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. 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.
Read more about this topic: Processor Supplementary Capability
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