Architectural Implications
Processor registers are typically divided into several groups: integer, floating-point, SIMD, control, and often special registers for address arithmetic which may have various uses and names such as address, index or base registers. However, in modern designs, these functions are often performed by more general purpose integer registers. In most processors, only integer and/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.
Most high performance 32-bit and 64-bit processors (some notable exceptions are most ARM and 32-bit MIPS CPUs) have integrated floating point hardware, which is often, but not always, based on 64-bit units of data. For example, although the x86/x87 architecture has instructions capable of loading and storing 64-bit (and 32-bit) floating-point values in memory, the internal floating point data and register format is 80 bits wide, while the general-purpose registers are 32 bits wide. In contrast, the 64-bit Alpha family uses a 64-bit floating-point data and register format (as well as 64-bit integer registers).
Read more about this topic: 64-bit Computing
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