write-back/write-through/write-allocate/write-no-allocate说明

Cache读写策略

• write-back
• write-through
• write-allocate
• write-no-allocate

CPU 读 Cache 时：

• 若 hit，则 CPU 直接从 Cache 中读取数据即可。

• 若 miss，有两种处理方式：

  • Read through，即直接从内存中读取数据；

• Read allocate，先把数据读取到 Cache 中，再从 Cache 中读数据。

执行写操作时：

先检查 cache 里是否有对应数据，如果有（write hit）：

• 根据是 write-back 还是 write-through 来具体操作：

  • write-back：将数据更新到 cache，并不更新到内存（DRAM），待后续 flush cache 时存入内存；

  • write-through：数据同时会更新到 cache 和内存；

如果没有（write miss）：

• 根据是write-allocate或是write-no-allocate：

  • write-allocate：将要写入的位置从内存读到cache，然后按照上述write hit继续操作；

  • write-no-allocate：不会将要写入的数据从内存读到cache，直接将要写的数据写入内存。

transient attribute

Another new memory attribute feature in the Armv8-M architecture is that Normal memory has a new Transient attribute. If an address region is marked as Transient it means the data within is unlikely to be frequently used. A cache design could, therefore, utilize this information to prioritize transient data for cacheline evictions. A cacheline eviction operation is needed when the processor needs to store a new piece of data into the cache but all of the cache-ways of the corresponding cache index have already been used by older valid data. In the case of the Cortex-M23 and Cortex-M33 processors, this attribute is not used as (a) there is no data cache support, and (b) the AHB interface does not have any signal for transient indication. Please note, even when an Armv8-M processor has a data cache transient support, is an optional feature. This is because this feature increases the SRAM area needed for cache tags and might, therefore, not be desirable for some designs.

from: https://www.sciencedirect.com/topics/engineering/memory-attribute

device memory attributes

https://blog.youkuaiyun.com/shenhuxi_yu/article/details/90617675

https://zhuanlan.zhihu.com/p/124946496