mirror of
https://github.com/vxunderground/MalwareSourceCode
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248 lines
14 KiB
NASM
248 lines
14 KiB
NASM
;This is a simple boot sector that will load either MS-DOS or PC-DOS. It is not
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;self-reproducing, but it will be used as the foundation on which to build a
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;virus into a boot sector.
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;This segment is where the first operating system file (IBMBIO.COM or IO.SYS)
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;will be loaded and executed from. We don't know (or care) what is there, but
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;we do need the address to jump to defined in a separate segment so we can
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;execute a far jump to it.
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DOS_LOAD SEGMENT AT 0070H
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ASSUME CS:DOS_LOAD
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ORG 0
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LOAD: DB 0 ;Start of the first operating system program
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DOS_LOAD ENDS
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MAIN SEGMENT BYTE
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ASSUME CS:MAIN,DS:MAIN,SS:NOTHING
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;This jump instruction is just here so we can compile this program as a COM
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;file. It is never actually executed, and never becomes a part of the boot
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;sector. Only the 512 bytes after the address 7C00 in this file become part of
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;the boot sector.
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ORG 100H
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START: jmp BOOTSEC
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;The following two definitions are BIOS RAM bytes which contain information
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;about the number and type of disk drives in the computer. These are needed by
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;the virus to decide on where to look to find drives to infect. They are not
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;normally needed by an ordinary boot sector.
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;
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; ORG 0410H
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;
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;SYSTEM_INFO: DB ? ;System info byte: Take bits 6 & 7 and add 1 to get number of
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; ;disk drives on this system (eg 01 = 2 drives)
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;
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; ORG 0475H
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;
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;HD_COUNT: DB ? ;Number of hard drives in the system
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;
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;This area is reserved for loading the first sector of the root directory, when
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;checking for the existence of system files and loading the first system file.
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ORG 0500H
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DISK_BUF: DW ? ;Start of the buffer
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;Here is the start of the boot sector code. This is the chunk we will take out
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;of the compiled COM file and put it in the first sector on a 360K floppy disk.
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;Note that this MUST be loaded onto a 360K floppy to work, because the
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;parameters in the data area that follow are set up to work only with a 360K
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;disk!
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ORG 7C00H
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BOOTSEC: JMP BOOT ;Jump to start of boot sector code
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ORG 7C03H ;This is needed because the jump will get coded as 2 bytes
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DOS_ID: DB 'EZBOOT ' ;Name of this boot sector (8 bytes)
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SEC_SIZE: DW 200H ;Size of a sector, in bytes
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SECS_PER_CLUST: DB 02 ;Number of sectors in a cluster
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FAT_START: DW 1 ;Starting sector for the first File Allocation Table (FAT)
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FAT_COUNT: DB 2 ;Number of FATs on this disk
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ROOT_ENTRIES: DW 70H ;Number of root directory entries
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SEC_COUNT: DW 2D0H ;Total number of sectors on this disk
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DISK_ID: DB 0FDH ;Disk type code (This is 360KB)
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SECS_PER_FAT: DW 2 ;Number of sectors per FAT
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SECS_PER_TRK: DW 9 ;Sectors per track for this drive
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HEADS: DW 2 ;Number of heads (sides) on this drive
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HIDDEN_SECS: DW 0 ;Number of hidden sectors on the disk
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DSKBASETBL:
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DB 0 ;Specify byte 1: step rate time, head unload time
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DB 0 ;Specify byte 2: Head load time, DMA mode
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DB 0 ;Wait time until motor turned off, in clock ticks
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DB 0 ;Bytes per sector (0=128, 1=256, 2=512, 3=1024)
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DB 12H ;Last sector number (we make it large enough to handle 1.2/1.44 MB floppies)
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DB 0 ;Gap length between sectors for r/w operations, in bytes
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DB 0 ;Data transfer length when sector length not specified
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DB 0 ;Gap length between sectors for format operations, in bytes
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DB 0 ;Value stored in newly formatted sectors
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DB 1 ;Head settle time, in milliseconds (we set it small to speed operations)
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DB 0 ;Motor startup time, in 1/8 seconds
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HEAD: DB 0 ;Current head to read from (scratch area used by boot sector)
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;Here is the start of the boot sector code
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BOOT: CLI ;interrupts off
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XOR AX,AX ;prepare to set up segments
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MOV ES,AX ;set ES=0
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MOV SS,AX ;start stack at 0000:7C00
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MOV SP,OFFSET BOOTSEC
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MOV BX,1EH*4 ;get address of disk
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LDS SI,SS:[BX] ;param table in ds:si
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PUSH DS
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PUSH SI ;save that address
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PUSH SS
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PUSH BX ;and its address
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MOV DI,OFFSET DSKBASETBL ;and update default
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MOV CX,11 ;values to the table stored here
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CLD ;direction flag cleared
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DFLT1: LODSB
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CMP BYTE PTR ES:[DI],0 ;anything non-zero
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JNZ SHORT DFLT2 ;is not a default, so don't save it
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STOSB ;else put default value in place
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JMP SHORT DFLT3 ;and go on to next
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DFLT2: INC DI
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DFLT3: LOOP DFLT1 ;and loop until cx=0
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MOV AL,AH ;set ax=0
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MOV DS,AX ;set ds=0 so we can set disk tbl
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MOV WORD PTR [BX+2],AX ;to @DSKBASETBL (ax=0 here)
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MOV WORD PTR [BX],OFFSET DSKBASETBL ;ok, done
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STI ;now turn interrupts on
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INT 13H ;and reset disk drive system
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ERROR1: JC ERROR1 ;if an error, hang the machine
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;Here we look at the first file on the disk to see if it is the first MS-DOS or
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;PC-DOS system file, IO.SYS or IBMBIO.COM, respectively.
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LOOK_SYS:
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MOV AL,BYTE PTR [FAT_COUNT] ;get fats per disk
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XOR AH,AH
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MUL WORD PTR [SECS_PER_FAT] ;multiply by sectors per fat
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ADD AX,WORD PTR [HIDDEN_SECS] ;add hidden sectors
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ADD AX,WORD PTR [FAT_START] ;add starting fat sector
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PUSH AX
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MOV WORD PTR [DOS_ID],AX ;root dir, save it
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MOV AX,20H ;dir entry size
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MUL WORD PTR [ROOT_ENTRIES] ;dir size in ax
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MOV BX,WORD PTR [SEC_SIZE] ;sector size
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ADD AX,BX ;add one sector
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DEC AX ;decrement by 1
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DIV BX ;ax=# sectors in root dir
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ADD WORD PTR [DOS_ID],AX ;DOS_ID=start of data
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MOV BX,OFFSET DISK_BUF ;set up disk read buffer at 0000:0500
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POP AX
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CALL CONVERT ;and go convert sequential sector number to bios data
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MOV AL,1 ;prepare for a disk read for 1 sector
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CALL READ_DISK ;go read it
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MOV DI,BX ;compare first file on disk with
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MOV CX,11 ;required file name
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MOV SI,OFFSET SYSFILE_1 ;of first system file for PC DOS
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REPZ CMPSB
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JZ SYSTEM_THERE ;ok, found it, go load it
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MOV DI,BX ;compare first file with
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MOV CX,11 ;required file name
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MOV SI,OFFSET SYSFILE_2 ;of first system file for MS DOS
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REPZ CMPSB
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ERROR2: JNZ ERROR2 ;not the same - an error, so hang the machine
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;Ok, system file is there, so load it
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SYSTEM_THERE:
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MOV AX,WORD PTR [DISK_BUF+1CH] ;get file size of IBMBIO.COM/IO.SYS
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XOR DX,DX
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DIV WORD PTR [SEC_SIZE] ;and divide by sector size
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INC AL ;ax=number of sectors to read
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MOV BP,AX ;store that number in BP
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MOV AX,WORD PTR [DOS_ID] ;get sector number of start of data
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PUSH AX
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MOV BX,700H ;set disk read buffer to 0000:0700
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RD_BOOT1: MOV AX,WORD PTR [DOS_ID] ;and get sector to read
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CALL CONVERT ;convert to bios Trk/Cyl/Sec info
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MOV AL,1 ;read one sector
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CALL READ_DISK ;go read the disk
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SUB BP,1 ;subtract 1 from number of sectors to read
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JZ DO_BOOT ;and quit if we're done
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ADD WORD PTR [DOS_ID],1 ;add sectors read to sector to read
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ADD BX,WORD PTR [SEC_SIZE] ;and update buffer address
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JMP RD_BOOT1 ;then go for another
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;Ok, the first system file has been read in, now transfer control to it
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DO_BOOT:
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MOV CH,BYTE PTR [DISK_ID] ;Put drive type in ch
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MOV DL,BYTE PTR [DRIVE] ;Drive number in dl
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POP BX
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JMP FAR PTR LOAD ;and transfer control to the first system file
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;Convert sequential sector number in ax to BIOS Track, Head, Sector information.
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;Save track number in DX, sector number in CH,
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CONVERT:
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XOR DX,DX
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DIV WORD PTR [SECS_PER_TRK] ;divide ax by sectors per track
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INC DL ;dl=sector number to start read on, al=track/head count
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MOV CH,DL ;save it here
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XOR DX,DX
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DIV WORD PTR [HEADS] ;divide ax by head count
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MOV BYTE PTR [HEAD],DL ;dl=head number, save it
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MOV DX,AX ;ax=track number, save it in dx
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RET
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;Read the disk for the number of sectors in al, into the buffer es:bx, using
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;the track number in DX, the head number at HEAD, and the sector
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;number at CH.
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READ_DISK:
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MOV AH,2 ;read disk command
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MOV CL,6 ;shift possible upper 2 bits of track number to
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SHL DH,CL ;the high bits in dh
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OR DH,CH ;and put sector number in the low 6 bits
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MOV CX,DX
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XCHG CH,CL ;ch (0-5) = sector, cl, ch (6-7) = track
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MOV DL,BYTE PTR [DRIVE] ;get drive number from here
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MOV DH,BYTE PTR [HEAD] ;and head number from here
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INT 13H ;go read the disk
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ERROR3: JC ERROR3 ;hang in case of an error
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RET
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;Move data that doesn't change from this boot sector to the one read in at
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;DISK_BUF. That includes everything but the DRIVE ID (at offset 7DFDH) and
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;the data area at the beginning of the boot sector.
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MOVE_DATA:
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MOV SI,OFFSET DSKBASETBL ;Move all of the boot sector code after the data area
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MOV DI,OFFSET DISK_BUF + (OFFSET DSKBASETBL - OFFSET BOOTSEC)
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MOV CX,OFFSET DRIVE - OFFSET DSKBASETBL
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REP MOVSB
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MOV SI,OFFSET BOOTSEC ;Move the initial jump and the sector ID
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MOV DI,OFFSET DISK_BUF
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MOV CX,11
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REP MOVSB
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RET
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SYSFILE_1: DB 'IBMBIO COM' ;PC DOS System file
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SYSFILE_2: DB 'IO SYS' ;MS DOS System file
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ORG 7DFDH
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DRIVE: DB 0 ;Drive number, used in disk reads, etc.
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BOOT_ID: DW 0AA55H ;Boot sector ID word
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MAIN ENDS
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END START
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