The Timer/Counter Interrupt Mask Registers are registers related to timer interruption.
The Timer/Counter Interrupt Mask Register related codes are defined in hardware/tools/avr/avr/include/avr/iom328p.h as below.
#define TIMSK0 _SFR_MEM8(0x6E) #define TOIE0 0 #define OCIE0A 1 #define OCIE0B 2 #define TIMSK1 _SFR_MEM8(0x6F) #define TOIE1 0 #define OCIE1A 1 #define OCIE1B 2 #define ICIE1 5 #define TIMSK2 _SFR_MEM8(0x70) #define TOIE2 0 #define OCIE2A 1 #define OCIE2B 2
They call _SFR_MEM8().