;------------------------------------------------------------------------------------------------------- ; Copyright (c) 2012, "asmdev" at board.flatassembler.net. All rights reserved. ; ; This file may be distributed and/or modified under the terms of the GNU GPLv2 License as published ; by the Free Software Foundation and appearing on www.gnu.org website ; ; This copyright notice MUST APPEAR in all copies of the script! ;------------------------------------------------------------------------------------------------------- org $7c00 use16 VBR: jmp short @f nop bpb_OEM db 'MSDOS5.0' bpb_SectSize dw 512 ; =512, Required bpb_SectPerClust db 1 bpb_ReservedSect dw 32 bpb_FATsCnt db 2 bpb_RootSize dw 0 bpb_TotalSect16 dw 0 bpb_Media db $f8 bpb_FatSize16 dw 0 bpb_TrackSect dw $3f bpb_Heads dw $ff bpb_hiddenSect dd $7d82 bpb_TotalSect dd $139C5 bpb_fatSz dd $26a bpb_extFlags dw 0 bpb_ver dw 0 bpb_rootClust dd 2 bpb_FSInfo dw 1 bpb_backupBoot dw 6 rb 12 bpb_DriveNum db $80 bpb_winSignature db 0 bpb_bootSig db $29 bpb_volID dd 0 bpb_volLabel db "NO NAME " db "FAT32 " offs = lbaPacket-VBR ;=================================================================================================== @@: xor ebp, ebp mov fs, dx ; FS[7:0] = drive # jmp 0:@f ; flush CS align 4 ; not really sure what alignment is for, might be for performace++ lbaPacket: dd $00'01'00'10 ; $10 = size of LBA struct; $01 = # of LBA sectors to read @@: mov ss, bp mov sp, VBR mov si, lbaPacket ; SI points to LBA packet struct mov ds, bp mov dword [si+12], ebp ; check int13 extensions (dl = drive # already) mov ah, $41 mov bx, $55aa int $13 jc .no_LBA cmp bx, $aa55 jnz .no_LBA test cl, 1 jnz .LBA_Legacy_done .no_LBA: not byte [si] ; IF [SI]=$10 then LBA, IF [SI] != $10 then Legacy ; get drive geometry for int13 ah=2 mov dx, fs mov ah, 8 int $13 ; return: DH = maxHead-1, CX[5:0] = max sector jc Error movzx bp, dh inc bp ; ebp[31:16] = 0 previously, ebp[15:0] = 9bit max head and ecx, 111111b ;=================================================================================================== .LBA_Legacy_done: push ebp ; save max head at [si-offs-4] push ecx ; save max sector at [si-offs-8] mov ebx, [si-offs+28] ; ebx = hidden sectors = start of partition mov bp, [si-offs+14] add ebp, ebx ; reserved sectors count += beginning of partition push ebp ; [si-offs-12] = beginning of FAT in LBA units (512b units) mov ebx, [si-offs+44] ; EBX = root cluster bts ebp, 31 ; EBP = index of cached 512b chunk inside FAT (=invalid index) ;------------------------------------ fileSearch: mov dword [si+4], $0000'8000 ; segment:offset where directory custers(one at a time) copied copyClusters: bsf cx, [si-offs+13] ; "a step" towards # of 512byte chunks in one cluster movzx eax, byte [si-offs+16] ; read FAT count mul dword [si-offs+36] ; *= FAT size (all in 512b units) lea edx, [ebx-2] ; edx = cluster shl edx, cl ; edx = cluster * secPerCluster add eax, [si-offs-12] ; eax: FATs size += beginning of FATs add eax, edx ; eax = beginning of specific clusters mov [si+8], eax ; 4byte LBA call readDisk .patch2: jmp short @f @@: mov di, $8000 find_file: cmp dword [di], "1234" jnz @f cmp dword [di+4], "5678" jnz @f cmp dword [di+8], "BIN" + $20'00'00'00 jnz @f mov bx, [di+20] ; top 16bit of cluster shl ebx, 16 mov bx, [di+26] ; low 16bit of cluster ; patch jumps mov byte [copyClusters.patch2+1], read_FAT_chunk-copyClusters.patch2-2 mov byte [read_FAT_chunk.patch1+2], (copyClusters-read_FAT_chunk.patch1-4) and $ff jmp fileSearch @@: add di, 32 cmp di, [si+4] jnz find_file ;------------------------------------ read_FAT_chunk: mov eax, ebx shr eax, 7 ; eax = index of desired 512b chunk inside FAT ; check if desired 512 byte chunk is cached already cmp eax, ebp jz @f mov ebp, eax push dword [si+4] add eax, [si-offs-12] ; 512b index += beginning of FAT in 512b units mov dword [si+4], $800 ; read FAT chunk to 0:$800 mov dword [si+8], eax ; update low 32bit of LBA xor cx, cx ; cl = 0, meaning one 512b chunk to read call readDisk pop dword [si+4] @@: and bx, 127 shl bx, 2 mov ebx, [$800+bx] ; read next cluster index cmp ebx, $0fff'fff0 ; check if we reached end of cluster chain .patch1: jb near fileSearch @@: cmp byte [.patch1+2], (copyClusters-read_FAT_chunk.patch1-4) and $ff jnz Error jmp $0:$8000 ;=================================================================================================== fileNotFound: db "12345678.bin missing" Error: mov ax, 3 int $10 .1: cld mov si, fileNotFound push $b800 pop es xor di, di mov ah, $a @@: lodsb cmp al, $b8 ; this byte is part of 'mov ax,3' instruction jz .1 stosw jmp @b ;=================================================================================================== ; temp = lba / max_sectors_per_track ; sector = (lba % max_sectors_per_track) + 1 ; head = temp % max_number_of_heads ; cylinder = temp / max_number_of_head ; "int13 ah=2" code bellow doesn't validate max cylinder supported by disk/bios ; There are no retry attepmts if read fails. ; max head at [si-offs-4] ; max sector at [si-offs-8] readDisk: push ebx ; Bochs 2.51 STILL messes with top 16bit for some registers mov byte [si+32], 1 shl byte [si+32], cl .read: sti cmp byte [si], $10 jz .read_42h ;------------------------------------ .readLegacy: mov eax, [si+8] ; read lba addr xor edx, edx div dword [si-offs-8] ; divide by max spt(sectors per track) inc dx mov cl, dl ; cl = sectors xor edx, edx div dword [si-offs-4] ; divide by max heads shl ah, 6 mov ch, al or cl, ah mov ax, fs xchg dl, dh ; dh = head mov dl, al ; dl = drive number push word [si+6] ; segment mov bx, [si+4] ; offset mov ax, $201 ; read one sector pop es int $13 jc short Error jmp .advanceSector ;------------------------------------ .read_42h: mov dx, fs mov ah, $42 int $13 jc short Error ;------------------------------------ .advanceSector: inc dword [si+8] ; source address ++ add word [si+4], 512 ; destination offset += sector size jnz @f add word [si+6], $1000 ; destination segment++ if offset=0 @@: dec byte [si+32] jnz .read pop ebx ret db (512-($-VBR)-2) dup('+') dw $aa55