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Creating a tiny x86 bootloader

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#1 Nuno Brito

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Posted 28 December 2016 - 09:58 PM

Good tutorial, explains in a really simply manner how to write the initial instructions of a boot loader.





We'll eschew the discussion of the intricacies of how the BIOS works, but here's what you need to know. When you turn your computer on, the processor immediately looks at physical address 0xFFFFFFF0 for the BIOS code, which is generally on some read-only piece of ROM somewhere in your computer. The BIOS then POSTs, and searches for acceptable boot media. The BIOS accepts some medium as an acceptable boot device if its boot sector, the first 512 bytes of the disk are readable and end in the exact bytes 0x55AA, which constitutes the boot signature for the medium. If the BIOS deems some drive bootable, then it loads the first 512 bytes of the drive into memory address 0x007C00, and transfers program control to this address with a jump instruction to the processor.







#2 AnonVendetta


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Posted 28 December 2016 - 11:58 PM

You'd better seriously sharpen your coding skills. Creating your own boot loader probably requires a set of skills similar to what you would need to write an OS from scratch.

#3 Wonko the Sane

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Posted 29 December 2016 - 08:47 AM

As said elsewhere that is NOT actually a bootloader.


It is a bootload, something that is loaded at boot and executes some code, namely is some very basic bootsector code.


A bootloader is loaded at boot and then it loads a payload (like some sectors on device, a second stage loader or an OS).


That - though nice -  is another "Hello World" thingy like there are many and - in the good ol' times - there were tens of similar bootsector codes used on non-system floppies to display messages or ASCII art or as "demo".


Similar bootsectors, often improperly called "bootloader":









And a previous post here:


All of these are nice :), introductory experiments in i386 assembler and BIOS interrupts, of course, but if anyone wants to learn how to write some actual MBR or bootsectors bootloader code he/she better study the source code of Grub, grub4dos, Syslinux, MakebootFat or mbldr.




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