.model tiny
.code
    org 100h

start:
    jmp install

; -----------------------------------------------------------------------------
; RESIDENT PART
; -----------------------------------------------------------------------------

old_int21_vector    dd 0

new_int21_handler:
    pushf
    call dword ptr cs:old_int21_vector ; Let DOS do its thang.
return_point:
    call print_star
    iret

print_star:
    push ax
    push bx
    mov ah, 0Eh
    mov al, '*'
    mov bx, 0
    int 10h
    pop bx
    pop ax
    ret

resident_end label byte

; -----------------------------------------------------------------------------
; INSTALLATION PART
; -----------------------------------------------------------------------------
install:
    ; Get the old vector
    mov ax, 3521h
    int 21h
    mov word ptr old_int21_vector[0], bx
    mov word ptr old_int21_vector[2], es

    ; Install the new vector
    mov dx, offset new_int21_handler
    mov ax, 2521h
    int 21h

    ; Leave the program resident
    mov ax, 3100h
    mov dx, offset resident_end
    add dx, 15
    mov cl, 4
    shr dx, cl
    int 21h

end start