Only primary partitions can be used to boot the operating system, and of these, only
the primary partition that is set to be bootable. Only one can be set bootable 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. DOS calls the bootable
partition the active partition.
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; AMI BIOSes are 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
older 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.
Most people are only going to have one primary partition on their PC, because most
people only use one operating system. If you are using more than one operating system
however--I mean incompatible ones that use different file formats, like Windows 95 with
UNIX, not DOS and Windows 95, 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. What a boot manager does is insert itself into the very beginning of the
boot process, normally 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.
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. One common boot
manager is IBM's boot manager, which ships as one component of PowerQuest's PartitionMagic 3.
Next: The DOS Boot Process
