Booting

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I'm confused. The 1st chapter of my A+ says that the data on the cmos chip tells the computer where to look for the operating system. That's fine, but what if the o/s isn't on the C: drive, who tells the CMOS chip where it is?

Does that make sense?

 

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Hopefully this might help explain the process.

 

im abit confused what you asking.. are you saying what tells the CMOS chip that on this drive is the operating system and thats what we booting?

PS what book you using?

 

If the OS has not yet been installed the machine will succesfully POST (Power On Self Test) but will display an error message on screen to that effect. A computer will (as long as all the hardware checks out ok, that's the purpose of the POST) boot with no OS.

To install the OS (on a new hardware build for instance) you'd need to set the boot order in the BIOS (in CMOS) so that the unit booted from CD first and then insert your bootable 2K, XP etc (yup, I know early '95 didn't boot from CD but I'm trying, and failing, to keep it simple).

Simply, the BIOS contained on the CMOS chip is the programming that allows the system to get as far as the OS and then the OS takes over the job.

Hope this helps,

J.

 

But how does the BIOS know where the O/S is?

Thanks

Lee

 

My understanding of it is that the BIOS looks for the system tracks on the hard disk, once it has located them it passes over to the HD to run up the boot loader.

 

It doesn't say "look on the C: drive" because at that point there is no C: drive. Drive letters are assigned by the OS. The BIOS simply looks at the boot sector (the first sector of a hard drive, not sure where it is on a CD-ROM) of whatever device it's cofigured to boot from first, be it a hard drive, CD-ROM or whatever. If it doesn't find an OS, it looks on the next device on the list and so on...

 

Let me see if I get this:

The Bios 1st looks in the boot sector of the hard drive, then looks for a boot file from a cd, floppy, flash memory, 2nd hard drive etc.

The order i which it searches these devices can be set in the bios.

Is this about right?

 

Yup - that's about right.

The BIOS is looking for a particular 'pattern' on each drive it checks, in the order set on one of the BIOS screens.

The first 'pattern' it finds it loads the first sector or so and hands control to that.

I say 'pattern' above, because bootable CDs behave a little differently from hard drives and floppies. And currenly it is a very rare machine that can boot from an USB pendrive.

Harry.

 

Sounds good to me.