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Every process has a read-only page mapped to its address space. This page is at a fixed location and keeps value of time, updated at every tick of clock. Since page is mapped into process memory, no system call is required. Process can directly read the page. This page is called vsyscall page and is filled by kernel at the start time.

It is part of kernel version 2.6+.

Reference: http://www.win.tue.nl/~aeb/linux/lk/lk-4.html