Editing USB (section)

===Keyboard Boot Protocol===
To avoid complexity in simpler USB stacks, such as those used in PC BIOS:es and bootloaders and [[KVM switch]]es, the USB HID standard specifies that keyboards (and mice) could also declare themselves to be "boot-capable".
Being "boot-capable" means that the keyboard (or mouse) would be able to switch to/from its regular fully-featured protocol to a special simpler hard-coded "Boot Protocol" that the simpler USB stack would understand. The switch would be done on special "Set Protocol" requests that only boot-capable devices would understand.

The boot protocol for keyboards imposes a limitation of six concurrent keys plus modifiers to be pressed at one time. This behaviour is referred to as 6-key rollover, or 6KRO.

====Protocol problem====
The USB HID specification is unfortunately vague about one detail of the boot protocols: the specification ''could'' be interpreted as requiring that a mouse or keyboard's regular (non-boot) protocol be a superset of the corresponding boot protocol, and to be limited to eight bytes in size. The ''keyboard boot protocol'' is however eight bytes long already, which means that boot-capable keyboards would in practice need to speak the boot protocol always.

Different engineers of either BIOS'es and keyboards have interpreted the specification differently on this point ... or engineered to be compatible with both interpretations:

* Some BIOS'es are overly simplified in that they would not send the required "Set Protocol" request to a Boot-capable keyboard. Such a BIOS could work incorrectly together with a keyboard designed according to the opposing interpretation of the standard.

* Many keyboards use only the simple 6KRO boot-protocol, as being able to enter BIOS is considered a vital feature and/or because supporting only one protocol is simpler and "good enough".

* Some keyboards that ''do'' support N-key rollover are not boot-capable, and do nothing in BIOS.

Other keyboards with NKRO work around this problem by allowing the user to select between 6KRO boot-protocol and NKRO with a DIP switch or special key combination.

====Workaround====
The trick for supporting both Boot Keyboard and NKRO is for the keyboard to expose ''two interfaces'' — as if it was two keyboards. One is a boot-capable keyboard interface and the other is a keyboard interface with N-key rollover, but with only one being active at the same time.
The change-overs happen when the boot keyboard receives "Set Protocol" requests, which (standards-compliant) BIOS:es and all fully-fledged operating systems send.

[[Soarer's Converter]] and firmware employ a variation of this trick, known to work with both Windows, MacOS and Linux and also with the non-compliant BIOS'es mentioned above.
The 6KRO "boot" protocol is sent always but the report descriptor for that interface describes it as containing no keys. Therefore, only a BIOS (which does not read report descriptors) would read from that interface but ignore the non-boot interface, and vice versa.<ref name="ghsoarer">Post by '''Soarer''' on Geekhack—[https://geekhack.org/index.php?topic=38786.msg1064755#msg1064755 Re:Are there any USB NKRO keyboards on the market yet?]. Posted 2013-10-04. Retrieved 2016-03-06.</ref>