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SamyPesse / How-to-Make-a-Computer-Operating-System

How to Make a Computer Operating System in C++
https://samypesse.gitbook.io/how-to-create-an-operating-system
Apache License 2.0
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Explain losetup offset in diskimage.sh #2

Open akkartik opened 10 years ago

akkartik commented 10 years ago

Is it to make room for the grub MBR?

SamyPesse commented 10 years ago

No it's not, it's the offset of the first partition: https://github.com/SamyPesse/How-to-Make-a-Computer-Operating-System/blob/master/Chapter-3/README.md#create-a-disk-image-for-our-kernel-and-grub

akkartik commented 10 years ago

Thanks. But I'm still confused: why isn't the offset 0? Why do we have to use the computation in your link?

I also am trying to go over the computation at your link. Locally, the output of fdisk -l -u c.img:

Disk sdk/c.img: 2 MB, 2097152 bytes
1 heads, 5 sectors/track, 819 cylinders, total 4096 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x9cbdff7d

    Device Boot      Start         End      Blocks   Id  System
sdk/c.img1   *           1           4           2   83  Linux

On the VM:

You must set cylinders.
You can do this from the extra functions menu.

Disk c.img: 0 MB, 0 bytes
1 heads, 5 sectors/track, 0 cylinders, total 0 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x9cbdff7d

Device Boot      Start         End      Blocks   Id  System
c.img1   *           1           4           2   83  Linux

I see where these show the size of each sector, but not where the 63 comes from. From http://en.wikipedia.org/wiki/Cylinder-head-sector#Sectors I learn that PCs usually have 63 sectors per track. Is that it?

SamyPesse commented 10 years ago

Did you change the disk geometry for your c.img1? or do you use the same geometry of c.img?

The offset is not 0 because the partition starts after some boot sectors.

akkartik commented 10 years ago

I didn't mean to change the geometry, I think I used the same geometry. And oddly enough the one with the 'must set cylinders' message is what shows hello correctly.

SamyPesse commented 10 years ago

Hum, in vagrant I have:

vagrant@lucid32:/vagrant/sdk$ fdisk -l -u c.img
You must set cylinders.
You can do this from the extra functions menu.

Disk c.img: 0 MB, 0 bytes
16 heads, 63 sectors/track, 0 cylinders, total 0 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0xe8c36f7f

Device Boot      Start         End      Blocks   Id  System
c.img1   *          63        4031        1984+  83  Linux

Locally what os are you using? do you have the same thing with vagrant?

akkartik commented 10 years ago

Host OS: Ubuntu 12.04.1 Vagrant OS: Ubuntu 10.04.4

Hmm, I installed vagrant from apt-get. Is this a problem?

mk270 commented 10 years ago

I have different results in Vagrant from when running on bare metal.

liuluheng commented 10 years ago

I have got the same message

16 heads, 63 sectors/track, 0 cylinders, total 0 sectors Units = sectors of 1 * 512 = 512 bytes

but why 0 cylinders?

vpereira commented 9 years ago

just something from fdisk:

the -u option, should receive the format wanted. The default is set if just "-u" is given. The default is different in different OS or Linux flavors. To avoid it you should use:

/sbin/fdisk -u=cylinder ...