21.(10 pts) Write two programs stored separately in two files. a) Main: 1) Defines an 8-bit global variable COUNTER. 2) Inializes the IRQ interrupt vector in the interrupt vector table. The name of the interrupt service routine is IRQ ISR 3) Sends the value of COUNTER every 10ms to an output port at address SD6. b) Write an interrupt service routine that counts the number of times itself is executed The name of the ISR is IRQ ISR. Every time the interrupt occurs, the 8-bit global variable COUNTER is incremented by one in this interrupt service routine