Bootloader works in emulators but not on real hard

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 bootloader starts off by writing a character to the video memory (for debugging), it then reads sector 2 off the drive and far jumps to the kernel. The kernel is then writing some characters to video memory.

On a real machine, I see the character from the bootloader on the screen, and there it hangs (blinking caret).

I have tried to set DS, ES, SI to zero, and I am also setting up a stack segment.

I am reading sector 2 off of the drive using bios int 13 function 2. I kind of suspect that it has something to do with the drive number. I have both tried to use the drive number passed to the bootloader on startup (in dl), and setting it manually to 0x0, 0x80 and 0x81.

One strange thing I noticed is that the labels I use to near jump, magically gets the correct address. Using objdump I see for example: jmp 0x2, while using gdb and qemu, it says: jmp 0x7c02. CS and all the other segment registers are zero. Whether I use -Ttext 0x0 or -Ttext 0x7c00 in the linking, the bootloader works fine on all the emulators. objdump says jmp 0x7c02 when I link with -Ttext 0x7c00.

EDIT, the bootloader looks like this:

.code16
.text
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

Edit, Second stage:

.code16
.text
    movw $0xb800, %ax
    movw %ax, %ds
    movw $0x0742, (0x2)

forever:
    jmp forever

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