Lion is the first operating system to require, and to fully take advantage of, 64-bit addressing modes in the Intel chips that power Apple’s Macintosh computers. One of the side-effects of this is that every object identifier in Mac OS X’s Cocoa programming framework (typically an address in memory), is now twice as long as it was in a 32-bit environment.<\/p>\n
Apple has apparently taken advantage of the 64-bit runtime in Lion by optimizing the Objective C runtime itself to use some of these extra bits for, shall we say, clever purposes.\u00a0Bavarious describes an optimization<\/a> through which Apple is able to replace previously full-fledged opaque objects such as NSNumber with an object-placeholder that exists entirely as the 64-bit “object address” itself. This means that, for a wide range of “simple” objects, no additional memory allocation is required, and no retain\/release memory management is required for the “object.”<\/p>\n The trick relies on a implementation detail of the system, that allocated blocks of memory will always be aligned at 16-byte offsets into the address space. This leaves a bunch of numbers that can be represented in 64-bits, that cannot reasonably be assigned to any other object. To understand this practically, imagine that your neighborhood’s postal addresses are all assigned at offsets of 10: 30, 40, 50, etc. A clever postal service could institute an addressing system that uses an “invalid” address such as “31,” to perhaps mean “deliver to 30 with expedited afternoon delivery.”<\/p>\n Cleverness like this with encoding extra information in memory addresses is a time-honored tradition. I recall the days of 24-bit addressing on classic Mac OS, where Apple, and many 3rd party developers, observed that the high 8 bits of a typical memory address could be tweaked and used to store additional information, because the system would never reference those bits when resolving a particular address.<\/p>\n