Underground/Underground CD1.iso/virii/zrodla/b/boot.asm

Assembly Source File | 1998-01-14 | 13.8 KB | 248 lines

;This is a simple boot sector that will load either MS-DOS or PC-DOS. It is not

;self-reproducing, but it will be used as the foundation on which to build a

;virus into a boot sector.



;This segment is where the first operating system file (IBMBIO.COM or IO.SYS)

;will be loaded and executed from. We don't know (or care) what is there, but

;we do need the address to jump to defined in a separate segment so we can

;execute a far jump to it.

DOS_LOAD        SEGMENT AT 0070H

                ASSUME  CS:DOS_LOAD



                ORG     0



LOAD:           DB      0               ;Start of the first operating system program



DOS_LOAD        ENDS





MAIN            SEGMENT BYTE

                ASSUME  CS:MAIN,DS:MAIN,SS:NOTHING



;This jump instruction is just here so we can compile this program as a COM

;file. It is never actually executed, and never becomes a part of the boot

;sector. Only the 512 bytes after the address 7C00 in this file become part of

;the boot sector.

                ORG     100H



START:          jmp     BOOTSEC



;The following two definitions are BIOS RAM bytes which contain information

;about the number and type of disk drives in the computer. These are needed by

;the virus to decide on where to look to find drives to infect. They are not

;normally needed by an ordinary boot sector.

;

;               ORG     0410H

;

;SYSTEM_INFO:    DB      ?                       ;System info byte: Take bits 6 & 7 and add 1 to get number of

;                                                ;disk drives on this system (eg 01 = 2 drives)

;

;                ORG     0475H

;

;HD_COUNT:       DB      ?                       ;Number of hard drives in the system

;

;This area is reserved for loading the first sector of the root directory, when

;checking for the existence of system files and loading the first system file.



                ORG     0500H



DISK_BUF:       DW      ?                       ;Start of the buffer



;Here is the start of the boot sector code. This is the chunk we will take out

;of the compiled COM file and put it in the first sector on a 360K floppy disk.

;Note that this MUST be loaded onto a 360K floppy to work, because the

;parameters in the data area that follow are set up to work only with a 360K

;disk!



                ORG     7C00H



BOOTSEC:        JMP     BOOT                    ;Jump to start of boot sector code



                ORG     7C03H                   ;This is needed because the jump will get coded as 2 bytes



DOS_ID:         DB      'EZBOOT  '              ;Name of this boot sector (8 bytes)

SEC_SIZE:       DW      200H                    ;Size of a sector, in bytes

SECS_PER_CLUST: DB      02                      ;Number of sectors in a cluster

FAT_START:      DW      1                       ;Starting sector for the first File Allocation Table (FAT)

FAT_COUNT:      DB      2                       ;Number of FATs on this disk

ROOT_ENTRIES:   DW      70H                     ;Number of root directory entries

SEC_COUNT:      DW      2D0H                    ;Total number of sectors on this disk

DISK_ID:        DB      0FDH                    ;Disk type code (This is 360KB)

SECS_PER_FAT:   DW      2                       ;Number of sectors per FAT

SECS_PER_TRK:   DW      9                       ;Sectors per track for this drive

HEADS:          DW      2                       ;Number of heads (sides) on this drive

HIDDEN_SECS:    DW      0                       ;Number of hidden sectors on the disk



DSKBASETBL:

                DB      0                       ;Specify byte 1: step rate time, head unload time

                DB      0                       ;Specify byte 2: Head load time, DMA mode

                DB      0                       ;Wait time until motor turned off, in clock ticks

                DB      0                       ;Bytes per sector (0=128, 1=256, 2=512, 3=1024)

                DB      12H                     ;Last sector number (we make it large enough to handle 1.2/1.44 MB floppies)

                DB      0                       ;Gap length between sectors for r/w operations, in bytes

                DB      0                       ;Data transfer length when sector length not specified

                DB      0                       ;Gap length between sectors for format operations, in bytes

                DB      0                       ;Value stored in newly formatted sectors

                DB      1                       ;Head settle time, in milliseconds (we set it small to speed operations)

                DB      0                       ;Motor startup time, in 1/8 seconds



HEAD:           DB      0                       ;Current head to read from (scratch area used by boot sector)



;Here is the start of the boot sector code



BOOT:           CLI                                     ;interrupts off

                XOR     AX,AX                           ;prepare to set up segments

                MOV     ES,AX                           ;set ES=0

                MOV     SS,AX                           ;start stack at 0000:7C00

                MOV     SP,OFFSET BOOTSEC

                MOV     BX,1EH*4                        ;get address of disk

                LDS     SI,SS:[BX]                      ;param table in ds:si

                PUSH    DS

                PUSH    SI                              ;save that address

                PUSH    SS

                PUSH    BX                              ;and its address



                MOV     DI,OFFSET DSKBASETBL            ;and update default

                MOV     CX,11                           ;values to the table stored here

                CLD                                     ;direction flag cleared

DFLT1:          LODSB

                CMP     BYTE PTR ES:[DI],0              ;anything non-zero

                JNZ     SHORT DFLT2                     ;is not a default, so don't save it

                STOSB                                   ;else put default value in place

                JMP     SHORT DFLT3                     ;and go on to next

DFLT2:          INC     DI

DFLT3:          LOOP    DFLT1                           ;and loop until cx=0



                MOV     AL,AH                           ;set ax=0

                MOV     DS,AX                           ;set ds=0 so we can set disk tbl

                MOV     WORD PTR [BX+2],AX              ;to @DSKBASETBL (ax=0 here)

                MOV     WORD PTR [BX],OFFSET DSKBASETBL ;ok, done

                STI                                     ;now turn interrupts on

                INT     13H                             ;and reset disk drive system

ERROR1:         JC      ERROR1                          ;if an error, hang the machine



;Here we look at the first file on the disk to see if it is the first MS-DOS or

;PC-DOS system file, IO.SYS or IBMBIO.COM, respectively.

LOOK_SYS:

                MOV     AL,BYTE PTR [FAT_COUNT]         ;get fats per disk

                XOR     AH,AH

                MUL     WORD PTR [SECS_PER_FAT]         ;multiply by sectors per fat

                ADD     AX,WORD PTR [HIDDEN_SECS]       ;add hidden sectors

                ADD     AX,WORD PTR [FAT_START]         ;add starting fat sector



                PUSH    AX

                MOV     WORD PTR [DOS_ID],AX            ;root dir, save it



                MOV     AX,20H                          ;dir entry size

                MUL     WORD PTR [ROOT_ENTRIES]         ;dir size in ax

                MOV     BX,WORD PTR [SEC_SIZE]          ;sector size

                ADD     AX,BX                           ;add one sector

                DEC     AX                              ;decrement by 1

                DIV     BX                              ;ax=# sectors in root dir

                ADD     WORD PTR [DOS_ID],AX            ;DOS_ID=start of data

                MOV     BX,OFFSET DISK_BUF              ;set up disk read buffer at 0000:0500

                POP     AX

                CALL    CONVERT                         ;and go convert sequential sector number to bios data

                MOV     AL,1                            ;prepare for a disk read for 1 sector

                CALL    READ_DISK                       ;go read it



                MOV     DI,BX                           ;compare first file on disk with

                MOV     CX,11                           ;required file name

                MOV     SI,OFFSET SYSFILE_1             ;of first system file for PC DOS

                REPZ    CMPSB

                JZ      SYSTEM_THERE                    ;ok, found it, go load it



                MOV     DI,BX                           ;compare first file with

                MOV     CX,11                           ;required file name

                MOV     SI,OFFSET SYSFILE_2             ;of first system file for MS DOS

                REPZ    CMPSB

ERROR2:         JNZ     ERROR2                          ;not the same - an error, so hang the machine



;Ok, system file is there, so load it

SYSTEM_THERE:

                MOV     AX,WORD PTR [DISK_BUF+1CH]      ;get file size of IBMBIO.COM/IO.SYS

                XOR     DX,DX

                DIV     WORD PTR [SEC_SIZE]             ;and divide by sector size

                INC     AL                              ;ax=number of sectors to read

                MOV     BP,AX                           ;store that number in BP

                MOV     AX,WORD PTR [DOS_ID]            ;get sector number of start of data

                PUSH    AX

                MOV     BX,700H                         ;set disk read buffer to 0000:0700

RD_BOOT1:       MOV     AX,WORD PTR [DOS_ID]            ;and get sector to read

                CALL    CONVERT                         ;convert to bios Trk/Cyl/Sec info

                MOV     AL,1                            ;read one sector

                CALL    READ_DISK                       ;go read the disk

                SUB     BP,1                            ;subtract 1 from number of sectors to read

                JZ      DO_BOOT                         ;and quit if we're done

                ADD     WORD PTR [DOS_ID],1             ;add sectors read to sector to read

                ADD     BX,WORD PTR [SEC_SIZE]          ;and update buffer address

                JMP     RD_BOOT1                        ;then go for another





;Ok, the first system file has been read in, now transfer control to it

DO_BOOT:

                MOV     CH,BYTE PTR [DISK_ID]           ;Put drive type in ch

                MOV     DL,BYTE PTR [DRIVE]             ;Drive number in dl

                POP     BX

                JMP     FAR PTR LOAD                    ;and transfer control to the first system file





;Convert sequential sector number in ax to BIOS Track, Head, Sector information.

;Save track number in DX, sector number in CH,

CONVERT:

                XOR     DX,DX

                DIV     WORD PTR [SECS_PER_TRK]         ;divide ax by sectors per track

                INC     DL                              ;dl=sector number to start read on, al=track/head count

                MOV     CH,DL                           ;save it here

                XOR     DX,DX

                DIV     WORD PTR [HEADS]                ;divide ax by head count

                MOV     BYTE PTR [HEAD],DL              ;dl=head number, save it

                MOV     DX,AX                           ;ax=track number, save it in dx

                RET





;Read the disk for the number of sectors in al, into the buffer es:bx, using

;the track number in DX, the head number at HEAD, and the sector

;number at CH.

READ_DISK:

                MOV     AH,2                            ;read disk command

                MOV     CL,6                            ;shift possible upper 2 bits of track number to

                SHL     DH,CL                           ;the high bits in dh

                OR      DH,CH                           ;and put sector number in the low 6 bits

                MOV     CX,DX

                XCHG    CH,CL                           ;ch (0-5) = sector, cl, ch (6-7) = track

                MOV     DL,BYTE PTR [DRIVE]             ;get drive number from here

                MOV     DH,BYTE PTR [HEAD]              ;and head number from here

                INT     13H                             ;go read the disk

ERROR3:         JC      ERROR3                          ;hang in case of an error

                RET



;Move data that doesn't change from this boot sector to the one read in at

;DISK_BUF. That includes everything but the DRIVE ID (at offset 7DFDH) and

;the data area at the beginning of the boot sector.

MOVE_DATA:

                MOV     SI,OFFSET DSKBASETBL            ;Move all of the boot sector code after the data area

                MOV     DI,OFFSET DISK_BUF + (OFFSET DSKBASETBL - OFFSET BOOTSEC)

                MOV     CX,OFFSET DRIVE - OFFSET DSKBASETBL

                REP     MOVSB

                MOV     SI,OFFSET BOOTSEC               ;Move the initial jump and the sector ID

                MOV     DI,OFFSET DISK_BUF

                MOV     CX,11

                REP     MOVSB

                RET





SYSFILE_1:      DB      'IBMBIO  COM'                   ;PC DOS System file

SYSFILE_2:      DB      'IO      SYS'                   ;MS DOS System file



                ORG     7DFDH



DRIVE:          DB      0                               ;Drive number, used in disk reads, etc.

BOOT_ID:        DW      0AA55H                          ;Boot sector ID word





MAIN            ENDS





                END START