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How Apple Boot Camp Works


Hard Drive Partitions

Boot Camp software relies on controlling how a Mac boots. To understand how Boot Camp takes control, let's first look at the Mac boot process. Specifically, we need to look at how a Mac reads and uses its hard drive, which stores the operating systems and all your data.

The hard drive is divided into one or more partitions. A partition is a range of physical addresses on the hard drive. In other words, the partition tells the computer where to read and write bits of data inside the hard drive. Information about the partitions on a hard drive is stored in a partition table.

When you boot your Mac, part of the boot process includes accessing the first few bytes of data of the hard drive. Those first bytes point to the partition table. From there, the partition table indicates which partition has the operating system and other data needed to finish booting the Mac.

Normally, when your Mac is fresh out of the box, it recognizes all the available storage space on your hard drive as one single partition. This is sufficient for most users, and it makes it easy to track your total available hard drive space.

However, if you want to install different operating systems on the same hard drive, you have to create different partitions for them. You could use any disk utility to create and format new partitions. Boot Camp, though, takes care of this partitioning for you. Boot Camp will resize your existing Mac OS partition and create and format a new partition for Windows. Boot Camp could also help in partitioning a separate hard drive if you had multiple hard drives in your Mac.

When you boot, how does the Mac know which partition to target? The partition table has an indicator of which partition to use when booting. Your Mac will look for its operating system on that partition. If you have both Mac and Windows, though, you need some way to select between those partitions. Boot Camp's role is to automate that selection so you don't have to worry about partition tables. Using Boot Camp, you'll have two options for switching between your Mac and Windows partitions:

  • Use the Boot Camp utility to indicate you want to switch to the other partition, and then reboot.
  • Use the Option key during the white splash screen while booting, and select the partition you want to use.

Now that you know what Boot Camp's doing, let's look at how to set it up on your Mac.