Incidentally, almost all computers contain some quantity of ROM (you’ll be able to produce a simple computer which contains no RAM — many microcontrollers do that by putting a number of RAM bytes around the processor nick itself — but generally impossible to produce one which contains no ROM). On the PC, the ROM is known as the BIOS (Fundamental Input/Output System). Once the micro-processor starts, it begins executing instructions it finds within the BIOS. The BIOS instructions do such things as test the hardware within the machine, after which it is going towards the hard disk drive to fetch the boot sector (observe how Hard Drives Work with details). This boot sector is yet another small program, and also the BIOS stores it in RAM after studying them back the disk. The micro-processor then begins executing the boot sector’s instructions from RAM. The boot sector program will inform the micro-processor to fetch another thing in the hard disk drive into RAM, that the micro-processor then executes, and so forth. This is the way the micro-processor loads and executes the whole operating-system.