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I am creating an operating system, and am able to create a simple multitasking kernel (though the code has been lost) and an IDE/ATA PIO driver without BIOS interrupts. I wish to create a boot loader that puts the system into Protected Mode, disables paging, creates a flat memory model, and then does a long jump to the 0x0 offset of a file named OS.SYS in the root directory of a FAT32 formatted hard drive. I know how do do everything except for one thing: change the boot sector of the disk. I know how to format a hard drive as FAT32 to get the correct BPB and such at the beginning of the file, but I do not know how to copy over the code so that the BPB is not corrupted. Even in Linux when DD was available, I was unable to overcome this hurdle - I always used a custom file system. I am now seeing the disadvantages to this, though, such as not being able to use simple tools like file managers to move files around for testing.

Basically, to sum it up, I'm asking how to change the boot sector code on an already working FAT32 file system without messing up the BPB.

Note: I am using Windows 10 and NASM, and intend to use virtual hard disks with QEMU or Virtualbox.

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    It should be obvious. Read the BPB from the existing bootsector, update your bootsector with the BPB, write the modified bootsector. – Ross Ridge Aug 25 '16 at 20:39
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    Write a program to read an existing BPB and write that BPB into your bootloader image.If dealing with partitioned media then you may want to preserve the partition table as well. – Michael Petch Aug 25 '16 at 20:40
  • I will try writing a program to do what you suggested. So basically I'm writing a tool to copy the current boot record into a copy of the binary output of the assembler, and then write the modified boot record to the disk –  Aug 25 '16 at 20:46
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    Yes, though you only want to copy part of the current boot sector (volume boot record). – Ross Ridge Aug 25 '16 at 20:52

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