Copy UART driver from xv6.

Makefile

@@ -3,7 +3,7 @@ LD=x86_64-elf-ld
 CC=x86_64-elf-gcc
 
 run: image.bin
-	qemu-system-i386 -drive format=raw,file=$<
+	qemu-system-i386 -drive format=raw,file=$< -serial mon:stdio
 
 debug-preboot: image.bin mbr.elf
 	qemu-system-i386 -drive format=raw,file=$< -s -S &
@@ -33,7 +33,8 @@ fs.img: kernel.bin tools/mkfs
 image.bin: mbr.bin fs.img
 	cat $^ >$@
 
-kernel.bin: kernel.o console.o drivers/vga.o drivers/keyboard.o string.o drivers/ata.o cpu/vectors.o cpu/idt.o
+kernel.bin: kernel.o console.o drivers/vga.o drivers/keyboard.o \
+	string.o drivers/ata.o cpu/vectors.o cpu/idt.o drivers/uart.o
 	$(LD) -m elf_i386 -o $@ -Ttext 0x1000 $^
 
 %.o: %.c

console.c

@@ -1,13 +1,18 @@
 #include "console.h"
 #include "drivers/vga.h"
+#include "drivers/uart.h"
 
 void printk(const char* msg) {
     vga_print_string(msg);
+    for (; *msg; ++msg) {
+        uartputc(*msg);
+    }
 }
 
 void panic(const char* msg) {
-    printk("Kernel panic: ");
+    printk("\nKernel panic: ");
     printk(msg);
+    asm("cli");
     while (1) {
         asm("hlt");
     }

cpu/idt.c

@@ -79,8 +79,7 @@ void trap(registers_t *r) {
         if (r->int_no < ARRLEN(exception_messages)) {
             msg = exception_messages[r->int_no];
         }
-        printk(msg);
-        printk("\n");
+        panic(msg);
     }
 
     /* Handle the interrupt in a more modular way */

drivers/uart.c

@@ -0,0 +1,44 @@
+#include "uart.h"
+#include "port.h"
+
+static int uart;
+
+enum {
+    COM1 = 0x3f8,
+};
+
+void uartinit() {
+    // Turn off the FIFO
+    port_byte_out(COM1+2, 0);
+
+    // 9600 baud, 8 data bits, 1 stop bit, parity off.
+    port_byte_out(COM1+3, 0x80);    // Unlock divisor
+    port_byte_out(COM1+0, 115200/9600);
+    port_byte_out(COM1+1, 0);
+    port_byte_out(COM1+3, 0x03);    // Lock divisor, 8 data bits.
+    port_byte_out(COM1+4, 0);
+    port_byte_out(COM1+1, 0x01);    // Enable receive interrupts.
+
+    // If status is 0xFF, no serial port.
+    if(port_byte_in(COM1+5) == 0xFF)
+        return;
+    uart = 1;
+
+    // Acknowledge pre-existing interrupt conditions;
+    // enable interrupts.
+    port_byte_in(COM1+2);
+    port_byte_in(COM1+0);
+}
+
+void
+uartputc(char c)
+{
+    int i;
+
+    if (!uart)
+        return;
+    for (i = 0; i < 128 && !(port_byte_in(COM1+5) & 0x20); i++) {
+        asm("pause");
+    }
+    port_byte_out(COM1+0, c);
+}

drivers/uart.h

@@ -0,0 +1,4 @@
+#pragma once
+
+void uartputc(char c);
+void uartinit();

kernel.c

@@ -6,9 +6,11 @@ asm(".asciz \"kernel start\"");
 #include "drivers/vga.h"
 #include "drivers/ata.h"
 #include "drivers/misc.h"
+#include "drivers/uart.h"
 
 void _start() {
     init_keyboard();
+    uartinit();
     load_idt();
     sti();
     char buf[512];
@@ -20,8 +22,7 @@ void _start() {
     printk(buf);
 
     while (1) {
-        asm("pause");
+        asm("hlt");
     }
-    asm("hlt");
     qemu_shutdown();
 }