forked from KolibriOS/kolibrios
184 lines
3.3 KiB
NASM
184 lines
3.3 KiB
NASM
; KolibriOS bootloader
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; bootsector for loading from FAT32 flash (or hard) drive
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; intended for use with mtldr_f file in root folder
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; this code has been written by diamond in 2005,2006,2007 specially for KolibriOS
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; this code is loaded by BIOS to 0000:7C00
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org 0x7C00
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jmp @f
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nop
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times 57h db 0
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; file 'bt2.dat':3,57h
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@@:
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xor eax, eax
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mov ds, ax
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mov ss, ax
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mov sp, 7C00h
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mov [boot_drive], dl
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cld
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sti
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push 800h
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pop es
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movzx ebx, word [7C0Eh] ; reserved_sect
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mov [fat_start], ebx
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mov al, byte [7C10h] ; num_fats
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mul dword [7C24h] ; sect_fat
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add eax, ebx
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; cluster 2 begins from sector eax
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movzx ebx, byte [7C0Dh] ; sects_per_clust
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add bx, bx
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sub eax, ebx
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mov [data_start], eax
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mov eax, [7C2Ch] ; root_cluster
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and eax, 0FFFFFFFh
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fat32_parse_dir:
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xor bx, bx
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mov di, bx
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push eax
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call read_cluster
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movzx cx, byte [7C0Dh] ; sects_per_clust
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shl cx, 4 ; *0x200/0x20
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scan_cluster:
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cmp byte [es:di], 0
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jz file_not_found
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push cx di
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mov cx, 11
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mov si, mtldr_f
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repz cmpsb
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pop di cx
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jz file_found
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add di, 20h
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loop scan_cluster
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pop eax
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call next_cluster
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jnc file_not_found
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jc fat32_parse_dir
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file_found:
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pop eax
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mov ax, [es:di+14h]
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and ax, 0FFFh
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shl eax, 10h
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mov ax, [es:di+1Ah]
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; eax contains first cluster
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@@:
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xor bx, bx
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push eax
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call read_cluster
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mov ax, es
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movzx cx, byte [7C0Dh]
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shl cx, 5
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add ax, cx
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mov es, ax
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pop eax
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call next_cluster
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jc @b
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jmp 0:8000h
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file_not_found:
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mov si, file_not_found_msg
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sayerr:
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call out_string
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jmp $
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read_cluster:
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; in: eax = cluster, bx->buffer
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movzx ecx, byte [7C0Dh]
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mul ecx
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add eax, [data_start]
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; read procedure
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; in: eax = absolute sector
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; cx = number of sectors
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; es:bx -> buffer
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read:
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add eax, [7C1Ch] ; hidden sectors
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push es
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read_loop:
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pushad
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; allocate disk address packet on the stack
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; qword +8: absolute block number
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push 0
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push 0 ; dword +C is high dword
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push eax ; dword +8 is low dword
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; dword +4: buffer address
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push es ; word +6 is segment
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push bx ; word +4 is offset
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; word +2: number of blocks, limited to 7Fh
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sub cx, 7Fh
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sbb ax, ax
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and ax, cx
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add ax, 7Fh
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push ax
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shl ax, 5
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mov cx, es
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add cx, ax
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mov es, cx
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; word +0: size of packet = 10h
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push 10h
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; now pair ss:sp contain address of disk address packet
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mov ax, 4200h
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mov dl, [boot_drive]
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mov si, sp
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int 13h
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mov si, disk_read_err
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jc sayerr
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popaw
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popad
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add eax, 7Fh
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sub cx, 7Fh
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ja read_loop
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pop es
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ret
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next_cluster:
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push es
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push ds
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pop es
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mov bx, 7E00h
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; sector is 200h bytes long, one entry in FAT occupies 4 bytes
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; => 80h entries in sector
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push eax
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shr eax, 7 ; div 80h
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cmp eax, [fat_cur_sector]
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jz @f
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mov [fat_cur_sector], eax
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add eax, [fat_start]
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mov cx, 1
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call read
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@@:
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pop eax
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and eax, 7Fh
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mov eax, [7E00h+eax*4]
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and eax, 0FFFFFFFh
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cmp eax, 0FFFFFF7h
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mov si, bad_cluster
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jz sayerr
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pop es
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ret
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out_string:
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lodsb
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test al, al
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jz .xxx
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mov ah, 0Eh
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mov bx, 7
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int 10h
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jmp out_string
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.xxx: ret
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file_not_found_msg db 'Cannot find file '
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mtldr_f db 'MTLD_F32 '
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db 13,10,0
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disk_read_err db 'Disk read error',13,10,0
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bad_cluster db 'Bad cluster',13,10,0
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fat_cur_sector dd -1
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times (7DFEh - $) db 0
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db 55h, 0AAh
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virtual at 7A00h
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fat_start dd ?
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data_start dd ?
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boot_drive db ?
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end virtual
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