RAM 6.0 BUX II Series Guia do Utilizador descarregar pdf (Página 30)

VxWorks

Hardware Considerations Guide, 6.0

block now maps to the area of flash that contains the modified data. This mapping

information is protected, even during a fault. For more information on fault

recovery and mapping information, see Recovering Mapping Information, p.29.

Erase Cycles and Garbage Collection

The write operation is intrinsically linked to the erase operation, since data cannot

be “over-written.” Data must be erased, and then those erased units must be

reclaimed before they are made available to a write operation. This section

describes that process, as well as the consequences of over-erasing sections of flash.

Erasing Units

Once data is written to an area of flash memory, modifying blocks leaves behind

block-sized regions of flash memory that no longer contain valid data. These

regions are also un-writable until erased. However, the erase cycle does not

operate on individual bytes or even blocks. Erasure is limited to much larger

regions called erase units. The size of these erase units depends on the specific flash

technology, but a typical size is 64 KB. For more information on erase units, see the

VxWorks Device Driver Developer’s Guide: Flash File System Support with TrueFFS.

Reclaiming Erased Blocks

To reclaim (erase) blocks that no longer contain valid data, TrueFFS uses a

mechanism called garbage collection. This mechanism copies all the valid data

blocks, from a source erase unit, into another erase unit known as a transfer unit.

TrueFFS then updates the block-to-flash map and afterward erases the old (erase)

unit. The virtual block presented to the outside world still appears to contain the

same data even though that data now resides in a different part of flash.

For details on the algorithm used to trigger garbage collection, see Garbage

Collection, p.28. For information about garbage collection and fault recovery, see

Recovering During Garbage Collection, p.30.

Over-Programming

As a region of flash is constantly erased and rewritten, it enters an over-programmed

state, in which it responds only very slowly to write requests. With rest, this

condition eventually fixes itself, but the life of the flash memory is shortened.

Eventually the flash begins to suffer from sporadic erase failures, which become

more and more frequent until the medium is no longer erasable and thus no longer

writable (data that is already resident on the flash is still readable). Consequently,

flash limits how often you can erase and rewrite the same area. This number,

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