LinuxQuestions.org
Visit Jeremy's Blog.
Go Back   LinuxQuestions.org > Forums > Linux Forums > Linux - Newbie
User Name
Password
Linux - Newbie This Linux forum is for members that are new to Linux.
Just starting out and have a question? If it is not in the man pages or the how-to's this is the place!

Notices



Reply
 
Search this Thread
Old 11-20-2012, 06:35 AM   #1
RaviTezu
Member
 
Registered: Nov 2012
Location: India
Distribution: Fedora, CentOs, RHEL
Posts: 164

Rep: Reputation: 24
Why MBR is limited to 512 bytes?


446+64+2=512(MBR)

446bytes for boot loader(commonly grub for most linux systems)

64bytes for partition table informtaion

2bytes for magic number.

i got the above information by some investigation on MBR :P

My Questions:
1.Why MBR is limited to 512bytes?

2.Why 446 byes is allotted to boot loader?(i know this contains the information for the next stages to boot the system)

3.How the system make use of these 2bytes??

Please dont think i'm rude in asking questions like this.
Please answer me..as it may help other people in this site as well. :P

Thanks a ton in advance,
Ravi



Note: Got some more questions.will ask you once my mind has a clear pic on these questions first.
 
Old 11-20-2012, 06:48 AM   #2
acid_kewpie
Moderator
 
Registered: Jun 2001
Location: UK
Distribution: Gentoo, RHEL, Fedora, Centos
Posts: 43,415

Rep: Reputation: 1968Reputation: 1968Reputation: 1968Reputation: 1968Reputation: 1968Reputation: 1968Reputation: 1968Reputation: 1968Reputation: 1968Reputation: 1968Reputation: 1968
It's basically historical. That was all that was felt was required way back 25 or so years ago. That was a LOT of space back then, and without a new standard blowing it away, like GUID partition tables, it can't just be changed trivially. Check the wikipedia page (or a millions google hits) for a lot more discussion of this

Last edited by acid_kewpie; 11-20-2012 at 06:51 AM.
 
Old 11-20-2012, 06:49 AM   #3
druuna
LQ Veteran
 
Registered: Sep 2003
Posts: 10,532
Blog Entries: 7

Rep: Reputation: 2374Reputation: 2374Reputation: 2374Reputation: 2374Reputation: 2374Reputation: 2374Reputation: 2374Reputation: 2374Reputation: 2374Reputation: 2374Reputation: 2374
In more detail: https://en.wikipedia.org/wiki/Master_boot_record
 
Old 11-20-2012, 07:27 AM   #4
pixellany
LQ Veteran
 
Registered: Nov 2005
Location: Annapolis, MD
Distribution: Arch/XFCE
Posts: 17,802

Rep: Reputation: 729Reputation: 729Reputation: 729Reputation: 729Reputation: 729Reputation: 729Reputation: 729
You might also want to know why the partition table has entries for cylinder, head, and sector**---or why the tradition system allows only 4 "primary" partitions, one of which can be an "extended" partition. Or why the extended partition is commonly described as a container for the "logical" partitions--even though is is really just a **pointer** to the first logical partition (which in turn has another **extended** partition to point to the next logical---etc.)

The answer to all of this: "That's the way it was designed." As already suggested, the design was driven by resources--in this case, space on the hard disk*. The history of technology is full of cases where something was designed to meet the constraints of the time, the original reasons were forgotten by most, and the design continued to morph and evolve over many years. If we were to start with clean sheet of paper in 2012, would there ever be an internal combustion engine? ----Probably not.




*I remember when getting a 10 MEGAbyte hard drive for the original MAC was a really big deal

**Warning: trying to relate the CHS notation to a simple offset on the drive or partition can cause temporary insanity or even brain damage.
 
Old 11-20-2012, 07:49 AM   #5
RaviTezu
Member
 
Registered: Nov 2012
Location: India
Distribution: Fedora, CentOs, RHEL
Posts: 164

Original Poster
Rep: Reputation: 24
:) Thanks for the replies!

3. Can any one tell me the use of last 2bytes in MBR?
&
4. I need the difference between a volume and a partition?
In windows i guess...they just differ in physical & logical notations..as volumes are represented with C:,D:,..etc.

How about linux???

Thanks,
ravi
 
Old 11-20-2012, 07:51 AM   #6
acid_kewpie
Moderator
 
Registered: Jun 2001
Location: UK
Distribution: Gentoo, RHEL, Fedora, Centos
Posts: 43,415

Rep: Reputation: 1968Reputation: 1968Reputation: 1968Reputation: 1968Reputation: 1968Reputation: 1968Reputation: 1968Reputation: 1968Reputation: 1968Reputation: 1968Reputation: 1968
Dude, you really can just google these... You don't seem to have tried to find this information at all.

*I* just found out from searching that the 2 bytes helps indentify bigendian and little endian systems, and identifies the end of the boot sector.

Last edited by acid_kewpie; 11-20-2012 at 07:52 AM.
 
Old 11-20-2012, 07:58 AM   #7
RaviTezu
Member
 
Registered: Nov 2012
Location: India
Distribution: Fedora, CentOs, RHEL
Posts: 164

Original Poster
Rep: Reputation: 24
acid_kewpie, sorry...

& thanks..
 
Old 11-20-2012, 08:53 AM   #8
pixellany
LQ Veteran
 
Registered: Nov 2005
Location: Annapolis, MD
Distribution: Arch/XFCE
Posts: 17,802

Rep: Reputation: 729Reputation: 729Reputation: 729Reputation: 729Reputation: 729Reputation: 729Reputation: 729
Quote:
Originally Posted by RaviTezu View Post
3. Can any one tell me the use of last 2bytes in MBR?
&
4. I need the difference between a volume and a partition?
In windows i guess...they just differ in physical & logical notations..as volumes are represented with C:,D:,..etc.

How about linux???

Thanks,
ravi
All of these terms tend to get used ambiguously....

Note, for example, that a partition cannot be mounted or otherwise used by the system unless it first has a filesystem on it. When we speak of "mounting" a partition, volume, drive, whatever----it really means we are mounting (AKA connecting) the filesystem contained on it.

In Unix and Linux, the correct conventions are used---In Windows, they tend to be incorrect. (They even have the slash backwards in pathnames..... )
 
Old 11-21-2012, 08:47 AM   #9
RaviTezu
Member
 
Registered: Nov 2012
Location: India
Distribution: Fedora, CentOs, RHEL
Posts: 164

Original Poster
Rep: Reputation: 24
Thank you very munch. pixellany
 
  


Reply

Tags
mbr


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is Off
HTML code is Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Question: Does the MBR still occupy the first 512 KB on new computers? crowlpa Linux - Newbie 2 03-05-2012 04:50 PM
Which program can boot a copy of the MBR (fake MBR) using mbr.bin Xeratul Linux - Software 6 12-04-2010 12:40 AM
UDP client / server program in c++ suddenly limited to 1468 bytes bricedebrignaisplage Linux - Networking 5 07-25-2008 01:13 AM
Restoring MBR :partitions are detected only after the reboot after MBR restoration? sharathksin Linux - Newbie 2 04-14-2006 08:48 AM
TX bytes vs. httpd bytes ovrload Linux - Networking 3 10-12-2005 05:19 PM


All times are GMT -5. The time now is 10:24 PM.

Main Menu
Advertisement
My LQ
Write for LQ
LinuxQuestions.org is looking for people interested in writing Editorials, Articles, Reviews, and more. If you'd like to contribute content, let us know.
Main Menu
Syndicate
RSS1  Latest Threads
RSS1  LQ News
Twitter: @linuxquestions
identi.ca: @linuxquestions
Facebook: linuxquestions Google+: linuxquestions
Open Source Consulting | Domain Registration