addition(bootsector)
This commit is contained in:
commit
757457d456
GPG Key ID:
441B6068B1C67B40
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@ -0,0 +1,252 @@
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PARTITION_IDENTIFIER equ 0x9e
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bits 16
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org 0x7c00
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entry:
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mov bx, 0x7e00
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mov ss, bx
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mov bp, 512
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mov sp, 512
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push dx
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push bp
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mov bp, sp
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call find_partition
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mov sp, bp
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pop bp
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cmp ax, 0xff
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je write_error.partition_not_found
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pop dx
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push bp
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mov bp, sp
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push ax
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push dx
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call load_partition
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mov sp, bp
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pop bp
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jmp jump_to_partition
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find_partition:
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xor si, si
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.finder_loop:
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mov ax, si
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shl ax, 4
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add ax, (0x7c00 + 446)
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mov bx, ax
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add bx, 4
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cmp [bx], byte PARTITION_IDENTIFIER
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je .partition_found
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inc si
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cmp si, 4
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jb .finder_loop
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.partition_not_found:
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mov ax, 0xff
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ret
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.partition_found:
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mov ax, si
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ret
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; Arguments:
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; FURTHEST FROM BP
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; 4) Start LBA, less significant half
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; 3) Output Segment
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; 2) Output Offset
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; 1) Disk ID
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; NEAREST TO BP
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load_sector:
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push ax
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push bx
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push cx
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push dx
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push di
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push si
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; Allocate Disk Address Packet
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sub sp, 16
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mov bx, sp
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mov [bx], byte 0x10 ; Length of DAP
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mov [bx + 1], byte 0 ; Unused byte
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mov [bx + 2], word 1 ; Sector count
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; Write offset of the output buffer
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mov ax, [bp - 4]
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mov [bx + 4], ax
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; Write segment of the output buffer
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mov ax, [bp - 6]
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mov [bx + 6], ax
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; Write LBA of sector to read
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; mov [bx + 8], word 0
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; mov [bx + 10], word 0
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; mov ax, [bp - 8]
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; mov [bx + 12], ax
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; mov ax, [bp - 9]
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; mov [bx + 14], ax
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mov si, [bp - 8]
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mov [bx + 8], si
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mov [bx + 10], word 0
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mov [bx + 12], word 0
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mov [bx + 14], word 0
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; Perform the interrupt
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mov si, bx
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mov dl, [bp - 2]
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mov ah, 0x42 ; Extended Read from sectors
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int 0x13
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add sp, 16
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pop si
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pop di
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pop dx
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pop cx
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pop bx
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pop ax
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ret
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load_partition:
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sub sp, 16
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mov di, sp
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mov [di], word 0 ; Sector Index
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; Store the start sector
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mov bx, [bp - 2] ; Partition Index
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shl bx, 4 ; Get the offset from the MBR-table's tsart
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add bx, (0x7c00 + 446) ; Point into MBR table
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mov [di + 2], bx ; Partition Table Entry Pointer
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mov ax, [bx + 8]
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mov [di + 4], ax ; Lower Partition Start LBA
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mov ax, [bx + 10]
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mov [di + 6], ax ; Higher Partition Start LBA
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mov ax, [bx + 12]
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mov [di + 8], ax ; Lower Partition Sector Count
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; mov ax, [bx + 6]
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; mov [di + 10], ax ; Higher Partition Sector Count
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.loader_loop:
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mov bx, [di] ; Sector Index
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mov dx, [bp - 4] ; Disk Identifier
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; Segment Address
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mov cx, [di]
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shl cx, 5
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add cx, 0x0a00
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; mov ax, [di]
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; shl ax, 4
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; sub cx, ax
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mov bx, 0
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; This alternative snippet uses only the address into the segment
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; at 0x0000, rather than only the segment address. Both achieves
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; exactly the same, though.
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;
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; Output Buffer Address
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; mov bx, [di]
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; shl bx, 7
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; add bx, 0x0a00
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push bp
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mov bp, sp
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push dx
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push bx
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push cx
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mov ax, [di + 4]
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add ax, [di]
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push ax
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call load_sector
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mov sp, bp
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pop bp
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mov ax, [di]
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cmp ax, [di + 8]
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; Increment and write-back the sector index.
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inc ax
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mov [di], ax
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jb .loader_loop
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.finished:
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add sp, 16
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ret
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jump_to_partition:
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jmp 0x0000:0xa000
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cli
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hlt
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write_error:
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.partition_not_found:
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mov si, .partition_not_found_text
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jmp .write_text
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.partition_not_found_text:
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db "Failed finding Partition!", 0x00
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; SI: Text-Address
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.write_text:
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; Reset Display
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mov ah, 0x00
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mov al, 0x03
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int 0x10
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xor di, di
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.text_writer_loop:
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; Move Cursor
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mov ah, 0x02 ; Interrupt Function (move cursor)
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mov bh, 0 ; Display Page
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mov dx, 3 ; Start Column
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add dx, di
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mov dh, 2 ; Line
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int 0x10
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; Gather character and check if it is the null-terminator.
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mov bx, si
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add bx, di
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mov al, [bx]
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cmp al, 0x00
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je .halt
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; Display the character
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mov ah, 0x0a
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mov bh, 0
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mov cx, 1
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int 0x10
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inc di
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jmp .text_writer_loop
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.halt:
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cli
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hlt
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times 446 - ($ - $$) nop
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loader_partition:
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db 0x80
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db 0x00, 0x00, 0x00
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db PARTITION_IDENTIFIER
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db 0x00, 0x00, 0x00
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dd 1
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dd 63
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system_partition:
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db 0x80
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db 0x00, 0x00, 0x00
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db 1 ; FAT12 partition ID
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db 0x00, 0x00, 0x00
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dd 64
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dd 1024
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times 510 - ($ - $$) db 0x00
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db 0x55, 0xaa
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