

Great isn't it?

And here it is the Pascal equivalent code, ready for use:

Temp := PORTA and BUTTON_MASK;
if Temp = 0 then exit; // exit if no button
if ((Temp - 1) and Temp) <> 0 then exit; // Exit if more than one at once
// Here Temp has the bit of the pressed button.