OSDev.org

Posted: **Wed Dec 02, 2020 8:40 am**

Up to this point, I have been using an ISO produced by grub-mkrescue, and booting from CDROM. But now I am trying to start exploring file system concepts / implementation, and as part of that I wanted to toy around with some FAT filesystem stuff. To that end, I am moving to working off of a disk image.

I followed the Wiki article for producing a disk image (https://wiki.osdev.org/GRUB_2#Disk_image_instructions), but I am having some trouble getting bochs to load the img file; it gets an IDE time out.
I looked at the hex makeup of the img file, and the MBR code is empty after installing grub; the partition entry exists from fdisk operations and it is bootable with value 0x80, but no actual bootloader code -- bytes 0-446 are all zeroed. This seems, uh, problematic -- I assume this is the root of this issue, but I didn't know if maybe that gets filled in at some point by some process I'm not aware of. Am I just missing a step? I should note that grub-install successfully installs a bunch of stuff under /boot/grub and /boot/efi.

bochsrc disk entry (from bximage)

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ata0-master: type=disk, mode=flat, path=myos_disk.img

------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

For completeness, here are the exact steps I took:

Create disk image:

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$ dd if=/dev/zero of=myos_disk.img bs=512 count=131072

Format myos_disk.img:

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$ fdisk myos_disk.img
Command (m for help): n
Partition type
   p   primary (0 primary, 0 extended, 4 free)
   e   extended (container for logical partitions)
Select (default p): p
Partition number (1-4, default 1): 1
First sector (2048-131071, default 2048): 
Last sector, +/-sectors or +/-size{K,M,G,T,P} (2048-131071, default 131071): 

Created a new partition 1 of type 'Linux' and of size 63 MiB.

Command (m for help): a
Selected partition 1
The bootable flag on partition 1 is enabled now.

Command (m for help): w
The partition table has been altered. Syncing disks.

Setup disk image on 2 loopback devices -- one for setting up GRUB and the MBR, the other for setting up the OS’s filesystem.

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$ sudo losetup /dev/loop0 myos_disk.img
$ sudo losetup /dev/loop1 myos_disk.img -o 1048576

Format the file system partition

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$ sudo mkdosfs -F32 -f 2 /dev/loop1

Mount the newly formatted partition:

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$ sudo mount /dev/loop1 /mnt

Install GRUB on the first device -- NOT THE OS’s formatted partition

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$ sudo grub-install --root-directory=/mnt --no-floppy --modules="normal part_msdos ext2 multiboot" /dev/loop0
Installing for x86_64-efi platform.
Installation finished. No error reported.
$ sync

Copy kernel, GRUB config, and any other desired files to the filesystem

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$ sudo cp isodir/boot/myos.bin /mnt/boot/myos.bin
$ sudo cp isodir/boot/grub/grub.cfg /mnt/boot/grub/grub.cfg

Unmount the partition, and remove the loopback devices

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$ sudo umount /mnt
$ losetup -d /dev/loop0; losetup -d /dev/loop1

Run bochs, which has the disk entry and a boot order starting with disk

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bochs -q

Thanks in advance for any help or suggestions. I have been looking through the forums/wiki grub docs but have not been able to really find anything

Posted: **Wed Dec 02, 2020 9:11 am**

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Installing for x86_64-efi platform.

Your host OS was booted from 64-bit UEFI, and grub-install thinks that's what you want. Add "--target=i386-pc" to your grub-install command to get the BIOS version, or boot Bochs in UEFI mode.

Posted: **Wed Dec 02, 2020 9:21 am**

Argghh, glossed right over that, of course it's a mistake like that -- I might make a note of the flag in the wiki page if that's okay.
Thanks so much for pointing it out, that fixed it. Also thank you for the link, that is nice to know.

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GRUB, MBR, and Booting From Disk

GRUB, MBR, and Booting From Disk

Re: GRUB, MBR, and Booting From Disk

Re: GRUB, MBR, and Booting From Disk