FAQ: SPI/Cache :: Line-Wrap mode vs. Line-base-first mode. Which one to choose?

Document created by PrasanthR Employee on Jul 25, 2013
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Wrap mode

Wrap mode is a specific feature supported by most flash devices in market. It enables the host processor to ask for the critical word first in a cache line. If enabled, the core is delivered with the critical word it asked for, as soon as possible (for instance, branching address). When using this mode, the Cache Controller, SPI Controller and the Flash device, all three blocks must be pre-programmed to support the same

 

Line-Wrap mode vs. Line-base-first mode

In Line-base-first mode, the line is always fetched starting at offset 0 (line base) while in line-wrap mode, the critical word is returned first; the rest of the line is fetched with wrap-around addressing. In Line-wrap mode, merging is not possible (not linear). Usually, line-base-first will be faster due to merging.

 

How to decide

Decision is application dependent. User has to experiment with his software, before deciding up on the mode. Some application require that critical instruction be executed very fast, so in such cases Wrap mode maybe used. For typical cases where one wants better linear throughput, Line base first mode maybe a better choice.

 

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