Add serial port input/output.

Delete parallel port output.
Works well with qemu -nographic mode.

uart.c

@@ -0,0 +1,76 @@
+// Intel 8250 serial port (UART).
+
+#include "types.h"
+#include "defs.h"
+#include "param.h"
+#include "traps.h"
+#include "spinlock.h"
+#include "dev.h"
+#include "mmu.h"
+#include "proc.h"
+#include "x86.h"
+
+#define COM1	0x3f8
+
+static int uart;	// is there a uart?
+
+void
+uartinit(void)
+{
+	char *p;
+
+	// Turn off the FIFO
+	outb(COM1+2, 0);
+	
+	// 9600 baud, 8 data bits, 1 stop bit, parity off.
+	outb(COM1+3, 0x80);	// Unlock divisor
+	outb(COM1+0, 115200/9600);
+	outb(COM1+1, 0);
+	outb(COM1+3, 0x03);	// Lock divisor, 8 data bits.
+	outb(COM1+4, 0);
+	outb(COM1+1, 0x01);	// Enable receive interrupts.
+
+	// If status is 0xFF, no serial port.
+	if(inb(COM1+5) == 0xFF)
+		return;
+	uart = 1;
+
+	// Acknowledge pre-existing interrupt conditions;
+	// enable interrupts.
+	inb(COM1+2);
+	inb(COM1+0);
+	picenable(IRQ_COM1);
+	ioapicenable(IRQ_COM1, 0);
+	
+	// Announce that we're here.
+	for(p="xv6...\n"; *p; p++)
+		uartputc(*p);
+}
+
+void
+uartputc(int c)
+{
+	int i;
+
+	if(!uart)
+		return;
+	for(i = 0; i < 128 && !(inb(COM1+5) & 0x20); i++)
+		microdelay(10);
+	outb(COM1+0, c);
+}
+
+static int
+uartgetc(void)
+{
+	if(!uart)
+		return -1;
+	if(!(inb(COM1+5) & 0x01))
+		return -1;
+	return inb(COM1+0);
+}
+
+void
+uartintr(void)
+{
+	consoleintr(uartgetc);
+}