USART0（Universal Synchronous and Asynchronous serial Receiver and Transmitter) related registers.

The USART0 related codes are defined in hardware/tools/avr/avr/include/avr/iom328p.h as below.

#define UCSR0A _SFR_MEM8(0xC0)
#define MPCM0 0
#define U2X0 1
#define UPE0 2
#define DOR0 3
#define FE0 4
#define UDRE0 5
#define TXC0 6
#define RXC0 7

#define UCSR0B _SFR_MEM8(0xC1)
#define TXB80 0
#define RXB80 1
#define UCSZ02 2
#define TXEN0 3
#define RXEN0 4
#define UDRIE0 5
#define TXCIE0 6
#define RXCIE0 7

#define UCSR0C _SFR_MEM8(0xC2)
#define UCPOL0 0
#define UCSZ00 1
#define UCPHA0 1
#define UCSZ01 2
#define UDORD0 2
#define USBS0 3
#define UPM00 4
#define UPM01 5
#define UMSEL00 6
#define UMSEL01 7

#define UBRR0 _SFR_MEM16(0xC4)

#define UBRR0L _SFR_MEM8(0xC4)
#define UBRR0_0 0
#define UBRR0_1 1
#define UBRR0_2 2
#define UBRR0_3 3
#define UBRR0_4 4
#define UBRR0_5 5
#define UBRR0_6 6
#define UBRR0_7 7

#define UBRR0H _SFR_MEM8(0xC5)
#define UBRR0_8 0
#define UBRR0_9 1
#define UBRR0_10 2
#define UBRR0_11 3

#define UDR0 _SFR_MEM8(0xC6)
#define UDR0_0 0
#define UDR0_1 1
#define UDR0_2 2
#define UDR0_3 3
#define UDR0_4 4
#define UDR0_5 5
#define UDR0_6 6
#define UDR0_7 7

They call _SFR_MEM8().