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I thought there was also a mandatory fixed-layout "boot" profile for mice and keyboards? There was some controversy because vendors interpreted it as only being

I thought there was also a mandatory fixed-layout "boot" profile for mice and keyboards? There was some controversy because vendors interpreted it as only being allowed to support the boot profile, resulting in most USB keyboards having 6-key rollover maximum.

Findecanor 2y ago
In a nutshell, yes. There's a flag for it (actually "subclass") for indicating being able to switch to the boot protocol. Some hosts, especially BIOS:es, did check that flag but never sent the command to switch. Keyboard vendors catered to the lowest common denominator because it was essential that users be able to enter the PC BIOS at boot. BTW. Apple chiclet keyboards (before the "Magic") got an interested workaround to this problem for its proprietary Fn key. It uses a variation of the boot protocol but only a 5-byte array (5KRO). When the Fn key is pressed, the sixth byte will contain a code that is otherwise an error code if interpreted as the boot protocol.