Add functions to be able to boot with BIOSs help

These functions are special: They are running in the 16 bit real mode world
to bring up barebox on an x86 box.

Signed-off-by: Juergen Beisert <jbe@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

1 files changed, 58 insertions, 0 deletions

diff --git a/arch/x86/boot/boot_main.S b/arch/x86/boot/boot_main.S
new file mode 100644
index 0000000000..f3d248ae5b
--- /dev/null
+++ b/arch/x86/boot/boot_main.S
@@ -0,0 +1,58 @@
+/*
+ * Copyright (C) 2009 Juergen Beisert, Pengutronix
+ *
+ * This code was inspired by the GRUB2 project.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of
+ * the License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 59 Temple Place, Suite 330, Boston,
+ * MA 02111-1307 USA
+ *
+ */
+
+/**
+ * @file
+ * @brief Common boot sector main routine to be entered by the BIOS
+ */
+/**
+ * @fn void _start(void)
+ *
+ * @brief Fix segment:offset settings of some buggy BIOSs
+ */
+
+#ifndef DOXYGEN_SHOULD_SKIP_THIS
+
+	.file "boot_main.S"
+	.code16
+
+	.extern real_start
+
+	.section .boot_start, "ax"
+	.type _start, @function
+
+	/*
+	 * The BIOS loads this code to address 0x00007c00.
+	 * The code should be called with CS:IP 0:0x7c00 (hopefully).
+	 */
+	.globl _start
+_start:
+	cli		/* we're not safe here! */
+	/*
+	 * It seems there are implementations in the wild which call this
+	 * code with CS:IP 0x07C0:0000 instead. We fix it immediately.
+	 */
+	ljmp $0, $real_start
+
+	.size _start, .-_start
+
+#endif