Content deleted Content added
Rewriting inaccurate paragraph, plus supplementary sourcing. |
Sourcing improved throughout this segment. |
||
Line 67:
There are two policies which define the way in which a modified cache block will be updated in the main memory: write through and write back.<ref name=":0" />
In the case of write through policy, whenever the value of the cache block changes, it is further modified in the lower-level memory hierarchy as well.<ref>David A. Patterson; John L. Hennessy; 2017. Computer Organization and Design RISC-V Edition: The Hardware Software Interface. Elsevier Science. pp. 386–387. ISBN 978-0-12-812276-1.</ref> This policy ensures that the data is stored safely as it is written throughout the hierarchy.
However, in the case of the write back policy, the changed cache block will be updated in the lower-level hierarchy only when the cache block is evicted. A "dirty bit" is attached to each cache block and set whenever the cache block is modified.<ref>Stefan Goedecker; Adolfy Hoisie; 2001. Performance Optimization of Numerically Intensive Codes. SIAM. p. 11. ISBN 978-0-89871-484-5.</ref> During eviction, blocks with a set dirty bit will be written to the lower-level hierarchy. Under this policy, there is a risk for data-loss as the most recently changed copy of a datum is only stored in the cache and therefore some corrective techniques must be observed.
In case of a write where the byte is not present in the cache block, the byte may be brought to the cache as determined by a write allocate or write no-allocate policy.<ref name=":0" /> Write allocate policy states that in case of a write miss, the block is fetched from the main memory and placed in the cache before writing.<ref>Harvey G. Cragon, 1996. Memory Systems and Pipelined Processors. Jones & Bartlett Learning. p. 47. ISBN 978-0-86720-474-2.</ref> In the write no-allocate policy, if the block is missed in the cache it will write in the lower-level memory hierarchy without fetching the block into the cache.<ref>David A. Patterson; John L. Hennessy; 2007. Computer Organization and Design, Revised Printing, Third Edition: The Hardware/Software Interface. Elsevier. p. 484. ISBN 978-0-08-055033-6.</ref>
The common combinations of the policies are [[Cache (computing)#Writing policies|"write block", "write allocate", and "write through write no-allocate"]].
| |||