Boot media refers to the physical storage device that contains the operating system files and data needed for a computer to boot up and load the operating system. This media allows the computer to start itself and load the OS without needing the OS files already installed on a storage drive.
What are the different types of boot media?
There are several common types of boot media used in computers:
- CD/DVD – Optical discs like CDs and DVDs can contain bootable operating system images. A CD/DVD has to be bootable, meaning it is formatted with boot files.
- USB flash drive – USB drives can be formatted with a bootable OS image and used to boot a computer.
- External hard drives – External HDDs connect via USB or eSATA and can boot computers if properly formatted.
- SD cards – SD cards used in devices like cameras can be set as bootable media.
- Network PXE boot – Network cards that support PXE allow booting from a network server.
- BIOS/UEFI boot menu – The BIOS itself has a boot selection menu that can call OS loaders.
Additionally, the following older media types can also be used as boot media:
- Floppy disk
- ZIP disk
- Jaz drive
- LS-120/240 SuperDisk
Why is boot media required to start a computer?
Boot media is required because when a computer first turns on, there is no operating system loaded into memory. The CPU does not know how to load the OS files from a hard drive or SSD yet. Boot media contains initial boot files like bootloaders and kernel images to get the OS startup process going.
The boot media starts this process by loading just enough executable code into RAM to access the proper boot device. This is known as bootstrapping. From there, further OS startup files stored on the boot device are loaded. The boot media ultimately allows the full OS to be loaded from its main storage location.
Without this external media, the computer would not know how to start loading system files and would not startup properly. Boot media contains the mandatory initial instructions to allow the CPU to access the OS.
What are the steps used during the boot process?
Here is an overview of the common steps in the boot process that boot media facilitates:
- BIOS/firmware initialization – The BIOS firmware initializes hardware components like RAM, devices, buses, interrupts, etc.
- POST testing – The Power-On Self-Test sequence checks hardware is working.
- Boot device selection – A boot device is selected from BIOS settings.
- Master boot record executed – The MBR code stored on boot media is loaded and run.
- Boot loader executed – The boot loader like GRUB is loaded into memory from media.
- Kernel initialization – The kernel is loaded into memory and initialized.
- Startup processes begin – Systemd and other init processes launch.
- Login prompts – User login prompts are presented.
As you can see, the boot media starts the loading of the MBR boot code and boot loader. These then continue the booting steps by loading the kernel from storage. The kernel initializes and starts core system processes. Boot media starts this whole sequence.
What goes on bootable media?
For media like a CD or USB drive to be bootable and start the boot process, it needs the following contents:
- Boot loader – Such as GRUB, LILO, or Windows bootmgr.
- OS kernel – The core executable kernel file like linux or ntldr.
- Initial RAM disk – Temporary files needed before mounting file systems.
- Device drivers – Drivers for media, storage, input to allow accessing OS image.
- Boot configuration – Boot settings, preferences, recovery tools.
The bootable media needs boot instructions, kernels, and OS files tailored for the boot process. The media is formatted and written with special boot sector data to make it bootable. Typically a disk image file or ISO contains all theboot files and is written to the boot media.
How is boot media created?
Boot media is created a few different ways depending on the type of media:
- CD/DVD – An ISO file or optical disc image with boot data is written to the CD/DVD.
- USB – Tools like Rufus, UNetbootin or dd write a bootable image to a flash drive.
- External drives – Imaging tools copy an OS install disc image.
- PXE – A network boot server provides boot images to PXE clients.
Many Linux distributions offer tools to write their live CD/USB images. On Windows, tools like Rufus can write bootable media. Specific bootable ISO files are written to various media in order to make that media bootable when used to startup a computer.
Why is network PXE boot used?
PXE network booting allows computers to startup and load an OS from a server on the local network. This is useful in the following cases:
- Diskless workstations – No hard drive needed locally.
- Thin clients – Lower hardware requirements.
- Operating system deployment – Loading images during installs.
- Recovery – Booting troubled systems from the network.
Enterprise environments make heavy use of PXE booting for scalable OS provisioning and recovery. It eliminates the need for physical media at each machine.
In summary, boot media like CDs, USB drives or PXE network booting provide the initial boot instructions that computers require to load their operating systems. Without boot media, the computer would not know how to startup and locate the rest of the OS files. Boot media starts the chain of loading additional boot components like bootloaders and kernels until the OS can fully load.