ATmega328P chip.
The arduino uno board (official version) has two main control chips, namely ATmega328P and ATmega16U2. ATmega328P is the core main control (MCU) of the board, which is responsible for program storage and operation. Typically arduino IDE programming is written to ROM in the ATMEGA328P. The ATmega16U2 is responsible for writing the program transmitted from the host computer through the serial port into the ATmega328P.