Using memory-mapped I/O and polling, write a function read_string that reads a string directly from the keyboard. The function will get characters from the keyboard and stores them in an array pointed by register $a0. The function should continue until n-1 characters are read or the newline character is pressed. The parameter n should be passed in register $a1. The function should insert a NULL byte at the end of the string. It should return the actual number of characters read in register $v0. Make sure to activate the Keyboard and Display MMIO Simulator. Write a main function to test read_string repeatedly, and to print the string after each call.