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