Assembly
BIOS-based disk I-O to access MS-DOS file structure
BIOS-based disk I-O to access MS-DOS file structure ; rawread.asm ; ; this program reads a DOS cluster using only BIOS disk calls. All ; of the tasks usually done by DOS, e.g. FAT lookup, cluster to ; logical sector translation, logical to physical translation, are ; all done by this program instead. The idea is to be able to create ; a program that can access DOS disks from a bootable floppy without ; having to have DOS. ; ; well, that's what it used to do. Now it's supposed to do something ; completely different. Its job is to scan the entire surface of the ; hard drive, looking for the specified string. If that string is ; found, it is to print the full path and directory entry, including ; the file date and time. ; ; but wait! There's more. Now what we have is a number of raw ; routines which could prove useful for manipulating a DOS file ; structure outside of the DOS environment. The main routine still ; should be kept (if renamed), since the order in which these things ; are done is important (e.g. later calls depend on data set up by ; earlier calls). ; ; get filename ; parse filename into subdirs ; locate root dir and cluster size ; follow subdir routing to filename ; report file size, date & time ; .MODEL small .STACK 0200h .586P .DATA PartEntry STRUC Bootable db ? ;80h = bootable, 00h = nonbootable BeginHead db ? ;beginning head BeginSector db ? ;beginning sector BeginCylinder db ? ;beginning cylinder FileSystem db ? ;name of file system EndHead db ? ;ending head EndSector db ? ;ending sector EndCylinder db ? ;ending cylinder StartSector dd ? ;starting sector (relative to beg. of disk) PartSectors dd ? ;number of sectors in partition PartEntry ENDS BootSector STRUC Jump db ? ;E9 xx xx or EB xx 90 JumpTarget dw ? ;E9 xx xx or EB xx 90 OemName db '????????' ;OEM name & version ;Start of BIOS parameter block BytesPerSec dw ? ;bytes per sector SecPerClust db ? ;sectors per cluster ResSectors dw ? ;number of reserved sectors FATs db ? ;number of file allocation tables RootDirEnts dw ? ;number of root-dir entries Sectors dw ? ;total number of sectors Media db ? ;media descriptor byte FATsecs dw ? ;number of sectors per FAT SecPerTrack dw ? ;sectors per track Heads dw ? ;number of heads HiddenSecs dd ? ;number of hidden sectors HugeSectors dd ? ;num sectors if Sectors==0 ;End of BIOS parameter block BootSector ENDS DirEntry STRUC FileName db '????????' ;name Extension db '???' ;extension Attributes db ? ;attributes Reserved db 10 dup (?) ;reserved Time dw ? ;time stamp Date dw ? ;date stamp StartCluster dw ? ;starting cluster FileSize dd ? ;file size DirEntry ENDS BootFileName db "CONFIG SYS" ;the boot loader for this OS MBR DB 0200h DUP (?) buff DB 0200h * 40h DUP (?) ClustOffs dd ? CR EQU 0DH LF EQU 0AH .CODE main PROC STARTUPCODE ;initialize stuff call FetchMBR C ;fetch the master boot record jc @@exit mov cx,4 ;search up to four partitions add bx,01aeh ;point to partition table (-10h) @@FindBootable: add bx,10h ;point to next entry cmp BYTE ptr [bx],80h ;is it a bootable partition? loopnz @@FindBootable call FetchSector C, \ WORD ptr [(PartEntry PTR bx).BeginHead], \ WORD ptr [(PartEntry PTR bx).BeginSector], \ WORD ptr [(PartEntry PTR bx).BeginCylinder], \ OFFSET MBR, ds ;SEG MBR ; ; here's the point at which our OS loader would begin, with the ; BootSector structure in memory. ; mov bx, OFFSET MBR call CalcClustOff C, \ WORD ptr [(BootSector PTR bx).ResSectors], \ WORD ptr [(BootSector PTR bx).FATsecs], \ WORD ptr [(BootSector PTR bx).FATs], \ WORD ptr [(BootSector PTR bx).RootDirEnts], \ WORD ptr [(BootSector PTR bx).BytesPerSec], \ WORD ptr [(BootSector PTR bx).SecPerClust] mov WORD ptr [ClustOffs],ax mov WORD ptr [ClustOffs+2],dx call CalcClust2 C, \ WORD ptr [(BootSector PTR bx).ResSectors], \ WORD ptr [(BootSector PTR bx).FATsecs], \ WORD ptr [(BootSector PTR bx).FATs] ; now dx:ax contains the logical sector for cluster 2 call LsectToGeom C, \ ax, dx, \ WORD ptr [(BootSector PTR bx).HiddenSecs] , \ WORD ptr [((BootSector PTR bx).HiddenSecs)+2],\ [(BootSector PTR bx).Heads], \ [(BootSector PTR bx).SecPerTrack] mov dl,80h mov bx,offset buff mov al,[(BootSector PTR MBR).SecPerClust] mov ah,2h ; get ready to read int 13h ; now find our desired filename within buffer (which has the root dir) call FindFile C, \ bx, 200h * 40h, offset BootFileName xor dh,dh mov dl,[(BootSector PTR MBR).SecPerClust] mov si,ax mov ax,[(DirEntry PTR si).StartCluster] mul dx add ax,WORD ptr [ClustOffs] adc dx,WORD ptr [ClustOffs+2] ; now dx:ax contains logical sector number for start of file call LsectToGeom C, \ ax, dx, \ WORD ptr [(BootSector PTR MBR).HiddenSecs] , \ WORD ptr [((BootSector PTR MBR).HiddenSecs)+2],\ [(BootSector PTR MBR).Heads], \ [(BootSector PTR MBR).SecPerTrack] mov dl,80h mov ax,204h ; read in 2k worth of data int 13h @@exit: EXITCODE ;exit to DOS ENDP main ; ; FetchMBR - fetches the Master Boot Record from the first physical ; hard disk and stores it in the location MBR. ; ; INPUT: none ; OUTPUT: AX is error code if CY set, ES:BX ==> MBR ; DESTROYED: none ; FetchMBR PROC C USES cx, dx ;save registers we'll use mov dx,80h ;first physical disk mov cx,1 ;head 1, sector 0 mov bx,ds ; mov es,bx ;point to boot record buffer mov bx,OFFSET MBR ;read into boot record mov ax,0201h ;read one sector int 13h ;BIOS read ret ;return to main FetchMBR ENDP ; ; FetchSector - fetches the physical sector described by the passed ; parameters and stores it in the named buffer ; ; INPUT: head, sector, cylinder, buffer ; OUTPUT: AX is error code if CY set, ES:BX ==> Boot ; DESTROYED: none ; FetchSector PROC C head:BYTE, sector:BYTE, cylinder:BYTE, buffer:DWORD USES cx, dx ;save registers we'll use mov ch, [cylinder] ; mov cl, [sector] ; mov dh, [head] ; mov dl, 80h ;first physical hard drive les bx, [buffer] ; mov ax,0201h ;read one sector int 13h ;BIOS read ret ;return to main FetchSector ENDP ; ; GeomToLsect - converts to logical sector number from the physical ; geometry (head, cylinder, track). See LsectToGeom. ; ; INPUT: cx, dx are set with cylinder/track, and head respectively ; HiddenSecs, Heads, SecPerTrack ; OUTPUT: lsect ; DESTROYED: none ; GeomToLsect PROC C lsect:DWORD, dHiddenSecs:DWORD, \ dHeads:WORD, dSecPerTrack:WORD, buffer:DWORD USES ax ;save registers we'll use mov ax, WORD ptr [lsect] ;load lsect into DX:AX mov dx, WORD ptr [lsect+2] ; stc ;add one additional adc ax, WORD ptr [dHiddenSecs] ;add starting sector adc dx, WORD ptr [dHiddenSecs+2] ; div [dSecPerTrack] ; mov cl,dl ;store sector in cl xor dx,dx ; div [dHeads] ; mov dh,dl ;store head in dh mov ch,al ;store low 8 bits of cylinder in ch shr ax,1 ; shr ax,1 ; and al,0c0h ;pass through two hi bits only or cl,ah ;mov bits into location ret ; GeomToLsect ENDP ; ; LsectToGeom - converts from logical sector number to the physical ; geometry (head, cylinder, track) in the form required ; by the BIOS (Int 13h) disk read and write calls. ; ; INPUT: lsect, HiddenSecs, Heads, SecPerTrack ; OUTPUT: cx, dx are set with cylinder/track, and head respectively ; DESTROYED: none ; LsectToGeom PROC C lsect:DWORD, lHiddenSecs:DWORD, \ lHeads:WORD, lSecPerTrack:WORD, buffer:DWORD USES ax ;save registers we'll use mov ax, WORD ptr [lsect] ;load lsect into DX:AX mov dx, WORD ptr [lsect+2] ; stc ;add one additional adc ax, WORD ptr [lHiddenSecs] ;add starting sector adc dx, WORD ptr [lHiddenSecs+2] ; div [lSecPerTrack] ; mov cl,dl ;store sector in cl xor dx,dx ; div [lHeads] ; mov dh,dl ;store head in dh mov ch,al ;store low 8 bits of cylinder in ch shr ax,1 ; shr ax,1 ; and al,0c0h ;pass through two hi bits only or cl,ah ;mov bits into location ret ; LsectToGeom ENDP ; ; CalcClust2 - calculates the starting logical sector number of ; cluster 2, (the beginning of data space for ; partitions). ; ; INPUT: ResSectors, FATsecs, FATs ; OUTPUT: dx:ax contains the starting logical sector number ; DESTROYED: none ; CalcClust2 PROC C cResSectors:WORD, cFATsecs:WORD, cFATs:BYTE xor dx,dx ; mov ax,[cFATsecs] ; mul [cFATs] ; add ax,[cResSectors] ; adc dx,0 ; ret CalcClust2 ENDP ; ; CalcClustOff - calculates the starting logical sector number of ; cluster 0, which isn't really a cluster, but the ; number returned is useful for calculations converting ; cluster number to logical sector ; ; INPUT: ResSectors, FATsecs, FATs ; OUTPUT: dx:ax contains the starting logical sector number ; DESTROYED: none ; CalcClustOff PROC C dResSectors:WORD, dFATsecs:WORD, dFATs:BYTE, \ dRootDirEnts:WORD, dBytesPerSec:WORD, dSecPerClust:BYTE LOCAL clustLo:WORD, clustHi:WORD xor dh,dh mov ax,[dFatSecs] mov dl,[dFATs] mul dx add ax,[dResSectors] adc dx,0 ; call CalcClust2 C, [dResSectors], [dFATsecs], [dFATs] ; now dx:ax = FATs * FATsecs + ResSectors mov [clustLo],ax mov [clustHi],dx mov dx,20h ; bytes per dir entry mov ax,[dRootDirEnts] ; mul dx ; multiply 'em out div [dBytesPerSec] ; and divide by bytes/sec add [clustLo],ax ; adc [clustHi],dx ; create the aggregate mov al,[dSecPerClust] ; xor ah,ah ; shl ax,1 ; AX = SecPerClust * 2 sub [clustLo],ax ; sbb [clustHi],0 ; propagate carry flag mov ax,[clustLo] ; mov dx,[clustHi] ; ret CalcClustOff ENDP ; ; FindFile - given a memory buffer containing the directory data ; and a static file name for which to search, this routine ; finds the file and returns a pointer to its directory ; entry in ds:si ; ; INPUT: dirbuffer, filespec ; OUTPUT: ax contains pointer to directory entry (or NULL) ; DESTROYED: none ; FindFile PROC C dirbuffer:WORD, limit:WORD, filespec:WORD USES cx, dx, di, si, es mov cx,ds ; mov es,cx ; es and ds point to same segment cld ; always count forward mov ax,[dirbuffer] ; load 'em up add [limit],ax mov dx,[filespec] ; keepsearching: mov cx,11 ; size of dos filename (8.3) mov si,dx ; mov di,ax ; repe cmpsb ; compare 'em jz foundit ; add ax,20h ; size of directory entry cmp ax,[limit] jb keepsearching xor ax,ax foundit: ret FindFile ENDP END
BIOS-based disk I-O to access MS-DOS file structure
