install bootloader on usb zero fat sectors

INSTALL BOOTLOADER ON USB ZERO FAT SECTORS INSTALL
INSTALL BOOTLOADER ON USB ZERO FAT SECTORS CODE
INSTALL BOOTLOADER ON USB ZERO FAT SECTORS WINDOWS

The partition boot sector would be none the wiser that there already was a bootsector loaded before, and could actually load yet another bootsector - which is why it's called chainloading. Then, your (relocated) MBR bootsector would load that partition boot sector to 0x0000:0x7c00, and jump there. Now, the thing your MBR bootsector can do is to relocate itself (copying from 0x0000:0x7c00 to, traditionally, 0x0060:0x0000), parse the partition table, display some kind of menu and let the user choose which partition to boot from. LILO or GRUB to the partition boot sector instead of the MBR is also an option.

INSTALL BOOTLOADER ON USB ZERO FAT SECTORS WINDOWS

Windows stores something akin to a second-stage bootloader in the boot sector of the partition it was installed in. The easiest way to boot another OS is a mechanism called chainloading. This can be achieved by inserting a '512-bytes' break in the ASM code, making sure the rest of the loader is put after the bootsector. GRUB uses two (or arguably, three) stages.Īnother way to avoid the 512-byte barrier is to split the bootloader in two parts, where the first half (512 bytes) can load the rest.

INSTALL BOOTLOADER ON USB ZERO FAT SECTORS CODE

The second one can then contain all the code needed for loading the kernel. The first being small with the sole purpose of loading the second one. Also, a lot of this size may be dedicated to bios structures and FAT headers, which leaves even less space to work withĪ two-stage bootloader actually consists of two bootloaders after each other. However, on the x86 you are usually limited to 512 bytes for a first stage (An exception is no-emulation El-Torito), which is not much. This image then performs the steps described above to start the kernel. Virtually any bootloader follows a common design.Ī single stage bootloader consists of a single file that is loaded entirely by the BIOS. Keep in mind that debugging paging initialization code without the help of exception handlers may quickly become a nightmare! Bootloader design Note: take time to think about whether or not you'll enable paging here.

It's common for the loader to keep interrupts disabled (the kernel will enable them later when an IDT is properly set up). In general, anything that involves a BIOS call is easier to do in Real Mode, so better do them while in real mode than trying to come back to real mode for a trip later. Other popular queries regard video modes. Pretty useful information to have is a map of the address space - effectively a map of where physical memory is and where it's not. For example, you'll need to tell Linux the root partition to start from. Some kernels require some extra information to run. Same goes for 'microkernel services' like disk/files/memory services, etc. If your kernel is modular and if Filesystems are understood by some modules, you need to load the modules along with the kernel. Linux, for instance, requires an additional 'initrd' file that will contain the 'initialization process' (as user level). What needs to be loaded mainly depends on what's in your kernel. The 'advantage' of being fully-FAT is that you don't need to re-write the bootsector every time you rewrite the kernel. The field reserved sectors in the BPB is a perfect place for this. Maybe the one I'd choose myself would be to reserve enough space on a FAT file system to store the list of sectors used by the kernel file.

INSTALL BOOTLOADER ON USB ZERO FAT SECTORS INSTALL

Question is: where on the disk? Is it a regular file on a FAT-formatted partition? Is it a collection of consecutive sectors in the 'reserved area' of the FAT file system (in which case you may need a dedicated tool to format the disk and install the kernel on it)? Or is the disk / partition simply left unformatted and the kernel pasted directly with a disk image tool?Īll the above options are possible. The bits of your kernel are somewhere on some disk (presumably the booting disk, but this is not mandatory). The boot loader will collect this information and present it in a way the kernel will be able to understand. Therefore it's a good place to perform memory map detection, detection of available video modes, loading of additional files, etc. Consequently it has easy access to BIOS resources and functions. On the x86, the boot loader runs in Real Mode.

Switch to an environment that the kernel will like.

Provide the kernel with the information it needs to work correctly.

Bring the kernel (and all the kernel needs to bootstrap) into memory.

Install bootloader on usb zero fat sectors