As a result, any 32-bit address that falls in the user segment kuseg, and which must have a sign bit of 0, is extended to a 64-bit integer with 32 high-order 0 bits. This automatically places the 32-bit kuseg in the bottom of the 64-bit xkuseg, as shown in Figure 1-7.
A 32-bit kernel address, which must have a sign bit of 1, is automatically extended to a 64-bit integer with 32 high-order 1 bits. This places all kernel segments shown in Figure 1-5 at the extreme top of the 64-bit address space. However, these 32-bit kernel spaces are not used by a kernel operating in 64-bit mode.