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A processor supplementary capability is a feature that has been added to an existing central processing unit design after the initial introduction of that design to the marketplace. A supplementary capability increases the usefulness of the processor design, allowing it to compete more favorably with competitors and giving consumers a reason to upgrade, while retaining backwards compatibility with the original design. The CPU supplementary instruction capability does not as a rule apply to 8 or 16 bit CPUs, as many of these CPUs are used mostly as microcontrollers. On modern 32 and 64 bit CPUs the ''processor supplementary capability'' does not extend to Floating Point Units (FPUs) or Memory Management Units (MMUs) as these are considered to be fundamental core functionalities. Extensions to the core functionalities of the MMU and FPU may be considered CPU extensions however. == Historical reasoning == The supplementary instructions feature has always been assumed to mean fixed sets of instructions that are not obligatory across all CPUs in a CPU family. Supplementary instructions will simply not be found on all processors within that family.〔http://markhobley.yi.org/glossary/supplementarycapability.html〕 A programmer who wishes to use a supplementary feature of a CPU is faced with a couple of choices. Supplemental instruction programming options * The ''operating system'' (kernel) and ''systems programmer'' (programs) may choose to design the systems software so that it mandatorily uses that feature and therefore can only be run on the more recent processors that have that feature. * On the other hand the system programmer may write or use existing software libraries to determine whether the processor it is running on has a particular feature (or set of instructions). Should the needed instructions not be there a fall back to a (presumably slower or otherwise less desirable) alternative technique can be initiated or else the program may be set to run with reduced functionality. * In other cases, an operating system may mimic the new features for older processors, though often with reduced performance. By using a lowest common denominator strategy (avoiding use of processor supplementary capabilities), programs can be kept portable across all machines of the same architecture.〔http://markhobley.yi.org/glossary/supplementarycapability.html〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「processor supplementary capability」の詳細全文を読む スポンサード リンク
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