Bootloader works in emulators but not on real hardware

Question

I am writing a bootloader in assembly and it seems to work fine on qemu, bochs and virtualbox. However, it is not loading the kernel on real hardware (it seems).

The

Answer 1

If your hardware is using floppy disk emulation for the USB drive it may be possible that without a proper BIOS Parameter Block (BPB) in your MBR that it is failing to boot properly. Many BIOSes will attempt to detect a BPB at the start of the bootloader and may even update the values with proper drive geometry after the bootloader is loaded into memory. It is possible your bootloader was not being detected as a proper bootable drive or it was but the BIOS overwrote some of your code with drive geometry information before executing it.

The following adds a BPB that happens to look like a 2.88MB floppy.

.global _start
.code16
.text

_start:
    jmp     main
    .space 3 - (.-_start)

    /* Configuration for a 2.88MB floppy using FAT 12 */
    OEMname:            .ascii      "MYBOOT  "
    bytesPerSector:     .word       512
    sectPerCluster:     .byte       1
    reservedSectors:    .word       1
    numFAT:             .byte       2
    numRootDirEntries:  .word       240
    numSectors:         .word       5760
    mediaType:          .byte       0xf0
    numFATsectors:      .word       9
    sectorsPerTrack:    .word       36
    numHeads:           .word       2
    numHiddenSectors:   .long       0
    numSectorsHuge:     .long       0
    driveNum:           .byte       0
    reserved:           .byte       0x00
    signature:          .byte       0x29
    volumeID:           .long       0x54428E71
    volumeLabel:        .ascii      "NO NAME    "
    fileSysType:        .ascii      "FAT12   "

main:
    movw $0xb800, %ax
    movw %ax, %ds
    movw $0x0741, (0x0)

    xorw %ax, %ax
    movw %ax, %ds
    movw %ax, %si
    movw %ax, %es

    movw  $0x8000, %ax
    movw  %ax, %ss
    movw  $0, %sp

    movb $2, %ah
    movb $1, %al
    movw $0x02, %cx
    movb $0x00, %dh

    movw $0x5000, %bx
    movw %bx, %es
    movw $0x0, %bx
    int $0x13

    ljmpw $0x5000, $0x0000

.space 510-(.-_start)
.word 0xaa55