Boot Question: CPU and BIOS

Boot Question: CPU and BIOS

Post by Roger Anderso » Sat, 03 Mar 2001 15:04:49



Hello All,

Reading about the boot process on x86 PCs. Here is my understanding of
the initial steps:

1. Power-on
2. CPU fetches and executes instruction at 0xfffffff0
3. Jump occurs to start of BIOS code

At this point, does the CPU execute the BIOS code? Does it hand control
over to the BIOS, essentially blocking until the BIOS returns? What
actually performs the work of reading the MBR, loading the boot sector?
The BIOS chip? Does the CPU execute BIOS code? What exactly is the BIOS?
Code? Chip? Is the CPU involved in the MBR read and boot sector load?
Does it wait until the BIOS returns to start executing LILO
instructions?

Hmm... No problems really. Just wanting to understand how this all
works.

- Roger

Roger Anderson

 
 
 

Boot Question: CPU and BIOS

Post by Dances With Cro » Sun, 04 Mar 2001 11:01:26


On Thu, 01 Mar 2001 22:04:49 -0800, Roger Anderson staggered into the
Black Sun and said:

Quote:>Reading about the boot process on x86 PCs. Here is my understanding of
>the initial steps:

>1. Power-on
>2. CPU fetches and executes instruction at 0xfffffff0
>3. Jump occurs to start of BIOS code

>At this point, does the CPU execute the BIOS code? Does it hand control
>over to the BIOS, essentially blocking until the BIOS returns? What
>actually performs the work of reading the MBR, loading the boot sector?

At boot time on an x86, a section of flash ROM is mmapped to FFFF:0000
(note the segment:offset notation; x86s wake up in real mode), and the
CPU begins executing that code.  The BIOS code contains a number of
motherboard-specific features, including the POST and the BIOS Setup
program.  It also contains *very* rudimentary device drivers for
supporting a keyboard and an IDE hard disk.  The BIOS contains
instructions that basically say, "Test the hardware, offer a chance to
go into the Setup program, then load the first sector from the boot
device and give control of the CPU to that code."

Quote:>The BIOS chip? Does the CPU execute BIOS code? What exactly is the BIOS?
>Code? Chip? Is the CPU involved in the MBR read and boot sector load?
>Does it wait until the BIOS returns to start executing LILO
>instructions?

BIOS = Basic Input/Output System, but has been overloaded to mean either
all the code residing in flash ROM on the motherboard, or the Setup
program residing in flash ROM which you enter by pressing {F1,Del,F2,F8,
whatever} upon boot.

If you want to know a reasonable amount about how these things work, I
suggest _Upgrading and Repairing PCs_ by Scott Mueller, ISBN
0-7897-1903-7 .  The 11th edition is a bit dated; see if a 12th edition
is out.  The book is very comprehensive (read: heavy enough to seriously
injure someone) and comes with some useful stuff on a CD-ROM.

--
Matt G|There is no Darkness in Eternity/But only Light too dim for us to see
Brainbench MVP for Linux Admin /  Workin' in a code mine, hittin' Ctrl-Alt
http://www.brainbench.com     /   Workin' in a code mine, whoops!
-----------------------------/    I hit a seg fault....

 
 
 

1. Boot Question: CPU and BIOS

Hello All,

Reading about the boot process on x86 PCs. Here is my understanding of
the initial steps:

1. Power-on
2. CPU fetches and executes instruction at 0xfffffff0
3. Jump occurs to start of BIOS code

At this point, does the CPU execute the BIOS code? Does it hand control
over to the BIOS, essentially blocking until the BIOS returns? What
actually performs the work of reading the MBR, loading the boot sector?
The BIOS chip? Does the CPU execute BIOS code? What exactly is the BIOS?
Code? Chip? Is the CPU involved in the MBR read and boot sector load?
Does it wait until the BIOS returns to start executing LILO
instructions?

No problems really. Just wanting to understand how this work exactly.
Thanks!

- Roger

Roger Anderson

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