Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/stm32/stm32h7rs/STM32H7R/EXTI/EXTI_EMR3#0x0
MR77=B_0x0
EXTI event mask register
CPU event mask on event input x+64
0 (B_0x0): Event request from line x is masked
1 (B_0x1): Event request from line x is unmasked
https://github.com/modm-io/cmsis-svd-stm32