On Mon, 16 Jul 2012, Pete Batard wrote: > On 2012.07.16 11:43, Markus wrote: > > Is it correct that the root hub of a host controller has always > > address 255 on the given USB bus? > > To answer your question, no that is not the case, at least across > platforms. On Windows however root hubs will always have 255 as their > address.
It's a little more complicated than that. Let's start by making a careful distinction between a "device address" and a "device number". The device address is the actual value used on the USB bus to identify the device. It is a 7-bit number, and it can take on any value from 0 to 127. However 0 is reserved for newly-connected devices; by the time a device has been enumerated its address will be > 0. The device number is the value used by the operating system or other software to identify the device. Valid ranges will depend on the operating system. Quite often it is the same as the device address, but not always. (In USB-3, for example, addresses are assigned automatically by the host controller hardware and not revealed to the operating system. Consequently the OS has to make up a device number, which bears no relation to the device address.) Getting back to the original question... Root hubs do not have device addresses at all, because packets on the USB bus are never addressed to the root hub. No packet sent by the host is meant for the root hub, whereas every packet sent by an external device is (again, with a few special exceptions in USB-3); either way there's no need for a root-hub address. However root hubs do have device numbers. The value depends on the operating system. In Linux the root hub's device number is always 1; In Windows the value currently used by libusbx is always 255. The actual value is rather arbitrary, since it never appears on the USB bus. Alan Stern ------------------------------------------------------------------------------ Live Security Virtual Conference Exclusive live event will cover all the ways today's security and threat landscape has changed and how IT managers can respond. Discussions will include endpoint security, mobile security and the latest in malware threats. http://www.accelacomm.com/jaw/sfrnl04242012/114/50122263/ _______________________________________________ libusbx-devel mailing list libusbx-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/libusbx-devel
