The pairing between the process descriptor and the Kernel Mode stack offers a key benefit in terms of efficiency: the kernel can easily obtain the process descriptor pointer of the process currently running on the CPU from the value of the esp register. In fact, since the memory area is 8 KB (213 bytes) long, all the kernel has to do is mask out the 13 least significant bits of esp to obtain the base address of the process descriptor. This is done by the current macro.
