64 BIT COMPUTING

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64-BIT COMPUTING Introduction The question of why we need 64-bit computing is often asked but rarely answered in a satisfactory manner. There are good reasons for the confusion surrounding the question.That is why first of all; let's look through the list of users who need 64 addressing and 64-bit calculations today: oUsers of CAD, designing systems, simulators do need RAM over 4 GB. Although there are ways to avoid this limitation (for example, Intel PAE), it impacts the performance. Thus, the
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  64-BIT COMPUTINGIntroductionThe question of why we need 64-bit computing is often asked but rarely answered ina satisfactory manner. There are good reasons for the confusion surrounding thequestion.That is why first of all; let's look through the list of users who need64 addressing and 64-bit calculations today: oUsers of CAD, designing systems,simulators do need RAM over 4 GB. Although there are ways to avoid this limitation(for example, Intel PAE), it impacts the performance. Thus, the Xeon processorssupport the 36bit addressing mode where they can address up to 64GB RAM.The idea of this support is that the RAM is divided into segments, and an addressconsists of the numbers of segment and locations inside the segment. This approachcauses almost 30% performance loss in operations with memory. Besides, programmingis much simpler and more convenient for a flat memory model in the 64bit addressspace - due to the large address space a location has a simple address processedat one pass. A lot of design offices use quite expensive workstations on the RISCprocessors where the 64bit addressing and large memory sizes are used for a longtime already. oUsers of data bases.Any big company has a huge data base, and extension of the maximum memory size andpossibility to address data directly in the data base is very costly. Although inthe special modes the 32bit architecture IA32 can address up to 64GB memory, atransition to the flat memory model in the 64bit space is much more advantageousin terms of speed and ease of programming. oScientific calculations. Memory size,a flat memory model and no limitation for processed data are the key factors here.Besides, some algorithms in the 64bit representation have a much simpler form.oCryptography and safety ensuring applications get a great benefit from 64bitinteger calculations.The labels 16-bit, 32-bit or 64-bit, when applied to a microprocessor,characterize the processor's data stream. Although you may have heard the term 64-bit code, this designates code that operates on 64-bit data. In more specificterms, the labels 64-bit, 32-bit, etc. designate the number of bits that eachof the processor's general-purpose registers (GPRs) can hold. So when someone usesthe term 64-bit processor, what they mean is a processor with GPRs that store64-bit numbers. And in the same vein, a 64-bit instruction is an instructionthat operates on 64-bit numbers. In the diagram above black boxes are code, whiteboxes are data, and gray boxes are results. The instruction and code sizes arenot to be taken literally, since they're intended to convey a general feel forwhat it means to widen a processor from 32 bits to 64 bits.Not all the data either in memory, the cache, or the registers is 64-bit data.Rather, the data sizes are mixed, with 64 bits being the widest. Note that in the64-bit CPU pictured above, the width of the code stream has not changed; the same-sized opcode could theoretically represent an instruction that operates on 32-bitnumbers or an instruction that operates on 64-bit numbers, depending on what theopcode's default data size is. On the other hand, the width of the data stream hasdoubled. In order to accommodate the wider data stream, the sizes of theprocessor's registers and the sizes of the internal data paths that feed thoseregisters must be doubled.Now let's take a look at two programming models, one for a 32-bit processor andanother for a 64-bit The registers in the 64-bit CPU pictured above are twice aswide as those in the 32- bit CPU, but the size of the instruction register (IR)that holds the currently executing instruction is the same in both processors.Again, the data stream has doubled in size, but the instruction stream has not.Finally, the program counter (PC) has also doubled in size.  For the simple processor pictured above, the two types of data that it can processare integer data and address data. Ultimately, addresses are really just integersthat designate a memory address, so address data is just a special type of integerdata. Hence, both data types are stored in the GPRs and both integer and addresscalculations are done by the ALU.Many modern processors support two additionaldata types: floating-point data and vector data. Each of these two data types hasits own set of registers and its own execution unit(s).
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