Hello World!

Now it's time to print "Hello World!" in our kernel. Normally, we would use the println macro. But since we are in no_std, there is no println macro. We can print to the COM1 serial port instead. More info: https://os.phil-opp.com/testing/#serial-port. Serial ports let us read and write data. Most modern computers don't have serial ports, but QEMU does. We can access COM1 through x86 I/O ports (also see https://wiki.osdev.org/Port_IO). We can read and write to I/O ports using in and out instructions.

Let's use the uart_16550 crate. In kernel/Cargo.toml dependencies, add

uart_16550 = "0.3.2"

In kernel/main.rs, add

use uart_16550::SerialPort;

and

use core::fmt::Write;

Then we can write to COM1 using

let mut serial_port = unsafe { SerialPort::new(0x3F8) };
serial_port.init();
writeln!(serial_port, "Hello World!\r").unwrap();

Now run it, and in the QEMU window, select View -> serial0. It should look like this:

We can make QEMU not create a window and print COM1 to the terminal by using --nographic. Let's also update tasks.json. Note that when you specify --nographic, you can't close QEMU with Ctrl + C and instead have to use Ctrl + A and then press X.