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@@ -0,0 +1,190 @@
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+#define BOOT_SEG 0x07c0
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+#define EXEC_SEG 0x0100
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+#define STACK_SEG 0x0200
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+#define STACK_SIZE 0x2000
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+
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+ .text
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+ .arch i386
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+ .section ".prefix", "awx", @progbits
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+ .code16
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+
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+/*
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+ * Find active partition
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+ *
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+ * Parameters:
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+ * %dl : BIOS drive number
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+ * %bp : Active partition handler routine
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+ */
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+find_active_partition:
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+ /* Set up stack at STACK_SEG:STACK_SIZE */
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+ movw $STACK_SEG, %ax
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+ movw %ax, %ss
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+ movw $STACK_SIZE, %sp
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+ /* Relocate self to EXEC_SEG */
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+ pushw $BOOT_SEG
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+ popw %ds
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+ pushw $EXEC_SEG
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+ popw %es
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+ xorw %si, %si
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+ xorw %di, %di
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+ movw $0x200, %cx
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+ rep movsb
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+ ljmp $EXEC_SEG, $1f
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+1: pushw %ds
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+ popw %es
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+ pushw %cs
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+ popw %ds
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+ /* Read and process root partition table */
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+ xorb %dh, %dh
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+ movw $0x0001, %cx
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+ xorl %esi, %esi
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+ xorl %edi, %edi
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+ call process_table
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+ /* Print failure message */
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+ movw $10f, %si
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+ movw $(20f-10f), %cx
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+1: movw $0x0007, %bx
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+ movb $0x0e, %ah
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+ lodsb
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+ int $0x10
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+ loop 1b
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+ /* Boot next device */
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+ int $0x18
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+10: .ascii "Could not locate active partition\r\n"
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+20:
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+
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+/*
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+ * Process partition table
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+ *
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+ * Parameters:
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+ * %dl : BIOS drive number
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+ * %dh : Head
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+ * %cl : Sector (bits 0-5), high two bits of cylinder (bits 6-7)
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+ * %ch : Low eight bits of cylinder
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+ * %esi:%edi : LBA address
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+ * %bp : Active partition handler routine
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+ *
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+ * Returns:
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+ * CF set on error
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+ */
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+process_table:
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+ xchgw %bx,%bx
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+ pushal
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+ movw $446, %bx
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+1: call read_sector
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+ jc 99f
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+ call process_partition
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+ addw $16, %bx
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+ cmpw $510, %bx
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+ jne 1b
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+99: popal
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+ ret
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+
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+/*
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+ * Process partition
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+ *
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+ * Parameters:
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+ * %dl : BIOS drive number
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+ * %dh : Head
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+ * %cl : Sector (bits 0-5), high two bits of cylinder (bits 6-7)
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+ * %ch : Low eight bits of cylinder
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+ * %esi:%edi : LBA address
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+ * %bx : Offset within partition table
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+ * %bp : Active partition handler routine
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+ */
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+process_partition:
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+ pushal
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+ /* Load C/H/S values from partition entry */
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+ movb %es:1(%bx), %dh
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+ movw %es:2(%bx), %cx
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+ /* Update LBA address from partition entry */
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+ addl %es:8(%bx), %edi
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+ adcl $0, %esi
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+ /* Check active flag */
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+ testb $0x80, %es:(%bx)
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+ jz 1f
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+ call read_sector
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+ jc 99f
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+ jmp *%bp
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+1: /* Check for extended partition */
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+ movb %es:4(%bx), %al
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+ cmpb $0x05, %al
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+ je 2f
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+ cmpb $0x0f, %al
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+ je 2f
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+ cmpb $0x85, %al
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+ jne 99f
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+2: call process_table
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+99: popal
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+ ret
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+
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+/*
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+ * Read single sector to 0000:7c00 and verify 0x55aa signature
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+ *
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+ * Parameters:
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+ * %dl : BIOS drive number
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+ * %dh : Head
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+ * %cl : Sector (bits 0-5), high two bits of cylinder (bits 6-7)
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+ * %ch : Low eight bits of cylinder
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+ * %esi:%edi : LBA address
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+ *
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+ * Returns:
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+ * CF set on error
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+ */
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+read_sector:
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+ pushal
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+ /* Check for LBA extensions */
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+ call check_lba
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+ jnc read_lba
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+read_chs:
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+ /* Read sector using C/H/S address */
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+ movw $0x0201, %ax
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+ xorw %bx, %bx
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+ stc
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+ int $0x13
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+ sti
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+ jmp 99f
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+read_lba:
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+ /* Read sector using LBA address */
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+ movb $0x42, %ah
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+ movl %esi, (lba_desc + 12)
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+ movl %edi, (lba_desc + 8)
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+ movw $lba_desc, %si
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+ int $0x13
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+99: /* Check for 55aa signature */
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+ jc 99f
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+ cmpw $0xaa55, %es:(510)
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+ je 99f
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+ stc
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+99: popal
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+ ret
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+
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+lba_desc:
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+ .byte 0x10
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+ .byte 0
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+ .word 1
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+ .word 0x0000
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+ .word 0x07c0
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+ .long 0, 0
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+
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+/*
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+ * Check for LBA extensions
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+ *
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+ * Parameters:
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+ * %dl : BIOS drive number
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+ *
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+ * Returns:
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+ * CF clear if LBA extensions supported
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+ */
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+check_lba:
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+ pushal
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+ movb $0x41, %ah
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+ movw $0x55aa, %bx
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+ stc
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+ int $0x13
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+ jc 99f
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+ cmpw $0xaa55, %bx
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+ je 99f
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+ stc
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+99: popal
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+ ret
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