A boot sector virus is a particular kind of virus named after the location it can be found.
That would be the boot sector of floppy disks or the Master Boot Record of more modern hard disks.
In some cases, they can infect the boot sector of said hard disks instead of the MBR.
The code that makes up the virus runs when whatever is on the disk or drive is booted up.
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What Do Boot Sector Viruses Do?
The downside is that they rely on DOS commands to spread.
That way, they achieve the same effect of launching early in the boot process.
Other viruses can execute from removable media, such as thumb drives, but bootkits cannot.
What Does a Boot Sector Virus Look Like?
A boot sector always has to have 0x55 and 0xAA as the last two bytes of data, respectively.
Without them there, the computer will either refuse to boot entirely or at least show an oops message.
Firstly, by its actions.
A boot sector virus infects the part of storage media loaded by the BIOS when booting up.
It also actively infects all other storage media attached to the infected computer.
Its worth remembering that modern bootkits work slightly differently and dont automatically infect devices.
The other way to identify a boot sector virus is with anti-virus software.
Note:Boot sector viruses are essentially obsolete, relying on DOS-era technology.
These operating systems likely see minimal use, particularly legacy systems.
Finding an antivirus product that can run on such an operating system would be challenging now.
More modern bootkits can be extremely hard to detect and remove as they infect areas of memory typically restricted.
Both can be defeated by reformatting the drive entirely.
This process, however, wipesalldata on the drive and so isnt ideal.
Its also theoretically possible for the bootkit to infect the motherboard itself, specifically the UEFI BIOS.
Especially if the virus could reinfect the image to which the motherboard was flashed.
The 100% surefire way to eliminate any virus is to throw away the infected component.
That is your hard drive, motherboard, etc., not necessarily the whole computer.
Conclusion
A boot sector virus is a classic key in from the DOS era.
The boot sector was the portion of the storage rig loaded first by the BIOS.
As such, the malware was immediately launched.
As they relied upon the BIOS and DOS commands, they died out when Windows was introduced.
A modern version is known as a bootkit.
It acts similarly, infecting the boot loader that calls the operating system.
