Write a main program that uses these system calls to echo the typed characters. The pseudo code will look something like:
void traphandler () {
if (R0 == 0) { // read system call
R1 = getchar(); // poll the status register of the IO terminal until a char is available then load it into R1
} else if (R0 == 1) { // write system call
putchar(R1); // just store R1 to memory location 0xfff0
}
enableInterrupts();
}
void main() {
char c;
while (1) {
c = read(); // set R0 to 0 can issue the 'trap' instruction, result will be in R1
write(c); // set R0 to 1 and set R1 to 'c' and issue the 'trap' instruction
}
}