A vendor's 64-bit operating systems may have a different page size from the vendor's 32-bit operating system. For example, IRIX 5.3 has a 4 kilobyte page size, while IRIX 6.0.1 has a 16 kilobyte page size. If your code is tuned to a particular page size or depends on proper page alignment of data structures, you may need to adjust to a new page size when you port to a 64-bit operating system. Most Unix operating systems implement a getpagesize routine to return the number of bytes in a page so that you can portably deal with different page sizes.