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Active Partitions and Boot Managers
Only primary partitions can be used to boot the operating system, and of these, only the specific primary partition that is set to be bootable. DOS calls the bootable partition the active partition. Only one partition can be set active at a time because otherwise, the master boot record does not know to which volume's boot code to give control of the boot process when the machine is turned on.
If you partition a new hard disk and create a primary DOS partition using the standard DOS utility FDISK, but forget to set the primary partition active, the BIOS will be unable to boot the operating system. This usually results in an error message like "No boot device available". Some BIOSes will give much more cryptic messages; older AMI BIOSes are (in)famous for giving the bizarre "NO ROM BASIC - SYSTEM HALTED" message when it cannot find a boot device. The reason for this error is that early IBM systems had a hard-coded version of the BASIC language built into its BIOS ROM. If no boot device could be found, the BIOS would execute this hard-coded BASIC interpreter instead. Since non-IBM systems don't have this BASIC ROM, their BIOSes must display an error message instead of going into BASIC. Why AMI chose this confusing message is a mystery to me, but at least you understand its history now. :^)
Most people are only going to have one primary partition on their PC, because most people only use one operating system. Remember that even if you want to split your disk up into multiple FAT file system partitions, only the first will be a primary partition--the rest will be logical drives within an extended partition. However, if you are using more than one operating system--meaning ones that use different file formats, like Linux and Windows ME, not DOS and Windows ME, which use the same file systems generally--then you may want to set up multiple primary partitions, one per operating system. You then have the problem of telling the system at boot time which operating system you want to use.
There are programs specifically designed for this task; they are usually called boot managers or boot loaders. What a boot manager does is insert itself into the very beginning of the boot process, sometimes by setting up a special boot manager partition and making itself the active partition. When you boot up the PC, the code in this partition runs. It analyzes the primary partitions on the disk and then presents a menu to you and asks which operating system you want to use. Whichever one you select, it marks as active, and then continues the boot process from there. Other methods may also be used to accomplish the same general objectives.
Boot managers are in many ways indispensable when working with multiple operating systems. However, you still want to take care when using one, since it does modify the disk at a very low level. Some boot managers require their own, dedicated partitions to hold their own code, which complicates slightly the setup of the disk. There are now a variety of different boot manager products on the market, including some that come included with utilities like Partition Magic.
Next: The DOS Boot Process