<Tr> <Th> Operating system </Th> <Th> 2012 (millions of units) </Th> <Th> 2013 (million of units) </Th> </Tr> <Tr> <Th> Android </Th> <Td> 504 </Td> <Td> 878 </Td> </Tr> <Tr> <Th> Windows </Th> <Td> 346 </Td> <Td> 328 </Td> </Tr> <Tr> <Th> iOS / Mac OS </Th> <Td> 214 </Td> <Td> 267 </Td> </Tr>

What is an operating system explain the major function of os