D4.8.1 WriteNoSnoop

A WriteNoSnoop transaction is used in a region of memory that is not Shareable with other masters. A WriteNoSnoop transaction can result from:
• A program action, such as a store operation.
• An update of main memory for a cache line that is in a Non-shareable region of memory.

WriteNoSnoop 用于一段不和其他master share的内存。用于：

1. store操作

2. Non-shareable的内存的cache update到主存中。

writenosnoop是针对不共享的内存的操作，所以不需要给其他的master发送snoop的消息。

可以是直接写内存，或者是将cache中的内容同步到内存。

D4.8.2 WriteUnique

A WriteUnique transaction is used in a region of memory that is Shareable with other masters. A single write occurs that is required to propagate to main memory or a downstream cache.

和其他master共享的内存，writeunique是一个写操作，会将内容写入主存或downstream cache。

这个不是更新cache到主存的命令，是cpu直接发出的写的命令，这个时候如果有cache的话，cache的数据必须是处于clean的状态

In the case of master holding a line in a Clean state while performing a WriteUnique transaction, the cache line must be updated to the new value when the WriteUnique transaction response is received.

如果有clean cache的话，更新之后cache的内容也必须得到更新

D4.8.3 WriteLineUnique

A WriteLineUnique transaction is used in a region of memory that is Shareable with other masters. A single write occurs, that is required to propagate to main memory or a downstream cache.	写道主存或downstream cache
A WriteLineUnique transaction must be a full cache line store and all bytes within the cache line must be updated	一次必须是写整个cache的量
In the case of master holding a line in a Clean state while performing a WriteLineUnique transaction, the cache line must be updated to the new value when the WriteLineUnique transaction response is received.	如果有clean cache的话，cache中的内容也必须得到更新。

D4.8.4 WriteBack

A WriteBack transaction is a write that can be used in Shareable and Non-shareable regions of memory. A WriteBack transaction is a write of a dirty cache line to update main memory or a downstream cache.

writeback属于cache维护命令，将dirty cache写道主存或downstream cache。

The difference between a WriteBack and a WriteClean transaction is whether the cache line remains allocated in the cache for a Shareable region of memory. After a WriteBack transaction, the cache line is no longer allocated. After a WriteClean transaction, the cache line remains allocated.

writeback和writeclean的区别：

writeback 写之后cache将变为invalid，writeclean写之后 cache依然有效。

D4.8.5 WriteClean

A WriteClean transaction is a write operation that can be used in Shareable and Non-shareable regions of memory. A WriteClean transaction is a write of a dirty cache line to update main memory or a downstream cache.

同writeback，也属于cache 维护类操作，将dirty cache更新到主存或downstream cache。

D4.8.6 WriteEvict

A WriteEvict transaction can be used when evicting a clean cache line. This transaction is used to write the line to a lower level of the cache hierarchy, such as an L3 or system level cache. A WriteEvict transaction is not required to update main memory.

A WriteEvict transaction must only be used in the following circumstances:
• When the cache line is held in a UniqueClean state.
• When the cache line has not been speculatively fetched from a different shareability domain.

必须时unique clean，为啥？

能看出来这条操作是将upstream的cleancache写道downstream，为下次使用做准备。

Note
It is important that a cache line that could have been speculatively fetched, so that it was located outside of its shareability domain, could become out-of-date as the cache line is not required to be updated by subsequent stores to the cache line. If a cache line could be a stale copy, then it must not be written back into its shareability domain by the use of a WriteEvict transaction.

D4.8.7 Restrictions on WriteUnique and WriteLineUnique usage

Typically, WriteUnique and WriteLineUnique transactions are used by a non-cached component that is writing to a Shareable region of memory. However, WriteUnique and WriteLineUnique transactions can be used by a cached component that meets the requirements.	WriteUnique and WriteLineUnique的典型使用场景是一个没有cache的master对shareable内存的写操作。对有cache的master也可以，只是有下面一些限制：
A cached component must be able to complete any incoming snoop transaction while a WriteUnique or WriteLineUnique transaction is in progress. A cached component must:	在WriteUnique or WriteLineUnique执行时，component必须能完成其他master的snoop的操作，需要按如下规则操作：
Complete any outstanding WriteBack, WriteClean, WriteEvict, or Evict transactions before issuing a WriteUnique or WriteLineUnique transaction	在发出WriteUnique or WriteLineUnique传输之前，必须完成所有的WriteBack, WriteClean, WriteEvict, or Evict 传输。why？
No additional WriteBack, WriteClean, WriteEvict, or Evict transactions can be issued until all outstanding WriteUnique or WriteLineUnique transactions are completed.	在WriteUnique or WriteLineUnique完成之前不能发出额外的WriteBack, WriteClean, WriteEvict, or Evict 这些传输
Complete any incoming snoop transactions without the use of WriteBack, WriteClean, WriteEvict, or Evict transactions while a WriteUnique or WriteLineUnique transaction is in progress.	当 WriteUnique or WriteLineUnique正在执行时，直接完成所有的snoop，而不需要执行WriteBack, WriteClean, WriteEvict, or Evict transactions
WriteNoSnoop transactions can also be blocked behind WriteUnique and WriteLineUnique transactions. Therefore, the design of the master must ensure that an incoming snoop transaction can complete when a WriteNoSnoop transaction is blocked by an outstanding WriteUnique or WriteLineUnique transaction.
This is necessary, because earlier transactions that also might require earlier snoop transactions to complete, can prevent WriteUnique and WriteLineUnique transactions from progressing.
These requirements restrict the use of WriteUnique and WriteLineUnique transactions to components that can either: • Complete all snoop transactions without requiring any data to be supplied, for example write-through caches that do not keep dirty cache lines for Shareable data. • Complete snoop transactions by using the snoop data channel, CDDATA.

总结一下：

WriteNoSnoop	不与其他master共享内存，直接写内存或cache维护
WriteUnique	没有cache的master写（部分cache行）
WriteLineUnique	没有cache的master写（整个cache行）
WriteBack	cache维护指令，将cache写到downstream或主存中，写之后此cache变为invalid
WriteClean	cache维护指令，将cache写到downstream或主存中，写之后cache依然存在，变为clean
WriteEvict	将upstream的clean cache写道downstream，并且invalid upstream的cache

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D4.8 Write transactions