The ADCSRA(ADC Control and Status Register A) is a register to control AD conversion.

The ADCSRA is defined in hardware/tools/avr/avr/include/avr/iom328p.h as below.

#define ADCSRA _SFR_MEM8(0x7A)
#define ADPS0 0
#define ADPS1 1
#define ADPS2 2
#define ADIE 3
#define ADIF 4
#define ADATE 5
#define ADSC 6
#define ADEN 7

The 0x7A is the address where the ADCSRA register exists. It calls the _SFR_MEM8().