Page "Booting" Paragraph 35

Some computer systems, upon receiving a boot signal from a human operator or a peripheral device, may load a very small number of fixed instructions into memory at a specific location, initialize at least one CPU, and then point the CPU to the instructions and start their execution.

These instructions typically start an input operation from some peripheral device ( which may be switch-selectable by the operator ).

Other systems may send hardware commands directly to peripheral devices or I / O controllers that cause an extremely simple input operation ( such as " read sector zero of the system device into memory starting at location 1000 ") to be carried out, effectively loading a small number of boot loader instructions into memory ; a completion signal from the I / O device may then be used to start execution of the instructions by the CPU.