Otherwise benign bugs may sometimes be harnessed for malicious intent by an unscrupulous user writing an exploit, code designed to take advantage of a bug and disrupt a computer's proper execution. Since computers merely execute the instructions they are given, bugs are nearly always the result of programmer error or an oversight made in the program's design. The control unit, ALU, and registers are collectively known as a central processing unit . Since the 1970s, CPUs have typically been constructed on a single MOS integrated circuit chip called a microprocessor. A combination of the planisphere and dioptra, the astrolabe was effectively an analog computer capable of working out several different kinds of problems in spherical astronomy. An astrolabe incorporating a mechanical calendar computer and gear-wheels was invented by Abi Bakr of Isfahan, Persia in 1235. Abū Rayhān al-Bīrūnī invented the first mechanical geared lunisolar calendar astrolabe, an early fixed-wired knowl...