I created a simple bootloader that reads a value in memory and keeps running only if that value is the expected one.

For some reason, when the OS boots on QEMU x86_64 this code works (it jumps to keep_running) but on VirtualBox and on physical hardware it doesn’t. After doing some debugging I found that the problem was with memory addressing, but I don’t know why it occurs and how to solve this problem.

; BOOT SECTOR
bits 16
[org 0x7C00]

call disk_read

jmp extended_space_start

disk_read:              ; disk_read(al > Number of sectors, dl > Drive number, es:bx > Address)
    push ax             ; Push the number of sectors for checking later
    mov ah, 0x02        ; Disk read operation
    mov cl, 0x02        ; Sector (0x01 = BOOT)
    mov ch, 0x00        ; Cylinder
    mov dh, 0x00        ; Head
;   mov dl, 0x00        ; Drive number (may be set by the BIOS) 0x00: 1st Floppy, 0x01: 2nd Floppy, 0x80: 1st Hard Disk Drive, 0x81: 2nd Hard Disk Drive
    int 0x13
    jc .disk_error      ; If any error happens the Carry is set to 1
    pop bx
    cmp al, bl
    jne .sector_error
    ret
.disk_error:
.sector_error:
    jmp $

test: db 0xFF

times 510-($-$$) db 0
db 0x55, 0xAA

extended_space_start:
cmp byte [test], 0xFF
je keep_running
jmp $

keep_running:
    mov ax, 0xB800
    mov es, ax

    mov ax, 0x0000
    mov di, ax
    mov byte [es:di], 'X'
    jmp $

QEMU reads 0xFF but VBox reads 0x00.
Why does the bootloader work on QEMU but not on VirtualBox / physical hardware?