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the arm aarch64 virtual memory system architecture allows from 48 to 56 bits for virtual memory and for any given processor from 32 to 56 bits for physical memory 36 the dec alpha specification requires minimum of 43 bits of virtual memory address space 8 tb to be and hardware need to check and trap if the remaining unsupported bits are zero to support compatibility on future processors alpha 21064 supported 43 bits of virtual memory address space 8 tb and 34 bits of physical memory address space 16 gb alpha 21164 supported 43 bits of virtual memory address space 8 tb and 40 bits of physical memory address space 1 tb alpha 21264 supported user configurable 43 or 48 bits of virtual memory address space 8 tb or 256 tb and 44 bits of physical memory address space 16 tb 64 bit applications edit 32 bit vs 64 bit edit a change from a 32 bit to a 64 bit architecture is a fundamental alteration as most operating systems must be extensively modified to take advantage of the new architecture because that software has to manage the actual memory addressing hardware 37 other software must also be ported to use the new abilities older 32 bit software may be supported either by virtue of the 64 bit instruction set being a superset of the 32 bit instruction set so that processors that support the 64 bit instruction set can also run code for the 32 bit instruction set or through software emulation or by the actual implementation of a 32 bit processor core within the 64 bit processor as with some itanium processors from intel which included an ia 32 processor core to run 32 bit x86 applications the operating systems for those 64 bit architectures generally support both 32 bit and 64 bit applications 38 one significant exception to this is the ibm as 400 software for which is compiled into a virtual instruction set architecture isa called technology independent machine interface timi timi code is then translated to native machine code by low level software before being executed the translation software is all that must be rewritten to move the full os and all software to a new platform as when ibm transitioned the native instruction set for as 400 from the older 32 48 bit impi to the newer 64 bit powerpc as codenamed amazon the impi instruction set was quite different from even 32 bit powerpc so this transition was even bigger than moving a given instruction set from 32 to 64 bits on 64 bit hardware with x86 64 architecture amd64 most 32 bit operating systems and applications can run with no compatibility issues while the larger address space of 64 bit architectures makes working with large data sets in applications such as digital video scientific computing and large databases easier there has been considerable debate on whether they or their 32 bit compatibility modes will be faster than comparably priced 32 bit systems for other tasks a compiled java program can run on a 32 or 64 bit java virtual machine with no modification the lengths and precision of all the built in types such as char short int long float and double and the types that can be used as array indices are specified by the standard and are not dependent on the underlying architecture java programs that run on a 64 bit java virtual machine have access to a larger address space 39 speed is not the only factor to consider in comparing 32 bit and 64 bit processors applications such as multi tasking stress testing and clustering for high performance computing hpc may be more suited to a 64 bit architecture when deployed appropriately for this reason 64 bit clusters have been widely deployed in large organizations such as ibm hp and microsoft summary a 64 bit processor performs best with 64 bit software a 64 bit processor may have backward compatibility allowing it to run 32 bit application software for the 32 bit version of its instruction set and may also support running 32 bit operating systems for the 32 bit version of its instruction set a 32 bit processor is incompatible with 64 bit software pros and cons edit a common misconception is that 64 bit architectures are no better than 32 bit architectures unless the computer has more than 4 gb of random access memory 40 this is not entirely true some operating systems and certain hardware configurations limit the physical memory space to 3 gb on ia 32 systems due to much of the 3 4 gb region being reserved for hardware addressing see 3 gb barrier 64 bit architectures can address far more than 4 gb however ia 32 processors from the pentium pro onward allow a 36 bit physical memory address space using physical address extension pae which gives a 64 gb physical address range of which up to 62 gb may be used by main memory operating systems that support pae may not be limited to 4 gb of physical memory even on ia 32 processors however drivers and other kernel mode software more so older versions may be incompatible with pae this has been cited as the reason for 32 bit versions of microsoft windows being limited to 4 gb of physical ram 41 although the validity of this explanation has been disputed 42 some operating systems reserve portions of process address space for os use effectively reducing the total address space available for mapping memory for user programs for instance 32 bit windows reserves 1 or 2 gb depending on the settings of the total address space for the kernel which leaves only 3 or 2 gb respectively of the address space available for user mode this limit is much higher on 64 bit operating systems memory mapped files are becoming more difficult to implement in 32 bit architectures as files of over 4 gb become more common such large files cannot be memory mapped easily to 32 bit architectures as only part of the file can be mapped into the address space at a time and to access such a file by memory mapping the parts mapped must be swapped into and out of the address space as needed this is a problem as memory mapping if properly implemented by the os is one of the most efficient disk to memory methods some 64 bit programs such as encoders decoders and encryption software can benefit greatly from 64 bit registers citation needed while the performance of other programs such as 3d graphics oriented ones remains unaffected when switching from a 32 bit to a 64 bit environment citation needed some 64 bit architectures such as x86 64 and aarch64 support more general purpose registers than their 32 bit counterparts although this is not due specifically to the word length this leads to a significant speed increase for tight loops since the processor does not have to fetch data from the cache or main memory if the data can fit in the available registers example in c int a b c d e for a 0 a 100 a b a c b d c e d this code first creates 5 values a b c d and e and then puts them in a loop during the loop this code changes the value of b to the value of a the value of c to the value of b the value of d to the value of c and the value of e to the value of d this has the same effect as changing all the values to a if a processor can keep only two or three values or variables in registers it would need to move some values between memory and registers to be able to process variables d and e also this is a process that takes many cpu cycles a processor that can hold all values and variables in registers can loop through them with no need to move data between registers and memory for each iteration this behavior can easily be compared with virtual memory although any effects are contingent on the compiler the main disadvantage of 64 bit architectures is that relative to 32 bit architectures the same data occupies more space in memory due to longer pointers and possibly other types and alignment padding this increases the memory requirements of a given process and can have implications for efficient processor cache use maintaining a partial 32 bit model is one way to handle this and is in general reasonably effective for example the z os operating system takes this approach requiring program code to reside in 31 bit address spaces the high order bit is not used in address calculation on the underlying hardware platform while data objects can optionally reside in 64 bit regions not all such applications require a large address space or manipulate 64 bit data items so these applications do not benefit from these features software availability edit x86 based 64 bit systems sometimes lack equivalents of software that is written for 32 bit architectures the most severe problem in microsoft windows is incompatible device drivers for obsolete hardware most 32 bit application software can run on a 64 bit operating system in a compatibility mode also termed an emulation mode e g microsoft wow64 technology for ia 64 and amd64 the 64 bit windows native mode 43 driver environment runs atop 64 bit ntdll dll which cannot call 32 bit win32 subsystem code often devices whose actual hardware function is emulated in user mode software like winprinters because 64 bit drivers for most devices were unavailable until early 2007 vista x64 using a 64 bit version of windows was considered a challenge however the trend has since moved toward 64 bit computing more so as memory prices dropped and the use of more than 4 gb of ram increased most manufacturers started to provide both 32 bit and 64 bit drivers for new devices so unavailability of 64 bit drivers ceased to be a problem 64 bit drivers were not provided for many older devices which could consequently not be used in 64 bit systems driver compatibility was less of a problem with open source drivers as 32 bit ones could be modified for 64 bit use support for hardware made before early 2007 was problematic for open source platforms citation needed due to the relatively small number of users 64 bit versions of windows cannot run 16 bit software however most 32 bit applications will work well 64 bit users are forced to install a virtual machine of a 16 or 32 bit operating system to run 16 bit applications or use one of the alternatives for ntvdm 44 mac os x 10 4 tiger and mac os x 10 5 leopard had only a 32 bit kernel but they can run 64 bit user mode code on 64 bit processors mac os x 10 6 snow leopard had both 32 and 64 bit kernels and on most macs used the 32 bit kernel even on 64 bit processors this allowed those macs to support 64 bit processes while still supporting 32 bit device drivers although not 64 bit drivers and performance advantages that can come with them mac os x 10 7 lion ran with a 64 bit kernel on more macs and os x 10 8 mountain lion and later macos releases only have a 64 bit kernel on systems with 64 bit processors both the 32 and 64 bit macos kernels can run 32 bit user mode code and all versions of macos up to macos mojave 10 14 include 32 bit versions of libraries that 32 bit applications would use so 32 bit user mode software for macos will run on those systems the 32 bit versions of libraries have been removed by apple in macos catalina 10 15 linux and most other unix like operating systems and the c and c toolchains for them have supported 64 bit processors for many years many applications and libraries for those platforms are open source software written in c and c so that if they are 64 bit safe they can be compiled into 64 bit versions this source based distribution model with an emphasis on frequent releases makes availability of application software for those operating systems less of an issue 64 bit data models edit in 32 bit programs pointers and data types such as integers generally have the same length this is not necessarily true on 64 bit machines 45 46 47 mixing data types in programming languages such as c and its descendants such as c and objective c may thus work on 32 bit implementations but not on 64 bit implementations in many programming environments for c and c derived languages on 64 bit machines int variables are still 32 bits wide but long integers and pointers are 64 bits wide these are described as having an lp64 data model which is an abbreviation of long pointer 64 48 49 other models are the ilp64 data model in which all three data types are 64 bits wide 50 49 and even the silp64 model where short integers are also 64 bits wide 51 52 however in most cases the modifications required are relatively minor and straightforward and many well written programs can simply be recompiled for the new environment with no changes another alternative is the llp64 model which maintains compatibility with 32 bit code by leaving both int and long as 32 bit 53 49 ll refers to the long long integer type which is at least 64 bits on all platforms including 32 bit environments there are also systems with 64 bit processors using an ilp32 data model with the addition of 64 bit long long integers this is also used on many platforms with 32 bit processors this model reduces code size and the size of data structures containing pointers at the cost of a much smaller address space a good choice for some embedded systems for instruction sets such as x86 and arm in which the 64 bit version of the instruction set has more registers than does the 32 bit version it provides access to the additional registers without the space penalty it is common in 64 bit risc machines citation needed explored in x86 as x32 abi and has recently been used in the apple watch series 4 and 5 54 55 64 bit data models data model short int int long int long long pointer size_t sample operating systems ilp32 16 32 32 64 32 x32 and arm64ilp32 abis on linux systems mips n32 abi llp64 16 32 32 64 64 microsoft windows x86 64 ia 64 and arm64 using visual c or mingw plan 9 56 lp64 16 32 64 64 64 most unix and unix like systems e g solaris linux bsd macos windows when using cygwin z os ilp64 16 64 64 64 64 hal computer systems port of solaris to the sparc64 silp64 64 64 64 64 64 classic unicos 51 52 versus unicos mp etc a programming model is a choice made to suit a given compiler and several can coexist on the same os however the programming model chosen as the primary model for the os application programming interface api typically dominates another consideration is the data model used for device drivers drivers make up the majority of the operating system code in most modern operating systems citation needed although many may not be loaded when the operating system is running many drivers use pointers heavily to manipulate data and in some cases have to load pointers of a certain size into the hardware they support for direct memory access dma as an example a driver for a 32 bit pci device asking the device to dma data into upper areas of a 64 bit machine s memory could not satisfy requests from the operating system to load data from the device to memory above the 4 gigabyte barrier because the pointers for those addresses would not fit into the dma registers of the device this problem is solved by having the os take the memory restrictions of the device into account when generating requests to drivers f...
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