Hey bahman
DDRA is defined as INPUT port and DDRD is defined as OUTPUT port.DDRx is data direction register which define for initialize I/O pins.PINA register get the readings from input pin of controller.I suggest to you refer the book of AVR controller and PIC controller by mazidi.chek I/O configuration and register of PIC18F452 for AVR atmega16 to PIC18F452 translation.