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I am reading a book, learning about how booting in computer works. One of the snippets in book mentions about how firmware proceeds with boot process while booting a computer.

Some context on what i am reading :

BIOS The BIOS boot process begins with the computer reading a boot sector (typically the first sector) from a disk and then executing that code. Thus, boot options for BIOS-based computers are limited; you can only select the order in which various boot devices (hard disks, optical disks, USB devices, network boot, and so on) are examined to find a boot sector.

It later mentions about various issues associated with removable disks used for boot sequences (check below)

Although boot sequences involving removable disks are common, they have their problems. For instance, if somebody accidentally leaves a CD or DVD in the drive, this can prevent the system from booting.

The book does not elaborate on why keeping CD/DVD in the drive cause the system from booting. Can someone throw some light on this topic ?

Thanks.

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    You're on the wrong site, I'm afraid. This is Electrical Engineering and primarily concerned with electronics design and your question is all about software so it should probably be on StackOverflow. (Most systems will check the CD drive and if non-bootable media, such as a music CD, is found will try the next device listed in the BIOS boot order. I suspect the book is not correct.) – Transistor May 26 '18 at 8:59
  • Apologies. I am new. Would take care going forward. Thanks for info. – auto tech May 26 '18 at 9:09
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The book does not elaborate on why keeping CD/DVD in the drive cause the system from booting.

Let's say you've set the boot sequence to boot first from CD, as you're trying to fix a problem, and using a rescue disk. You boot off that sucessfully, and fix the problem. You take out the rescue CD, and restart the system.

Now the BIOS cannot find a CD in the slot, so it looks in the next place, your hard drive, and boots from there. All fine, and you run like this for ages, forgetting that your boot sequence is still set to CD first.

Now you play an audio CD in the drive, forget it's there, and switch off. Next time you come to boot, it will find a CD in the drive, try to boot from it, unsuccessfully. Depending on how smart the BIOS is, it may realise it's not valid and go on to the next device.

This is the best case scenario. The BIOS may hang, and eventually you'll realise the problem, remove the CD, and change the boot sequence back to HD first.

Unfortunately, there is a worse scenario. Some 'audio' CDs, and some freeware CDs, are maliciously bootable, a virus infection waiting for a forgetful/careless user.

  • Hahaha that hilarious and answers my question. Thanks. – auto tech May 26 '18 at 9:05
  • failing to boot from cd won't check next devices on boot list ? – johnger May 26 '18 at 9:06
  • @Neil_UK yeah thats a good question ? what will happen in such case ? – auto tech May 26 '18 at 9:08
  • @autotech that depends on who wrote the bios : either "if error stop" or "if error next" etc... – Solar Mike May 26 '18 at 11:23

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