[Document Write Back] What are the events that writes data in the memory ?

[khandelwaltanuj]

Hi @cfuguet

Is it possible to give details of the events that causes a write acces to the memory ? As per my understand, when a dirty is evicted, ou a read/amo operation is performed on a dirty bit, it causes a write to memory.

Are there any other cases ?

Thanks a lot

Regards Tanuj

[cfuguet]

The cache initiates a write to the memory on any of the following conditions:

• When replacing a cacheline on a miss (read or write) and the victim cacheline is dirty.
• When performing an AMO or LR/SC operation on a dirty cacheline.
• When performing a write-through write.
• When performing an uncacheable write.
• When performing a flush or flush-invalidate operation on a dirty cacheline.
• When performing a read or write operation with the write-through hint on a dirty cacheline.

[khandelwaltanuj]

Thanks for your reply @cfuguet

So as I understand, a load operation with a write back on a dirty line does not causes a memory write operation.

Do you think, it would be nice to have this table in the documentations please ? Even if I know it may add a lot of verification work for me :O.

Regards, Tanuj

[cfuguet]

Hi @khandelwaltanuj,

Yes, a load hit with the write-back hint does not trigger a write when reading a dirty cacheline.

A load can only trigger a write in two scenarios:

1. load, cache miss and the victim cacheline is dirty.
2. load with the write-through hint, cache hit and the target cacheline is dirty.

César