mirror of
https://github.com/vxunderground/MalwareSourceCode
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304 lines
10 KiB
NASM
304 lines
10 KiB
NASM
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; DISKSCAN.ASM -- Checks out disk by reading sectors
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; --------------------------------------------------
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CSEG Segment
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Assume CS:CSEG, DS:CSEG, ES:CSEG, SS:CSEG
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Org 100h
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Entry: Jmp Begin
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; All Data
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; --------
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db ' Copyright 1986 Ziff-Davis Publishing Co.'
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db ' Programmed by Charles Petzold '
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DriveError db 'Invalid Drive$'
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DosVersErr db 'Needs DOS 2.0+$'
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MemoryError db 'Needs 64K$'
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ReadSegment dw ?
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DriveNum db ?
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DiskBlock db 18 dup (?)
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TotalSectors dw ?
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SectorsIn64K dw ?
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StartSector dw 0
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SectorLabel2 db 9,'Sector $'
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SectorLabel db 13,'Sectors $'
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DashLabel db ' - $'
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ErrorLabel db ': Error!'
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CRLF db 13,10,'$'
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ErrorAddr dw Err0,Err1,Err2,Err3,Err4,Err5,Err6,Err7
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dw Err8,Err9,ErrA,ErrB,ErrC,ErrD,ErrD,ErrD
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Err0 db 'Write Protect$'
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Err1 db 'Unknown Unit$'
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Err2 db 'Drive Not Ready$'
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Err3 db 'Unknown Command$'
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Err4 db 'CRC Error$'
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Err5 db 'Request Length$'
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Err6 db 'Seek Error$'
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Err7 db 'Unknown Media$'
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Err8 db 'Sector Not Found$'
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Err9 db 'No Paper$'
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ErrA db 'Write Fault$'
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ErrB db 'Read Fault$'
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ErrC db 'General Failure$'
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ErrD db 'Undocumented Error$'
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BootSectMsg db 'Boot Sector$'
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RootDirMsg db 'Root Directory$'
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BadFatMsg db 'File Alloc. Table$'
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InUseMsg db 'Used by file$'
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NotInUseMsg db 'Unallocated$'
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BadFlagMsg db 'Flagged as bad$'
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FatReadMsg db "Can't Read FAT$"
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Divisors dw 10000, 1000, 100, 10, 1 ; For decimal conversion
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; Check Drive Parameter, DOS Version, and Enough Memory
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; -----------------------------------------------------
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ErrorExit: Mov AH,9 ; Write error message
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Int 21h ; through DOS
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Int 20h ; And terminate
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Begin: Mov DX, Offset DriveError ; Possible message
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Or AL, AL ; Check Drive Validity Byte
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Jnz ErrorExit ; If not zero, invalid drive
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Mov DX, Offset DosVersErr ; Possible message
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Mov AH, 30h
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Int 21h ; Get DOS Version Number
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Cmp AL, 2 ; Check for 2.0 or later
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Jb ErrorExit ; If not, terminate with message
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Mov DX, Offset MemoryError ; Possible error message
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Mov BX, 256+Offset EndProg ; Set beyond program
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Mov SP, BX ; Move stack closer to code
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Add BX, 15 ; Add 15 to round up
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Mov CL, 4 ; Divide BX by 16
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Shr BX, CL
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Mov AH, 4Ah ; Free allocated memory
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Int 21h ; by calling DOS Set Block
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Jc ErrorExit ; Terminate on error
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Mov BX, 1000h ; Ask for 64K bytes
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Mov AH, 48h ; by using DOS
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Int 21h ; Allocate Memory call
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Jc ErrorExit ; Terminate on error
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Mov [ReadSegment], AX ; Save segment of memory block
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; Get Disk Information From DOS
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; -----------------------------
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Mov DL, DS:[005Ch] ; Get Drive Parameter
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Push DS ; Save DS
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Mov AH, 32h ; Call DOS to
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Int 21h ; get DOS Disk Block (DS:BX)
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Mov SI, BX ; Now DS:SI points to Disk Block
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Mov DI, Offset DiskBlock ; DI points to destination
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Mov CX, 18 ; 18 bytes to copy'
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Cld ; Forward direction
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Rep Movsb ; Move 'em in
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Pop DS ; Get back DS
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Mov BX, Offset DiskBlock ; BX to address Disk Block
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Mov DX, 1 ; Set DX:AX to 65,536
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Sub AX, AX
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Div Word Ptr [BX + 2] ; Divide by Bytes Per Sector
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Mov [SectorsIn64K], AX ; Save that values
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Mov AX, [BX + 13] ; Last Cluster Number
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Dec AX ; AX = Number of Clusters
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Mov CL, [BX + 5] ; Cluster to Sector Shift
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Shl AX, CL ; AX = Number Data Sectors
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Add AX, [BX + 11] ; Add First Data Sector
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Mov [TotalSectors], AX ; AX = Number Total Sectors
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Mov AL, DS:[005Ch] ; Drive Number (0=def, 1=A)
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Dec AL ; Make it 0=A, 1=B
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Jns GotDriveNumber ; If no sign, not default drive
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Mov AH, 19h ; Get current disk
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Int 21h ; by calling DOS
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GotDriveNumber: Mov [DriveNum], AL ; Save Drive Number (0=A, 1=B)
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; Start Reading
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; -------------
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MainLoop: Mov DX, Offset SectorLabel ; String to display on screen
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Call StringWrite ; Display it
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Mov AX, [StartSector] ; Starting sector number
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Call WordWrite ; Display number on screen
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Mov DX, Offset DashLabel ; String containing a dash
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Call StringWrite ; Display it on the screen
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Mov CX, [SectorsIn64K] ; Number of sectors to read
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Add AX, CX ; Add it to starting sector
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Jc NumRecalc
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Cmp AX, [TotalSectors] ; See if bigger than total
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Jbe NumSectorsOK ; If so, proceed
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NumRecalc: Mov AX, [TotalSectors] ; Otherwise get total sectors
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Mov CX, AX ; Move it to CX also
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Sub CX, [StartSector] ; Now CX = sectors to read
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NumSectorsOK: Dec AX ; AX = last sector to read
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Call WordWrite ; Display it on screen
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Call ReadSectors ; Read the sectors
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Jnc NextSectors ; If no error, skip detail
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Call ReadSectors ; Repeat read
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Jnc NextSectors ; If still no error, skip
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DiskError: Mov DX, Offset ErrorLabel ; String saying "Error!"
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Call StringWrite ; Display it on screen
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ErrorLoop: Push CX ; Now save previous number
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Mov CX, 1 ; So we can read one at a time
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Call ReadSectors ; Read one sector
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Jnc NoError ; If no error, proceed
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Mov BL, AL ; Save error code
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Mov DX, Offset SectorLabel2 ; String with "Sector "
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Call StringWrite ; Display it on screen
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Mov AX, [StartSector] ; The sector we just read
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Call WordWrite ; Display it on screen
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Mov DX, Offset DashLabel ; String with a dash
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Call StringWrite ; Display it on screen
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And BL, 0Fh ; Blank out error top bits
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Sub BH, BH ; Now BX is error code
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Add BX, BX ; Double it for word access
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Mov DX, [ErrorAddr + BX] ; Get address of message
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Call StringWrite ; Display message on screen
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Call FindSector ; See where sector is
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Mov DX, Offset CRLF ; String for new line
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Call StringWrite ; Do carriage ret & line feed
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NoError: Inc [StartSector] ; Kick up the start sector
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Pop CX ; Get back counter
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Loop ErrorLoop ; And read next sector
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Mov AX, [StartSector] ; Sector of next group
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Jmp Short CheckFinish ; Check if at end yet
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NextSectors: Mov AX, [StartSector] ; For no error, increment
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Add AX, [SectorsIn64K] ; StartSector for next group
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Jc Terminate ; (If overflow, terminate)
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Mov [StartSector], AX ; And save it
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CheckFinish: Cmp AX, [TotalSectors] ; See if at then end
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Jae Terminate ; If so, just terminate
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Jmp MainLoop ; If not, do it again
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Terminate: Int 20h ; Terminate
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; Find Sector in FAT to see if used by file, etc.
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; -----------------------------------------------
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FindSector: Mov DX, Offset DashLabel ; Print dash
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Call StringWrite
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Mov AX, [StartSector] ; Sector with error
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Mov DX, Offset BootSectMsg ; Set up message
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Cmp AX, Word Ptr [DiskBlock + 6] ; See if sector boot
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Jb PrintMsg ; If so, print as such
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Mov DX, Offset BadFatMsg ; Set up message
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Cmp AX, Word Ptr [DiskBlock + 16] ; See if sector in FAT
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Jb PrintMsg ; If so, print as such
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Mov DX, Offset RootDirMsg ; Set up message
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Cmp AX, Word Ptr [DiskBlock + 11] ; See if sector in dir
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Jb PrintMsg ; If so, print as such
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Push [StartSector] ; Save the sector
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Mov AX, Word Ptr [DiskBlock + 6] ; Reserved sectors
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Mov [StartSector], AX ; Start of first FAT
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Mov CL, [DiskBlock + 15] ; Sectors for FAT
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Sub CH, CH ; Zero out top byte
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Call ReadSectors ; Read in FAT
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Pop [StartSector] ; Get back bad sector
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Mov DX, Offset FatReadMsg ; Set up possible msg
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Jc PrintMsg ; If read error, print
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Mov AX, [StartSector] ; Get bad sector
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Sub AX, Word Ptr [DiskBlock + 11] ; Subtract data start
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Mov CL, [DiskBlock + 5] ; Sector Shift
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Shr AX, CL ; Shift the sector
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Add AX, 2 ; AX is now cluster
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Push ES ; Save ES for awhile
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Mov ES, [ReadSegment] ; ES segment of FAT
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Cmp Word Ptr [DiskBlock + 13], 0FF0h; 12 or 16-bit FAT?
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Jge Fat16Bit ; And jump accordingly
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Mov BX, AX ; This is cluster number
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Mov SI, AX ; So is this
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Shr BX, 1 ; This is one-half cluster
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Mov AX, ES:[BX + SI] ; BX + SI = 1.5 CX
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Jnc NoShift ; If no CY from shift, got it
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Mov CL, 4 ; If CY from shift must
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Shr AX, CL ; shift word 4 bits right
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NoShift: Or AX, 0F000h ; Now put 1's in top bits
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Cmp AX, 0F000h ; See if zero otherwise
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Jmp Short CheckWord ; And continue checking
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Fat16Bit: Mov BX, AX ; This is cluster number
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Shl BX, 1 ; Double it
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Mov AX, ES:[BX] ; Pull out word from sector
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Or AX, AX ; See if zero (unallocated)
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CheckWord: Pop ES ; Get back ES
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Mov DX, Offset NotInUseMsg ; Set up possible message
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Jz PrintMsg ; If so, print message
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Mov DX, Offset BadFlagMsg ; Set up possible message
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Cmp AX, 0FFF7h ; See if cluster flagged bad
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Jz PrintMsg ; If so, print message
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Mov DX, Offset InUseMsg ; If not, cluster is in use
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PrintMsg: Call StringWrite ; Print cluster disposition
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Ret ; And return
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; Read Sectors (CX = Number of Sectors, Return CY and AL for error)
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; -----------------------------------------------------------------
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ReadSectors: Push BX ; Push all needed registers
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Push CX
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Push DX
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Push DS
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Mov AL, [DriveNum] ; Get the drive number code
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Sub BX, BX ; Buffer address offset
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Mov DX, [StartSector] ; Starting Sector
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Mov DS, [ReadSegment] ; Buffer address segment
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Int 25h ; Absolute Disk Read
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Pop BX ; Fix up stack
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Pop DS ; Get back registers
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Pop DX
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Pop CX
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Pop BX
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Ret ; Return to program
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; Screen Display Routines
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; -----------------------
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WordWrite: Push AX ; Push some registers
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Push BX ; AX contains word to display
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Push CX
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Push DX
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Push SI
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Mov SI, Offset Divisors ; SI points to divisors
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Mov CX, 4 ; CL counter; CH zero blanker
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WordWriteLoop: Mov BX, [SI] ; Get divisor
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Add SI, 2 ; Increment SI for next one
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Sub DX, DX ; Prepare for division
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Div BX ; Divide DX:AX by BX
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Push DX ; Save remainder
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Or CH, AL ; See if zero
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Jz LeadZero ; If so, do not display it
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Add AL, '0' ; Convert number to ASCII
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Mov DL, AL ; Print out character
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Mov AH, 2 ; by calling DOS
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Int 21h
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LeadZero: Pop AX ; Get back remainder
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Dec CL ; Decrement counter
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Jg WordWriteLoop ; If CL still > 0, do it again
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Mov CH, 1 ; No more zero blanking
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Jz WordWriteLoop ; Convert last digit to ASCII
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Pop SI ; Get back pushed registers
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Pop DX
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Pop CX
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Pop BX
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Pop AX
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Ret
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StringWrite: Push AX ; Displays string from DX
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Mov AH, 9 ; to screen by calling DOS
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Int 21h
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Pop AX
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Ret
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EndProg Label Byte ; End of program
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CSEG EndS
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End Entry
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