10 Chapter 1 code to check the system hardware and load the main operating system software from disk. It also contains some basic hardware control routines so that the keyboard and screen can be used before the main system has been loaded. The hard disk is a non-volatile, read and write storage device, consisting of a set of metal disks with a magnetic recording surface, read/write heads, motors and control hardware. It provides a large volume of data storage for the operating system, application and user files. The applications are stored on disk and then selected as required for loading into memory; because the disk is a read and write device, user files can be stored, applications added and software updates easily installed. Standard hard disk drives can now hold over 1 TB (1 terabyte ¼ 10 12 bytes) of data. The PC system quickly becomes ever more elaborate, and this description may well already be out of date in some respects. However, the basic principles of microprocessor system operation are the same as established in the earliest digital computers, and these also apply to microcontrollers, as we will see. 1.2. Word-Processor Operation In order to understand the operation of the PC microprocessor system, we will look at how the