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().