KeblaOS

Disk Image

Created: 14^th April 2025

Disk.img image

Introduction

A disk.img is a disk image file — a complete byte-for-byte copy of a physical disk (like a hard drive, SSD, USB stick, or CD/DVD). It acts as a virtual disk, storing everything the physical disk would: partitions, boot records, filesystems, and data files.

🔧 Key Properties:

It’s usually created using tools like dd, qemu-img, or disk utilities.
It can contain bootable operating systems, file structures, or even custom partition schemes.
Can be mounted as a loop device on Linux, or opened using tools like OSFMount on Windows.

🛠 Common Uses:

Operating System Development: Used to test or install OS builds (like you’re doing in KeblaOS).
Virtual Machines: Used by QEMU, VirtualBox, etc., as a virtual hard disk.
Disk Cloning & Backup: Used to back up or restore exact disk states.
Forensics & Analysis: Used to analyze disks without touching the original.

Overview

A disk image (disk.img) is a file that emulates a physical disk, containing all the data, partition tables, boot sectors, and filesystem metadata. Disk images are widely used for emulation, virtualization, OS development, backup, and software testing.

In this project, I am creating a disk.img inside the Disk directory, which will serve as a virtual disk drive for QEMU during the development and testing of the KeblaOS operating system.

Hard Drive image

The Makefile includes a command:

make build_disk

This command creates a 512 MB FAT32-formatted disk image and mounts it on the Disk/mnt directory.

This note demonstrates various use cases and steps related to working with disk.img.

📦 1. Creating a Blank Disk Image

We create a blank disk image of 512MB filled with zeros:

dd if=/dev/zero of=Disk/disk.img bs=1M count=512

🧱 2. Partitioning the Disk with MBR (Master Boot Record)

To add a partition table and create a FAT32 partition:

parted Disk/disk.img --script -- mklabel msdos
parted Disk/disk.img --script -- mkpart primary fat32 1MiB 100%

The 1MiB offset is to align the partition properly for performance and compatibility.

🔍 3. Check if `disk.img` Is Mounted

mount | grep disk.img

This command helps you verify if the image is already mounted.

🔄 4. Check Loop Device Associations

losetup -l

Output might look like:

NAME       SIZELIMIT OFFSET AUTOCLEAR RO BACK-FILE
/dev/loop0         0      0         0  0 /path/to/KeblaOS/Disk/disk.img

📌 5. Check Mounted Partition from Image

lsblk

Output (simplified):

loop0       7:0    0   512M  0 loop
️└─loop0p1 259:0    0   511M  0 part /mnt/disk

This tells us /dev/loop0p1 is the partition we want to mount.

🕵️ 6. Check Which Processes Are Using `disk.img`

sudo lsof | grep disk.img

Helpful when umount fails due to active use.

❌ 7. Unmount Disk Image (If Mounted)

sudo umount Disk/mnt || true

Use || true to avoid script failure if it’s already unmounted.

🔓 8. Detach the Loop Device

sudo losetup -d /dev/loop0 || true

This releases the loop device from the image file.

🛠️ 9. Mounting the Partition from `disk.img`

After partitioning, attach and mount it:

sudo losetup -fP Disk/disk.img
sudo mount /dev/loop0p1 Disk/mnt

📁 10. Browsing Contents Inside `disk.img`

Once mounted, you can directly browse or manipulate files inside Disk/mnt just like a normal filesystem.

1. Check the partitions inside disk.img
```
fdisk -l disk.img
```
  This will show you if disk.img contains partitions and where they start (sector offset).
Mount a specific partition inside disk.img Suppose you found a partition starting at sector 2048. To mount it:
```
sudo mount -o loop,offset=$((2048 * 512)) disk.img /mnt
```
Here, 2048 is the sector number and 512 is the sector size in bytes.
If it’s a raw filesystem (no partition table)
```
sudo mount -o loop disk.img /mnt
```

Use losetup (optional, advanced)

sudo losetup -fP disk.img   # Associate loop devices
lsblk                       # Check loop devices
sudo mount /dev/loopXpY /mnt

🪟 On Windows, or with GUI tools: Use OSFMount

OSFMount Screenshot

Free tool to mount .img files as a drive

Allows mounting individual partitions or whole image

Great for viewing FAT, NTFS, ext2/3 (with limitations)

🥪 11. Using `disk.img` with QEMU

You can use this image as a virtual hard disk:

qemu-system-x86_64 -drive file=Disk/disk.img,format=raw

✅ Summary

Disk images are essential for OS development. They simulate real disk environments, allowing for:

Bootloader installation
Filesystem formatting
Kernel deployment
Testing OS features without needing physical hardware

With tools like dd, parted, losetup, and mount, disk.img becomes a powerful part of your system programming toolkit.

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