void int_t1 interrupt 3

This is a function declaration in C that specifies that the function "int_t1" is an interrupt service routine (ISR) for interrupt 3.

In C, interrupts are used to handle asynchronous events that occur during program execution. When an interrupt occurs, the CPU suspends the current program execution and transfers control to the corresponding ISR to handle the interrupt.

The "void" keyword indicates that the ISR does not return a value. The "interrupt" keyword is used to specify that it is an ISR. The "3" after the "interrupt" keyword specifies the interrupt number, which determines which hardware or software event triggers the ISR