How to set the Stack segment
bootloader_start:

    mov ax, 07C0h           ; Set up 4K of stack space above buffer

    add ax, 544         ; 8k buffer = 512 paragraphs + 32 paragraphs

; NASM syntax
xor ax, ax ; or set ax to whatever segment you wish
mov ss, ax ; IIRC, you can't just go mov ss, 0
mov sp, 0xF000 ; or set sp to whatever you wish