In this book (understanding Linux kernel), the kernel can easily obtain the address of the thread_info structure of the process currently running on a CPU from the value of the esp register. In fact, if the thread_union structure is 8 KB (213 bytes) long, the kernel masks out the 13 least significant bits of esp to obtain the base address of the thread_info structure; on the other hand, if the thread_union struc- ture is 4 KB long, the kernel masks out the 12 least significant bits of esp. This is done by the current_thread_info() function, which produces assembly language instructions like the following: movl $0xffffe000,%ecx or 0xfffff000 for 4KB stacks andl %esp,%ecx movl %ecx,p Why is *"stack pointer(esp) & 0xffffe000"* equal to the process descriptor base address? That means the base address of process descriptor is always *0xXYZ...000*, right? It is weird.