The Timer/Counter Registers are registers incremented by each timer clock.
The Timer/Counter registers related codes are defined in hardware/tools/avr/avr/include/avr/iom328p.h as below.
#define TCNT0 _SFR_IO8(0x26) #define TCNT0_0 0 #define TCNT0_1 1 #define TCNT0_2 2 #define TCNT0_3 3 #define TCNT0_4 4 #define TCNT0_5 5 #define TCNT0_6 6 #define TCNT0_7 7 #define TCNT1 _SFR_MEM16(0x84) #define TCNT1L _SFR_MEM8(0x84) #define TCNT1L0 0 #define TCNT1L1 1 #define TCNT1L2 2 #define TCNT1L3 3 #define TCNT1L4 4 #define TCNT1L5 5 #define TCNT1L6 6 #define TCNT1L7 7 #define TCNT1H _SFR_MEM8(0x85) #define TCNT1H0 0 #define TCNT1H1 1 #define TCNT1H2 2 #define TCNT1H3 3 #define TCNT1H4 4 #define TCNT1H5 5 #define TCNT1H6 6 #define TCNT1H7 7 #define TCNT2 _SFR_MEM8(0xB2) #define TCNT2_0 0 #define TCNT2_1 1 #define TCNT2_2 2 #define TCNT2_3 3 #define TCNT2_4 4 #define TCNT2_5 5 #define TCNT2_6 6 #define TCNT2_7 7
They call _SFR_IO8(), _SFR_MEM8() and _SFR_MEM16().