mov ah, 0x0e

mov al, [the_secret]
int 0x10 ; shouldnt work

mov bx, 0x7c0 ; segment is automatically << 4
mov ds, bx ; set data segment offset to 0x7c0
; all memory references will be offset by ds implicitly
mov al, [the_secret]
int 0x10

mov al, [es:the_secret]
int 0x10

mov bx, 0x7c0
mov es, bx
mov al, [es:the_secret]
int 0x10

jmp $

the_secret:
    db "X"

times 510 - ($-$$) db 0
dw 0xaa55