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In computer architecture , bit computing is the use of processors that have datapath widths, integer size, and memory address widths of 64 bits eight octets. Also, bit computer architectures for central processing units CPUs and arithmetic logic units ALUs are those that are based on processor registers , address buses , or data buses of that size.
From the software perspective, bit computing means the use of code with bit virtual memory addresses. However, not all bit instruction sets support full bit virtual memory addresses; x and ARMv8 , for example, support only 48 bits of virtual address, with the remaining 16 bits of the virtual address required to be all 0's or all 1's, and several bit instruction sets support fewer than 64 bits of physical memory address.
The term bit describes a generation of computers in which bit processors are the norm. A bit register can store 2 64 over 18 quintillion or 1. The range of integer values that can be stored in 64 bits depends on the integer representation used. With no further qualification, a bit computer architecture generally has integer and addressing processor registers that are 64 bits wide, allowing direct support for bit data types and addresses.
However, a CPU might have external data buses or address buses with different sizes from the registers, even larger the bit Pentium had a bit data bus, for instance [2]. The term may also refer to the size of low-level data types, such as bit floating-point numbers.
Processor registers are typically divided into several groups: However, in modern designs, these functions are often performed by more general purpose integer registers. In most processors, only integer or address-registers can be used to address data in memory; the other types of registers cannot. The size of these registers therefore normally limits the amount of directly addressable memory, even if there are registers, such as floating-point registers, that are wider.
In contrast, the bit Alpha family uses a bit floating-point data and register format, and bit integer registers. Many computer instruction sets are designed so that a single integer register can store the memory address to any location in the computer's physical or virtual memory. Therefore, the total number of addresses to memory is often determined by the width of these registers.
Some supercomputer architectures of the s and s, such as the Cray-1 , [3] used registers up to 64 bits wide, and supported bit integer arithmetic, although they did not support bit addressing. In the mids, Intel i [4] development began culminating in a too late [5] for Windows NT release; the i had bit integer registers and bit addressing, so it was not a fully bit processor, although its graphics unit supported bit integer arithmetic.
A notable exception to this trend were mainframes from IBM, which then used bit data and bit address sizes; the IBM mainframes did not include bit processors until During the s, several low-cost bit microprocessors were used in consumer electronics and embedded applications.
Notably, the Nintendo 64 [7] and the PlayStation 2 had bit microprocessors before their introduction in personal computers. High-end printers, network equipment, and industrial computers, also used bit microprocessors, such as the Quantum Effect Devices R However, not all instruction sets, and not all processors implementing those instruction sets, support a full bit virtual or physical address space. The x architecture as of [update] allows 48 bits for virtual memory and, for any given processor, up to 52 bits for physical memory.
Thus the bit physical address provides ample room for expansion while not incurring the cost of implementing full bit physical addresses. The Power ISA v3. The Oracle SPARC Architecture allows 64 bits for virtual memory and, for any given processor, between 40 and 56 bits for physical memory. A change from a bit to a 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.
The operating systems for those bit architectures generally support both bit and bit applications. The IMPI instruction set was quite different from even bit PowerPC, so this transition was even bigger than moving a given instruction set from 32 to 64 bits. On bit hardware with x architecture AMD64 , most bit operating systems and applications can run with no compatibility issues. While the larger address space of 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 bit compatibility modes will be faster than comparably priced bit systems for other tasks.
A compiled Java program can run on a or 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 bit Java virtual machine have access to a larger address space. Speed is not the only factor to consider in comparing bit and bit processors. Applications such as multi-tasking, stress testing, and clustering — for high-performance computing HPC — may be more suited to a bit architecture when deployed appropriately.
The main disadvantage of bit architectures is that, relative to 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 bit model is one way to handle this, and is in general reasonably effective. Not all such applications require a large address space or manipulate bit data items, so these applications do not benefit from these features. The most severe problem in Microsoft Windows is incompatible device drivers for obsolete hardware.
Most bit application software can run on a bit operating system in a compatibility mode, also termed an emulation mode, e. DLL, which cannot call bit Win32 subsystem code often devices whose actual hardware function is emulated in user mode software, like Winprinters.
Because bit drivers for most devices were unavailable until early Vista x64 , using a bit version of Windows was considered a challenge. Most manufacturers started to provide both bit and bit drivers for new devices, so unavailability of bit drivers ceased to be a problem. Driver compatibility was less of a problem with open-source drivers, as bit ones could be modified for bit use.
Support for hardware made before early , was problematic for open-source platforms, [ citation needed ] due to the relatively small number of users. However, most bit applications will work well. Mac OS X This allowed those Macs to support bit processes while still supporting bit device drivers; although not bit drivers and performance advantages that can come with them.
On systems with bit processors, both the and bit macOS kernels can run bit user-mode code, and all versions of macOS include bit versions of libraries that bit applications would use, so bit user-mode software for macOS will run on those systems.
This source-based distribution model, with an emphasis on frequent releases, makes availability of application software for those operating systems less of an issue.
In bit programs, pointers and data types such as integers generally have the same length. This is not necessarily true on bit machines. In many programming environments for C and C-derived languages on 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.
Another alternative is the ILP64 data model in which all three data types are 64 bits wide, and even SILP64 where short integers are also 64 bits wide. Another alternative is the LLP64 model, which maintains compatibility with bit code by leaving both int and long as bit.
LL refers to the long long integer type, which is at least 64 bits on all platforms, including bit environments. Microsoft Windows uses an LLP64 model. The disadvantage of the LP64 model is that storing a long into an int may overflow.
In the LLP64 model, the reverse is true. These are not problems which affect fully standard-compliant code, but code is often written with implicit assumptions about the widths of data types. 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 bit PCI device asking the device to DMA data into upper areas of a bit machine's memory could not satisfy requests from the operating system to load data from the device to memory above the 4 gibibyte 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 for DMA, or by using an input—output memory management unit IOMMU.
Most architectures of 64 bits that are derived from the same architecture of 32 bits can execute code written for the bit versions natively, with no performance penalty. From Wikipedia, the free encyclopedia. For bit images in computer graphics, see Deep color. This section needs additional citations for verification. Please help improve this article by adding citations to reliable sources.
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