There is no preexisting set of INT1 instructions. There is a vector in low memory which points to a function that is to be executed when that interrupt occurs.
Depending on the compiler that you are using, there will be a way to designate a function as an interrupt function. It is then the job of the compiler and the linker to link that interrupt function with the vector which the processor goes to for that interrupt source. The compiler will also take care of saving and restoring the machine state and putting an interrupt return instruction at the end of the function.
I'm using CodeVision, so which compiler are you using?