Added active partition locator, and simple MBR built using it.

src/arch/i386/prefix/bootpart.S

@@ -0,0 +1,189 @@
+#define BOOT_SEG	0x07c0
+#define EXEC_SEG	0x0100
+#define STACK_SEG 	0x0200
+#define STACK_SIZE	0x2000
+	
+	.text
+	.arch i386
+	.section ".prefix", "awx", @progbits
+	.code16
+
+/*
+ * Find active partition
+ *
+ * Parameters:
+ *   %dl	: BIOS drive number
+ *   %bp	: Active partition handler routine
+ */
+find_active_partition:
+	/* Set up stack at STACK_SEG:STACK_SIZE */
+	movw	$STACK_SEG, %ax
+	movw	%ax, %ss
+	movw	$STACK_SIZE, %sp
+	/* Relocate self to EXEC_SEG */
+	pushw	$BOOT_SEG
+	popw	%ds
+	pushw	$EXEC_SEG
+	popw	%es
+	xorw	%si, %si
+	xorw	%di, %di
+	movw	$0x200, %cx
+	rep movsb
+	ljmp	$EXEC_SEG, $1f
+1:	pushw	%ds
+	popw	%es
+	pushw	%cs
+	popw	%ds
+	/* Read and process root partition table */
+	xorb	%dh, %dh
+	movw	$0x0001, %cx
+	xorl	%esi, %esi
+	xorl	%edi, %edi
+	call	process_table
+	/* Print failure message */
+	movw	$10f, %si
+	movw	$(20f-10f), %cx
+1:	movw	$0x0007, %bx
+	movb	$0x0e, %ah
+	lodsb
+	int	$0x10
+	loop	1b
+	/* Boot next device */
+	int	$0x18
+10:	.ascii	"Could not locate active partition\r\n"
+20:	
+
+/*
+ * Process partition table
+ *
+ * Parameters:
+ *   %dl	: BIOS drive number
+ *   %dh	: Head
+ *   %cl	: Sector (bits 0-5), high two bits of cylinder (bits 6-7)
+ *   %ch	: Low eight bits of cylinder
+ *   %esi:%edi	: LBA address
+ *   %bp	: Active partition handler routine
+ *
+ * Returns:
+ *   CF set on error
+ */
+process_table:
+	pushal
+	movw	$446, %bx
+1:	call	read_sector
+	jc	99f
+	call	process_partition
+	addw	$16, %bx
+	cmpw	$510, %bx
+	jne	1b
+99:	popal
+	ret
+
+/*
+ * Process partition
+ *
+ * Parameters:
+ *   %dl	: BIOS drive number
+ *   %dh	: Head
+ *   %cl	: Sector (bits 0-5), high two bits of cylinder (bits 6-7)
+ *   %ch	: Low eight bits of cylinder
+ *   %esi:%edi	: LBA address
+ *   %bx	: Offset within partition table
+ *   %bp	: Active partition handler routine
+ */
+process_partition:
+	pushal
+	/* Load C/H/S values from partition entry */
+	movb	%es:1(%bx), %dh
+	movw	%es:2(%bx), %cx
+	/* Update LBA address from partition entry */
+	addl	%es:8(%bx), %edi
+	adcl	$0, %esi
+	/* Check active flag */
+	testb	$0x80, %es:(%bx)
+	jz	1f
+	call	read_sector
+	jc	99f
+	jmp	*%bp
+1:	/* Check for extended partition */
+	movb	%es:4(%bx), %al
+	cmpb	$0x05, %al
+	je	2f
+	cmpb	$0x0f, %al
+	je	2f
+	cmpb	$0x85, %al
+	jne	99f
+2:	call	process_table
+99:	popal
+	ret
+
+/*
+ * Read single sector to 0000:7c00 and verify 0x55aa signature
+ *
+ * Parameters:
+ *   %dl	: BIOS drive number
+ *   %dh	: Head
+ *   %cl	: Sector (bits 0-5), high two bits of cylinder (bits 6-7)
+ *   %ch	: Low eight bits of cylinder
+ *   %esi:%edi	: LBA address
+ *
+ * Returns:
+ *   CF set on error
+ */
+read_sector:
+	pushal
+	/* Check for LBA extensions */
+	call	check_lba
+	jnc	read_lba
+read_chs:
+	/* Read sector using C/H/S address */
+	movw	$0x0201, %ax
+	xorw	%bx, %bx
+	stc
+	int	$0x13
+	sti
+	jmp	99f
+read_lba:
+	/* Read sector using LBA address */
+	movb	$0x42, %ah
+	movl	%esi, (lba_desc + 12)
+	movl	%edi, (lba_desc + 8)
+	movw	$lba_desc, %si
+	int	$0x13
+99:	/* Check for 55aa signature */
+	jc	99f
+	cmpw	$0xaa55, %es:(510)
+	je	99f
+	stc
+99:	popal
+	ret
+
+lba_desc:
+	.byte	0x10
+	.byte	0
+	.word	1
+	.word	0x0000
+	.word	0x07c0
+	.long	0, 0
+
+/*
+ * Check for LBA extensions
+ *	
+ * Parameters:
+ *   %dl	: BIOS drive number
+ *
+ * Returns:
+ *   CF clear if LBA extensions supported
+ */
+check_lba:
+	pushal
+	movb	$0x41, %ah
+	movw	$0x55aa, %bx
+	stc
+	int	$0x13
+	jc	99f
+	cmpw	$0xaa55, %bx
+	je	99f
+	stc
+99:	popal
+	ret

src/arch/i386/prefix/mbr.S

@@ -0,0 +1,13 @@
+	.text
+	.arch i386
+	.section ".prefix", "awx", @progbits
+	.code16
+	.org 0
+
+mbr:
+	movw	$exec_sector, %bp
+	jmp	find_active_partition
+exec_sector:
+	ljmp	$0x0000, $0x7c00
+
+#include "bootpart.S"