Merge pull request #41 from SamTebbs33/feature/serial-output

Add serial output

src/kernel/kmain.zig

@@ -5,6 +5,7 @@ const arch = if (builtin.is_test) @import("../../test/kernel/arch_mock.zig") els
 const multiboot = @import("multiboot.zig");
 const tty = @import("tty.zig");
 const vga = @import("vga.zig");
+const serial = @import("serial.zig");
 
 // Need to import this as we need the panic to be in the root source file, or zig will just use the
 // builtin panic and just loop, which is what we don't want
@@ -23,6 +24,8 @@ pub export fn kmain(mb_info: *multiboot.multiboot_info_t, mb_magic: u32) void {
         arch.init();
         vga.init();
         tty.init();
+        serial.init(serial.DEFAULT_BAUDRATE, serial.Port.COM1) catch unreachable;
+
         tty.print("Hello Pluto from kernel :)\n");
 
         // Enable interrupts

src/kernel/serial.zig

@@ -0,0 +1,80 @@
+const arch = @import("arch.zig").internals;
+
+// The LCR is the line control register
+const LCR = 3;
+// Maximum baudrate
+const BAUD_MAX = 115200;
+// 8 bits per serial character
+const CHAR_LEN = 8;
+// One stop bit per transmission
+const SINGLE_STOP_BIT = true;
+// No parity bit
+const PARITY_BIT = false;
+
+pub const DEFAULT_BAUDRATE = 38400;
+
+const SerialError = error {
+    InvalidBaud
+};
+
+pub const Port = enum(u16) {
+    COM1 = 0x3F8,
+    COM2 = 0x2F8,
+    COM3 = 0x3E8,
+    COM4 = 0x2E8
+};
+
+// Compute a value that encodes the serial properties
+// This is fed into the LCR (line control register)
+fn lcrValue(char_len: u8, comptime stop_bit: bool, comptime parity_bit: bool, msb: u8) u8 {
+    var val = char_len & 0x3;
+    val |= (if (stop_bit) 0 else 1) << 2;
+    val |= (if (parity_bit) 1 else 0) << 3;
+    val |= (msb & 0x1) << 7;
+    return val;
+}
+
+fn baudDivisor(baud: u32) SerialError!u16 {
+    if (baud > BAUD_MAX or baud <= 0) return SerialError.InvalidBaud;
+    return @intCast(u16, BAUD_MAX / baud);
+}
+
+fn transmitIsEmpty(port: Port) bool {
+    return arch.inb(@enumToInt(port) + 5) & 0x20 > 0;
+}
+
+///
+/// Initialise a serial port to a certain baudrate
+///
+/// Arguments
+///  IN baud: u32 - The baudrate to use. Cannot be more than MAX_BAUDRATE
+///  IN port: Port - The port to initialise
+///
+/// Throws a SerialError if the baudrate is invalid
+///
+pub fn init(baud: u32, port: Port) SerialError!void {
+    // The baudrate is sent as a divisor of the max baud rate
+    const divisor: u16 = try baudDivisor(baud);
+    const port_int = @enumToInt(port);
+    // Send a byte to start setting the baudrate
+    arch.outb(port_int + LCR, lcrValue(0, false, false, 1));
+    // Send the divisor's lsb
+    arch.outb(port_int, @truncate(u8, divisor));
+    // Send the divisor's msb
+    arch.outb(port_int + 1, @truncate(u8, divisor >> 8));
+    // Send the properties to use
+    arch.outb(port_int + LCR, lcrValue(CHAR_LEN, SINGLE_STOP_BIT, PARITY_BIT, 0));
+    // Stop initialisation
+    arch.outb(port_int + 1, 0);
+}
+
+pub fn write(char: u8, port: Port) void {
+    while (!transmitIsEmpty(port)) { arch.halt();  }
+    arch.outb(@enumToInt(port), char);
+}
+
+pub fn writeString(str: []const u8, port: Port) void {
+    for (str) |char| {
+        write(char, port);
+    }
+}