use16
      org 0x7c00
        ; ax bx cx dx -> 16bit general purpose registers
        ; mov ax, 0x12
        ; mov bx, ax
        ; mov cx, 0x1234 ; mov 16bit value 0x1234 into 16bit register cx
        ; mov dl, 0x12   ; mov 8bit value 0x12 into the low 8bit register dl of dx
        ; mov dh, 0x34

entry:

        mov ah, 0x0e     ; bios scrolling teletype
        mov al, [my_character1]

        int 0x10         ; cpu enters into an interrupt state, jumps to this interrupt handler in the IVT in memory setup by bios

        mov al, [my_character2]
        int 0x10

        my_character1:

                db 'X'

        my_character2:

                db 'Y'
        jmp $            ; endless jump
        times 510 - ($-$$)db 0
        dw 0xaa55 ;16bit word or 2 bytes - disc signature
    

entry directive sets the entry point for MZ executable, it should be followed by the far address (name of segment, colon and the offset inside segment) of desired entry point.