The vast majority of PC users today have no memory of what PC keyboards looked like before the standard 101/102-key layout arrived, even though various OEMs do their best to mangle the standard layout in order to minimize usability, especially on laptops. OEM-specific modifications aside, the basic layout of the main block of alphanumeric keys has not changed in over 30 years, since 1986.
However, up until that point the PC keyboard layout and the keyboard hardware changed quite a bit, and looking at the 1981-1986 IBM Technical References is key to understanding a) why the standard keyboard scan codes are so complex, and b) why there are so many seemingly odd vendor-specific modifications of the standard layout.
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