I need an FAT12 bootloader, but I am too lazy to make one. I want to get started on the kernel.

Yeah, O/S developement is not for lazy people!

Some advice on writing your own bootloader
Don't do it. At least, don't make the bootloader the first thing you work on. Reasons:
-A bootloader is not an OS.
-A bootloader is a VERY SMALL part of an OS.
-Writing a GOOD bootloader is as much work as writing a small OS (e.g. GRUB).
-Writing a robust and flexible bootloader is difficult. (Writing a crappy and inflexible bootloader is distressingly easy.)
-Writing a bootloader requires a lot of arcane knowledge (e.g. unreal mode, A20 gate, BIOS bugs and subtleties) that is otherwise useless.
-Bootloaders are very difficult to test and debug.
-Because of the hybrid 16- and 32-bit nature of x86 CPUs, x86 boot code is ugly.
-If booting your kernel fails, you won't know if the error is in your kernel or in your bootloader.

Because bootloaders are complex and difficult to make.

-Writing a GOOD bootloader is as much work as writing a small OS (e.g. GRUB).

-Writing a bootloader requires a lot of arcane knowledge

-Because of the hybrid 16- and 32-bit nature of x86 CPUs, x86 boot code is ugly.

I am called lazy by my family, you go to work, come home and sit at your PC all night

;----------------------------------------;
; start of 32-bit area.                  ;
;     flush segments and jump to kernel  ;
;----------------------------------------;
use32
flush:
          mov     eax, 0x10
          [b]mov     ds, eax[/b]
          mov     es, eax
          mov     fs, eax    

flat assembler  version 1.64
C:\projects\BOS_boot\BOS_boot.asm [341]:
          mov     ds, eax
error: invalid size of operand.
    

Change the eax's to ax's and you should be happy as a lark.