Make the serial console port and its baudrate configurable on the bootloader
(e.g GRUB) command line.

The parameter format is similar to Linux's. Examples:
console=ttyS0        # to enable serial console on the first serial port
console=ttyS1        # second serial port
console=ttyS0,38400  # with the given baudrate

Selectable parity, bits, flow control not implemented yet.

Index: memtest86+-1.70/config.h
===================================================================
--- memtest86+-1.70.orig/config.h
+++ memtest86+-1.70/config.h
@@ -15,6 +15,9 @@
 /*	to enable. */
 #define SERIAL_CONSOLE_DEFAULT 0
 
+/* SERIAL_TTY - The default serial port to use. 0=ttyS0, 1=ttyS1 */
+#define SERIAL_TTY 0
+
 /* SERIAL_BAUD_RATE - Baud rate for the serial console */
 #define SERIAL_BAUD_RATE 9600
 
Index: memtest86+-1.70/lib.c
===================================================================
--- memtest86+-1.70.orig/lib.c
+++ memtest86+-1.70/lib.c
@@ -11,6 +11,18 @@
 
 int slock = 0, lsr = 0;
 short serial_cons = SERIAL_CONSOLE_DEFAULT;
+
+#if SERIAL_TTY != 0 && SERIAL_TTY != 1
+#error Bad SERIAL_TTY. Only ttyS0 and ttyS1 are supported.
+#endif
+short serial_tty = SERIAL_TTY;
+const short serial_base_ports[] = {0x3f8, 0x2f8};
+
+#if ((115200%SERIAL_BAUD_RATE) != 0)
+#error Bad default baud rate
+#endif
+int serial_baud_rate = SERIAL_BAUD_RATE;
+
 char buf[18];
 
 struct ascii_map_str {
@@ -721,19 +733,9 @@ void ttyprint(int y, int x, const char *
 	serial_echo_print(p);
 }
 
-#if defined(SERIAL_BAUD_RATE)
-
-#if ((115200%SERIAL_BAUD_RATE) != 0)
-#error Bad ttys0 baud rate
-#endif
-
-#define SERIAL_DIV     (115200/SERIAL_BAUD_RATE)
-
-#endif /* SERIAL_BAUD_RATE */
-
 void serial_echo_init(void)
 {
-	int comstat, hi, lo;
+	int comstat, hi, lo, serial_div;
 
 	/* read the Divisor Latch */
 	comstat = serial_echo_inb(UART_LCR);
@@ -744,12 +746,11 @@ void serial_echo_init(void)
 
 	/* now do hardwired init */
 	serial_echo_outb(0x03, UART_LCR); /* No parity, 8 data bits, 1 stop */
-#if defined(SERIAL_BAUD_RATE)
+	serial_div = 115200 / serial_baud_rate;
 	serial_echo_outb(0x83, UART_LCR); /* Access divisor latch */
-	serial_echo_outb(SERIAL_DIV & 0xff, UART_DLL);  /* baud rate divisor */
-	serial_echo_outb((SERIAL_DIV>> 8) & 0xff, UART_DLM);
+	serial_echo_outb(serial_div & 0xff, UART_DLL);  /* baud rate divisor */
+	serial_echo_outb((serial_div >> 8) & 0xff, UART_DLM);
 	serial_echo_outb(0x03, UART_LCR); /* Done with divisor */
-#endif
 
 	/* Prior to disabling interrupts, read the LSR and RBR
 	 * registers */
@@ -974,6 +975,59 @@ void wait_keyup( void ) {
 		}
 	}
 }
+
+/*
+ * Handles "console=<param>" command line option
+ *
+ * Examples of accepted params:
+ *   ttyS0
+ *   ttyS1
+ *   ttyS0,115200
+ */
+void serial_console_setup(char *param)
+{
+	char *option, *end;
+	unsigned long tty;
+	unsigned long baud_rate;
+
+	if (strncmp(param, "ttyS", 4))
+		return;   /* not a serial port */
+
+	param += 4;
+
+	tty = simple_strtoul(param, &option, 10);
+
+	if (option == param)
+		return;   /* there were no digits */
+
+	if (tty > 1)
+		return;   /* only ttyS0 and ttyS1 supported */
+
+	if (*option == '\0')
+		goto save_tty; /* no options given, just ttyS? */
+
+	if (*option != ',')
+		return;  /* missing the comma separator */
+
+	/* baud rate must follow */
+	option++;
+	baud_rate = simple_strtoul(option, &end, 10);
+
+	if (end == option)
+		return;  /* no baudrate after comma */
+
+	if (baud_rate == 0 || (115200 % baud_rate) != 0)
+		return;  /* wrong baud rate */
+
+	if (*end != '\0')
+		return;  /* garbage at the end */
+
+	serial_baud_rate = (int) baud_rate;
+save_tty:
+	serial_tty = (short) tty;
+	serial_cons = 1;
+}
+
 #ifdef LP
 #define DATA            0x00
 #define STATUS          0x01
Index: memtest86+-1.70/main.c
===================================================================
--- memtest86+-1.70.orig/main.c
+++ memtest86+-1.70/main.c
@@ -31,6 +31,7 @@ const struct tseq tseq[] = {
 };
 
 char firsttime = 0;
+char cmdline_parsed = 0;
 
 struct vars variables = {};
 struct vars * const v = &variables;
@@ -135,12 +136,53 @@ static void run_at(unsigned long addr)
 	__run_at(run_at_addr);
 }
 
+/* command line passing using the 'old' boot protocol */
+#define MK_PTR(seg,off) ((void*)(((unsigned long)(seg) << 4) + (off)))
+#define OLD_CL_MAGIC_ADDR ((unsigned short*) MK_PTR(INITSEG,0x20))
+#define OLD_CL_MAGIC 0xA33F
+#define OLD_CL_OFFSET_ADDR ((unsigned short*) MK_PTR(INITSEG,0x22))
+
+static void parse_command_line(void)
+{
+	char *cmdline;
+
+	if (cmdline_parsed)
+		return;
+
+	if (*OLD_CL_MAGIC_ADDR != OLD_CL_MAGIC)
+		return;
+
+	unsigned short offset = *OLD_CL_OFFSET_ADDR;
+	cmdline = MK_PTR(INITSEG, offset);
+
+	/* skip leading spaces */
+	while (*cmdline == ' ')
+		cmdline++;
+
+	while (*cmdline) {
+		if (!strncmp(cmdline, "console=", 8)) {
+			cmdline += 8;
+			serial_console_setup(cmdline);
+		}
+
+		/* go to the next parameter */
+		while (*cmdline && *cmdline != ' ')
+			cmdline++;
+		while (*cmdline == ' ')
+			cmdline++;
+	}
+
+	cmdline_parsed = 1;
+}
+
 void do_test(void)
 {
 	int i = 0;
 	unsigned long chunks;
 	unsigned long lo, hi;
 
+	parse_command_line();
+
 	/* If we have a partial relocation finish it */
 	if (run_at_addr == (unsigned long)&_start) {
 		run_at_addr = 0xffffffff;
Index: memtest86+-1.70/serial.h
===================================================================
--- memtest86+-1.70.orig/serial.h
+++ memtest86+-1.70/serial.h
@@ -136,8 +136,8 @@
  */
 
 #include "io.h"
-#define serial_echo_outb(v,a) outb((v),(a)+0x3f8)
-#define serial_echo_inb(a)    inb((a)+0x3f8)
+#define serial_echo_outb(v,a) outb((v),(a)+serial_base_ports[serial_tty])
+#define serial_echo_inb(a)    inb((a)+serial_base_ports[serial_tty])
 #define BOTH_EMPTY (UART_LSR_TEMT | UART_LSR_THRE)
 /* Wait for transmitter & holding register to empty */
 #define WAIT_FOR_XMITR \
Index: memtest86+-1.70/test.h
===================================================================
--- memtest86+-1.70.orig/test.h
+++ memtest86+-1.70/test.h
@@ -103,6 +103,7 @@ void printpatn(void);
 void printpatn(void);
 void itoa(char s[], int n);
 void reverse(char *p);
+void serial_console_setup(char *param);
 void serial_echo_init(void);
 void serial_echo_print(const char *s);
 void ttyprint(int y, int x, const char *s);

--