Some keys, such as function keys, cursor keys, and ALT+KEY combinations, have no ASCII code. When a key is pressed, a microprocessor within the keyboard generates an "extended scan code" of two bytes.
The first (low-order) byte contains the ASCII code, if any. The second (high-order) byte has the scan code--a unique code generated by the keyboard when a key is either pressed or released. Because the extended scan code is more extensive than the standard ASCII code, programs can use it to identify keys which do not have an ASCII code.
For more details on key codes, see:
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