For a project I would like to invoke the MBR on the first harddisk directly from DOS. I've written a small assembler program that loads the MBR in memory at 0:7c00h an does a far jump to it. I've put my util on a bootable floppy. The disk (HD0, 0x80) i'm trying to boot has a TrueCrypt boot loader on it. It shows up the TrueCrypt screen, but after typing in the password it crashes the system. When I run my little utlility (w00t.com) on a normal WinXP machine it seams to crash immedealty.

Apparently I'm forgetting some crucial stuff the BIOS normally does, my guess is it's something trivial. Can someone with better bare-metal DOS and BIOS experience help me out?

Heres my code:

.MODEL tiny
.386
_TEXT SEGMENT USE16

INCLUDE BootDefs.i

ORG 100h

start:
    ; http://vxheavens.com/lib/vbw05.html
    ; Before DOS has booted the BIOS stores the amount of usable lower memory 
    ; in a word located at 0:413h in memory. We going to erase this value because
    ; we have booted dos before loading the bootsector, and dos is fat (and ugly).

    ; fake free memory  
    ;push ds
    ;push   0
    ;pop        ds
    ;mov        ax, TC_BOOT_LOADER_SEGMENT / 1024 * 16 + TC_BOOT_MEMORY_REQUIRED
    ;mov    word ptr ds:[413h], ax  ;ax = memory in K
    ;pop ds
    ;lea si, memory_patched_msg
    ;call print

    ;mov ax, cs
    mov ax, 0
    mov es, ax

    ; read first sector to es:7c00h (== cs:7c00)
    mov  dl, 80h
    mov  cl, 1
    mov  al, 1
    mov  bx, 7c00h ;load sector to es:bx
    call read_sectors

    lea si, mbr_loaded_msg
    call print

    lea si, jmp_to_mbr_msg
    call print

    ;Set BIOS default values in environment
    cli
    mov dl, 80h ;(drive C)
    xor ax, ax
    mov ds, ax
    mov es, ax
    mov ss, ax
    mov sp, 0ffffh
    sti

    push es
    push 7c00h
    retf            ;Jump to MBR code at 0:7c00h


    ; Print string
print:
    xor bx, bx
    mov ah, 0eh
    cld

@@: lodsb
    test al, al
    jz print_end

    int 10h
    jmp @B

print_end:
    ret

    ; Read sectors of the first cylinder
read_sectors:
    mov ch, 0           ; Cylinder
    mov dh, 0           ; Head
                        ; DL = drive number passed from BIOS
    mov ah, 2
    int 13h
    jnc read_ok

    lea si, disk_error_msg
    call print
read_ok:
    ret

memory_patched_msg      db 'Memory patched', 13, 10, 7, 0
mbr_loaded_msg          db 'MBR loaded', 13, 10, 7, 0
jmp_to_mbr_msg          db 'Jumping to MBR code', 13, 10, 7, 0
disk_error_msg          db 'Disk error', 13, 10, 7, 0

_TEXT ENDS
END start