The portModeRegister() is a macro that returns a mode register that controlls the mode of the specified port.
The portModeRegister() is defined in hardware/arduino/avr/cores/arduino/Arduino.h as below.
The input is P. The macro calls pgm_read_word() with an argument pin_to_mode_PGM + (P).
The port_to_mode_PGM is defined in hardware/arduino/variants/standard/pins_arduino.h(in case of Arduino Uno). It is an array shown below. As this array is declared as PROGMEM, it is placed on flash memory rather than SRAM.
As the port_to_mode_PGM is an array, adding (P) to the port_to_mode_PGM means the (P+1)th element of the array. In C++ language, the element of array x[i] can be accessed by *(x + i). For example, if the P is PB(2), the third element(note that the elements begins with 0) of the array, that is the address of DDRB, is returned.
DDRB, DDRC and DDRDare registers which determine if the deigital pin is output mode or input mode.
The pgm_read_word() is a macro to read a variable defined as PROGMEM.
As a result of portModeRegister(), it returns the (P+1)th element of port_to_mode_PGM that is placed on flash memory.
June 19, 2020