Added a gcc compilable MBR code - borrowed from FreeBSD. For later use with

the writembr utility (I was just too lazy to make it build right now).


git-svn-id: file:///srv/svn/repos/haiku/trunk/current@2522 a95241bf-73f2-0310-859d-f6bbb57e9c96

src/apps/bin/writembr/mbr.S

@@ -0,0 +1,164 @@
+/*
+** Copyright (c) 1999 Robert Nordier
+** All rights reserved.
+**
+** Redistribution and use in source and binary forms are freely
+** permitted provided that the above copyright notice and this
+** paragraph and the following disclaimer are duplicated in all
+** such forms.
+**
+** This software is provided "AS IS" and without any express or
+** implied warranties, including, without limitation, the implied
+** warranties of merchantability and fitness for a particular
+** purpose.
+/*
+
+/* A 512 byte MBR boot manager that simply boots the active partition. */
+
+		.set LOAD, 0x7c00				// Load address
+		.set EXEC, 0x600 				// Execution address
+		.set PT_OFF, 0x1be				// Partition table
+		.set MAGIC, 0xaa55				// Magic: bootable
+
+		.set NUM_HARD_DRIVES, 0x475		// Number of hard drives
+
+		.globl start					// Entry point
+		.code16
+
+/*
+ * Setup the segment registers for flat addressing and setup the stack.
+ */
+start:	cld								// String ops inc
+		xorw %ax, %ax					// Zero
+		movw %ax, %es					// Address
+		movw %ax, %ds					// data
+		movw %ax, %ss					// Set up stack
+		movw $LOAD, %sp					//
+/*
+ * Relocate ourself to a lower address so that we are out of the way when
+ * we load in the bootstrap from the partition to boot.
+ */ 
+		movw $main-EXEC+LOAD, %si		// Source
+		movw $main, %di					// Destination
+		movw $0x200-(main-start), %cx	// Byte count
+		rep								// Relocate code
+		movsb							//
+/*
+ * Jump to the relocated code.
+ */
+		jmp main-LOAD+EXEC				// Jump to relocated code
+
+/*
+ * Scan the partition table looking for an active entry.  Note that %ch is
+ * zero from the repeated string instruction above.  We save the offset of
+ * the active partition in %si and scan the entire table to ensure that only
+ * one partition is marked active.
+ */
+
+main:	xorw %si, %si					// No active partition
+		movw $partition_table, %bx		// Partition table
+		movb $0x4, %cl					// Number of entries
+main.1: cmpb %ch, (%bx)					// Null entry?
+		je main.2						// Yes
+		jg err_partition_table			// If 0x1..0x7f
+		testw %si, %si	 				// Active already found?
+		jnz err_partition_table			// Yes
+		movw %bx, %si					// Point to active
+main.2: addb $0x10, %bl					// Till
+		loop main.1						//  done
+		testw %si, %si	 				// Active found?
+		jnz main.3						// Yes
+		int $0x18						// BIOS: Diskless boot
+/*
+ * Ok, we've found a possible active partition.  Check to see that the drive
+ * is a valid hard drive number.
+ */
+main.3: cmpb $0x80, %dl					// Drive valid?
+		jb main.4						// No
+		movb NUM_HARD_DRIVES, %dh		// Calculate the highest
+		addb $0x80, %dh					//   drive number available
+		cmpb %dh, %dl					// Within range?
+		jb main.5						// Yes
+main.4: movb (%si), %dl					// Load drive
+/*
+ * Ok, now that we have a valid drive and partition entry, load the CHS from
+ * the partition entry and read the sector from the disk.
+ */
+main.5:	movw %sp, %di					// Save stack pointer
+		movb 0x1(%si), %dh				// Load head
+		movw 0x2(%si), %cx				// Load cylinder:sector
+		movw $LOAD, %bx					// Transfer buffer
+		cmpb $0xff, %dh					// Might we need to use LBA?
+		jnz main.7						// No.
+		cmpw $0xffff, %cx				// Do we need to use LBA?
+		jnz main.7						// No.
+		pushw %cx						// Save %cx
+		pushw %bx						// Save %bx
+		movw $MAGIC, %bx				// Magic
+		movb $0x41, %ah					// BIOS:	EDD extensions present?
+		int $0x13						//
+		jc main.6						// No.
+		cmpw $MAGIC, %bx				// Magic ok?
+		jne main.6						// No.
+		testb $0x1, %cl					// Packet mode present?
+		jz main.6						// No.
+		popw %bx						// Restore %bx
+		pushl $0x0						// Set the LBA
+		pushl 0x8(%si)					//  address
+		pushw %es						// Set the address of
+		pushw %bx						//  the transfer buffer
+		pushw $0x1						// Read 1 sector
+		pushw $0x10						// Packet length
+		movw %sp, %si					// Packer pointer
+		movw $0x4200, %ax				// BIOS:	LBA Read from disk
+		jmp main.8						// Skip the CHS setup
+main.6:	popw %bx						// Restore %bx
+		popw %cx						// Restore %cx
+main.7:	movw $0x201, %ax				// BIOS: Read from disk
+main.8:	int $0x13						// Call the BIOS
+		movw %di,%sp					// Restore stack
+		jc err_loading_os				// If error
+/*
+ * Now that we've loaded the bootstrap, check for the magic 0xaa55 signature.  If it
+ * is present, execute the bootstrap we just loaded.
+ */
+		cmpw $MAGIC, 0x1fe(%bx)			// Bootable?
+		jne err_missing_os				// No
+		jmp *%bx						// Invoke bootstrap
+/*
+ * Various error message entry points.
+ */
+err_partition_table:
+		movw $msg_partition_table, %si	// "Invalid partition table"
+		jmp putString					//
+
+err_loading_os:
+		movw $msg_loading_os, %si		// "Error loading operating system"
+		jmp putString					//
+
+err_missing_os:
+		movw $msg_missing_os, %si		// "Missing operating system"
+		jmp putString					//
+/*
+ * Output an ASCIZ string to the console via the BIOS.
+ */ 
+putString.0:
+		movw $0x7, %bx			 		// Page:attribute
+		movb $0xe, %ah					// BIOS: Display character
+		int $0x10						//
+putString:
+		lodsb							// Get character
+		testb %al,%al					// End of string?
+		jnz putString.0					// No
+putString.1:
+		jmp putString.1					// Await reset
+
+msg_partition_table: 	.asciz "Invalid partition table"
+msg_loading_os: 		.asciz "Error loading operating system"
+msg_missing_os: 		.asciz "Missing operating system"
+
+		.org PT_OFF
+
+partition_table:
+		.fill 0x10,0x4,0x0				// Partition table
+		.word MAGIC						// Magic number