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-   -   Why MBR is limited to 512 bytes? (http://www.linuxquestions.org/questions/linux-newbie-8/why-mbr-is-limited-to-512-bytes-4175437937/)

RaviTezu 11-20-2012 05:35 AM

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. :)

acid_kewpie 11-20-2012 05:48 AM

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

druuna 11-20-2012 05:49 AM

In more detail: https://en.wikipedia.org/wiki/Master_boot_record

pixellany 11-20-2012 06:27 AM

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.

RaviTezu 11-20-2012 06:49 AM

:) 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

acid_kewpie 11-20-2012 06:51 AM

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.

RaviTezu 11-20-2012 06:58 AM

acid_kewpie, sorry...

& thanks..

pixellany 11-20-2012 07:53 AM

Quote:

Originally Posted by RaviTezu (Post 4833218)
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.....;) )

RaviTezu 11-21-2012 07:47 AM

Thank you very munch. pixellany


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