ROM Purpose—Device Can Be Used In Boot Process

In order to boot the OS into memory, the system needs three devices:

• A mass storage device to load the OS from. This is sometimes referred to as the IPL (Initial Program Load) device and is typically an IDE or a SCSI hard drive.

• A display adapter to enable progress messages to be displayed during the boot process. In this context, this is typically referred to as the output device.

• A keyboard to allow the user to interact with the machine during the boot process. In this context, this is typically referred to as the input device.

The OS must locate three devices that fall into these categories and must also locate a device driver associated with each of the devices. Remember that the OS hasn't been booted into memory yet and therefore hasn't loaded any loadable device drivers into memory from disk! This is the main reason that device ROMs exist. It contains a device driver that permits the device to be used during the boot process.