UNIX for Dummies Questions & Answers

I installed ubuntu on a windows machine, but after restart the computer automatically booted windows without showing me boot option. I think I might have installed my ubuntu bootloader in the wrong partition. I previously have sda1,2,3,5 and 6. I partitioned sda6 into sda6, sda7 and sda8. I mounted / on sda6, swap on sda7, /home on sda8. When choosing where to install bootloader on the bottom, I selected sda6, instead of sda, where my windows bootloader is. I wonder if I should choose that disk? And now how can I repair it? I used a usb to install it. I can lunch ubuntu directly from my usb, so I tried grub-install /dev/sda on the usb lunched version but it displays: cp: cannot create regular file `/boot/grub/915resolution.mod': Permission denied. Don't know what to do.
Btw, in which disk is my ubuntu kernel installed? thanks a lot.

OK, first a few words about the booting process of a PC. That will make it clearer, hopefully:

When a PC starts, the BIOS holds the first program to be loaded. It scans several busses (IDE-controllers, SCSI-controllers, ...) for prospective devices to boot from and creates a list. Then this is worked from top to bottom (though it is possible to exclude certain devices or rearrange list entries - this is a common option in BIOSes).

Every device found that way - usually disks, USB sticks, CD-ROM-drives, diskette drives and so on - is in turn scanned for a so-called "Master boot record". This is a certain disk block, which contains a boot-loader program - if the disk is bootable. If not, then this space is simply left free and the process tries the next device in its list and searches there for a MBR.

If such a boot-loader is found, it is loaded by the BIOS and started. The boot-loader is NOT an OS by itself, it just loads an OS. GRUB, the older lilo and some other boot-loaders work that way. Usually the boot-loader presents a selection of kernels to boot from and a list of root volumes (aka "partitions" to use as the root of the FS - to mount as "/" in case of Unix-systems, as "C:" in case of DOS/Windows-systems or as "[SYS]" in case of VMS-systems.

Once a kernel image and/or boot partition is selected the boot-loader program tries to pass control to an OS-bootstrap program located in certain parts of the selected partition (this is why you need to flag a partition "active" to boot from - this certain space is set apart) and starts that. This in turn loads the OS kernel and the rest is OS specific. Note, that the whole process up to the initialization of the OS kernel happens in the CPUs "real mode", even for protected mode/virtual mode OSes. They have to switch the CPU into their respective mode of operation themselves.

So, to answer your question: yes, you will have to install the boot loader (perhaps "GRUB", less likely "lilo") to the MBR ("/dev/sda", which signifies the disk, rather than "/dev/sdaX", which denotes a certain partition on that disk), otherwise it will not get executed automatically. You need to do that as root, not as normal user, because files involved in the boot process have mostly permissions set to be accessible only by root.

I hope this helps.

bakunin

oh, I see, and I used EasyBCD to re-install grub. It works. Thanks a lot.

Hi, freedombird9!
Can you please post your procedure to reinstall your grub with EasyBCD? I met similar problem to boot my Ubuntu server (64bit) for my dual system. The other one is Win7 which can boot fine.
The problem with Ubuntu is it always boot into rescue mode and Googled out GRUB is broken. Tried liveCD and boot-repair, but not succeed. Thank you in advance!

yeah, you need to install the EasyBCD in your Windows, and it will automatically install the new grub to your MBR, you only need to select the version, it's very easy and straight forward.