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I am trying to develop an operating system. I have finished the Barebones tutorial from OSDev but still have some issues with multiboot.

As I understand, when I create a multiboot image following the tutorial, I have an ELF format binary file. This ELF executable is loaded into memory by QEMU when I specify the -kernel option. So I have a single executable program in memory and it has an entry point. It has some functions and libraries alongside it. There is nothing else in the memory. There is also no floppy or disk image from which I can read or write sectors. This is mainly because I haven't specified anything to QEMU in that regard.

However, what I want to achieve is after QEMU loads my ELF executable, I want to have a disk or floppy architecture from which I can read or write sectors. I also want to load some user programs from it or maybe set up a file system on it. So far, what I'm think is this:

qemu-system-i386 -kernel myKernel.bin -fda myDisk.image

Or maybe, I should take my myKernel.bin file, pad it with zeros until it is aligned to 512 bytes, then append the rest of the disk image to it.

My question is how can I achieve this functionality with QEMU. I need to know whether a task like this is possible before diving into writing a disk driver. If not possible, I can take a different approach but these are the ideas I have right now. I have already looked into -initrd and it isn't quite what I want. I also don't want to have any files loaded into the RAM by QEMU. Instead, I want to manipulate the disk image myself. Any suggestions on how to do that?

Max Leske
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    When you create a multiboot compliant bootloader and use the `-kernel` option a side effect is that you are placed into protected mode. You no longer have direct access to the BIOS. You can still access the [floppy disk controller](http://wiki.osdev.org/Floppy_Disk_Controller) directly (you effectively write a device driver). Alternatively you can switch back to real mode use Floppy Disk BIOS interrupts load data into memory and then jump back into protected mode. But in both cases you mount a virtual floppy disk with a QEMU command with -kernel and `-fda` as you have suggested. – Michael Petch May 21 '16 at 14:18

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You don't need both a floppy image and a multiboot kernel, unless you've written some kind of loader. The -kernel flag works similarly to grub and it's much faster than loading from virtual media. This is how many linux kernel and driver devs test their code rather than creating a virtual media image every time.