With the purpose of being fast, it is necessary to manipulate the microcontroller features with lower lever functions than the standard ones provided by the Arduino IDE. The internal functions are managed through some registers, that are collections of eight bits where each one governs something particular. Each register contains eight bits because the ATMega328P has an 8-bit architecture.
The registers have some names that are specified in the datasheet depending on their meanings, like ADCSRA for the ADC Setting Register A. Also each meaningful bit of the registers has a name, like ADEN for the ADC Enable Bit in the ADCSRA register.
To set their bits we could use the usual C syntax for binary algebra, but I found on the internet a couple of macros that are very nice and clean:
// Defines for setting and clearing register bits
#define cbi(sfr, bit) (_SFR_BYTE(sfr) &= ~_BV(bit))
#define sbi(sfr, bit) (_SFR_BYTE(sfr) |= _BV(bit))
Using them is very simple, if we want to set to 1 the Enable Bit of the ADC we can just write:
While if we want to set it to 0 (id est clear it) we can just write: