The EICRA/EIMSK are registers to control external interrupts.
The EICRA and EIMSK are defined in hardware/tools/avr/avr/include/avr/iom328p.h as below.
#define EICRA _SFR_MEM8(0x69) #define ISC00 0 #define ISC01 1 #define ISC10 2 #define ISC11 3 #define EIMSK _SFR_IO8(0x1D) #define INT0 0 #define INT1 1
They call _SFR_MEM8() and _SFR_IO8().