324 lines
4.8 KiB
NASM
324 lines
4.8 KiB
NASM
DISK_ID EQU 0x80
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KERNEL_START EQU 0x100000
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ELF_START EQU 0x8000
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NUM_SECTORS EQU 21
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org 0x7C00
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bits 16
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jmp .start
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[bits 16]
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; Function: check_a20
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;
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; Returns: 0 in ax if the a20 line is disabled (memory wraps around)
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; 1 in ax if the a20 line is enabled (memory does not wrap around)
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check_a20:
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cli
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xor ax, ax ; ax = 0
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mov es, ax
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not ax ; ax = 0xFFFF
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mov ds, ax
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mov di, 0x0500
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mov si, 0x0510
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mov al, byte [es:di]
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mov bx, ax
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mov al, byte [ds:si]
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mov byte [es:di], 0x00
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mov byte [ds:si], 0xFF
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cmp byte [es:di], 0xFF
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mov byte [ds:si], al
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mov ax, bx
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mov byte [es:di], al
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mov ax, 0
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je .check_a20__exit
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mov ax, 1
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.check_a20__exit:
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ret
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;;; Print string at bx
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puts:
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mov al, '!'
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mov ah, 0x0e
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int 0x10
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.puts_loop:
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mov al, [bx] ; load the character
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cmp al, 0 ; check for null byte
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je .end_puts_loop
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mov ah, 0x0e
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int 0x10 ; print charachter
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inc bx
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jmp .puts_loop
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.end_puts_loop:
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ret
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;;; Print bl as hex
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puthex:
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; Print high bits
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mov dl, bl
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shr dl, 4
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cmp dl, 9
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jg .puthex_high_alpha
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; It's < 10, print as digit
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mov al, dl
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add al, 0x30
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mov ah, 0x0e
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int 0x10
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jmp .puthex_low
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.puthex_high_alpha:
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; It's >= 10, print as letter
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mov al, dl
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add al, 0x57 ; 0x57 = (0x61 - 10)
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mov ah, 0x0e
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int 0x10
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.puthex_low:
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; Print low bits
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mov dl, bl
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and dl, 0b1111
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cmp dl, 9
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jg .puthex_low_alpha
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; It's < 10, print as digit
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mov al, dl
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add al, 0x30
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mov ah, 0x0e
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int 0x10
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jmp .puthex_end
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.puthex_low_alpha:
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; It's >= 10, print as letter
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mov al, dl
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add al, 0x57 ; 0x57 = (0x61 - 10)
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mov ah, 0x0e
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int 0x10
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.puthex_end:
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ret
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.start:
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; disable vga cursor
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mov ah, 0x01
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mov ch, 0b00100000
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mov cl, 0b00000000
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int 0x10
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; reset disk system
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mov ah, 0x00
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mov dl, DISK_ID
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int 0x13
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; print result on error
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jnc .reset_disk_no_error
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mov bl, ah
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call puthex
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.reset_disk_no_error:
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mov cx, 2 ; start at sector 2 (skip bootloader)
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mov bx, ELF_START ; write to ELF_START
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.read_disk_loop:
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; Read sector dl into memory
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mov ah, 0x02
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mov al, 0x01 ; number of sectors
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mov ch, 0x00 ; cylinder number, low 8 bits
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; mov cl, 0x01 ; cylinder number, high 2 bits + sector number (6 bits)
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mov dh, 0x00 ; head number
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mov dl, DISK_ID ; drive number
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int 0x13
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; exit on error
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jc .read_disk_error
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cmp cx, NUM_SECTORS
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jge .read_disk_end ; we have reached the sector limit, time to boot
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inc cl ; next sector
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add bx, 512 ; add 512 to data start so we don't overwrite sectors
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jmp .read_disk_loop
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.read_disk_error:
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mov bl, ah
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call puthex
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.read_disk_end:
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call check_a20
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cmp ax, 1
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je .a20_enabled
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mov bx, .str_no_A20
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call puts
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jmp .end
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.a20_enabled:
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; disable interrupts
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cli
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; reset ds
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xor ax, ax
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mov ds, ax
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; load the GDT
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lgdt [.gdt_desc]
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; switch to protected mode
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mov eax, cr0
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or eax, 1
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mov cr0, eax
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; jump to 32 bit code
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jmp 0x08:.clear_pipe
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[BITS 32]
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.clear_pipe:
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; set Data Segment and Stack segment
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mov ax, 10h
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mov ds, ax
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mov ss, ax
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; set up stack
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mov esp, 0x090000
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; parse elf file at ELF_START to KERNEL_START
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; verify ELF header
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mov esi, ELF_START
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cmp dword [esi], 464C457Fh ; ELF magic
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jne .invalid_elf
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cmp word [esi+4], 0101h ; lsb 32 bit, little endian
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jne .invalid_elf
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cmp word [esi+18], 03 ; x86 architecture
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jne .invalid_elf
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; read the entrypoint and store it
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mov eax, dword [esi+0x18] ; program entry position
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mov dword [.entrypoint], eax
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mov cx, word [esi+0x2C] ; read phnum (number of program headers)
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mov eax, dword [esi+0x1C] ; read phoff (offset of program header)
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; ebx is now ELF_START, esi jumps to the start of the program header
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mov ebx, esi
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add esi, eax
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; set up for loop
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sub esi, 0x20
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inc cx
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.elf_ph_loop:
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add esi, 0x20
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dec cx
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jz .invalid_elf ; there is no valid code block
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cmp word [esi], 1 ; check if p_type is loadable
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jne .elf_ph_loop
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; add offset to ebx (ebx = pointer to code)
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add ebx, dword [esi+0x04]
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; store p_filesz (ecx = size of the segment)
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mov ecx, dword [esi+0x10]
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; calculate size of bss (edx = size of bss)
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mov edx, dword [esi+0x14]
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sub edx, ecx
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; check if ecx is zero
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or ecx, ecx
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jz .invalid_elf
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; set source
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mov esi, ebx
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; set destination
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mov edi, KERNEL_START
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; repeat ecx/4 times (because it moves 4 bytes at a time)
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shr ecx, 2
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; copy
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repnz movsd
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; jump to start of kernel
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jmp [.entrypoint]
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.end:
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jmp .end
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.invalid_elf:
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mov byte [ds:0x0B8000], 'E'
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mov byte [ds:0x0B8001], 0x40
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mov byte [ds:0x0B8002], 'L'
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mov byte [ds:0x0B8003], 0x40
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mov byte [ds:0x0B8004], 'F'
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mov byte [ds:0x0B8005], 0x40
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jmp .end
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.data:
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.str_no_A20:
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db "A20 fault"
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db 0
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.gdt:
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dd 0
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dd 0
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dw 0FFFFh
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dw 0
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db 0
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db 10011010b
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db 11001111b
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db 0
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dw 0FFFFh
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dw 0
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db 0
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db 10010010b
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db 11001111b
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db 0
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.gdt_end:
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.gdt_desc:
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dw .gdt_end - .gdt - 1
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dd .gdt
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.entrypoint: dd 0
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; print padding nullbytes
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times 510 - ($ - $$) db 0
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; write magic string
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dw 0xAA55 |