the chapter on A Caching Memory in the book

[ns]

hello I am trying to form a conceptual picture of how all the different memory related specs in Chapter 5 of Specifying Systems relate to each other. First off is MemoryInterface which I understand to be essentially the signature of the  interface over which the processor and memory communicate (the dashed box in the figure) . This has a variable memInt which represents the "state of the interface". Then comes the spec InternalMemory which makes use of the MemoryInterface (specifically the Send and Reply actions). This is wrapped inside a module called Memory to hide the internal vars. Finally there's WriteThroughCache which it says "implements the memory specification". My questions are

• what does "state of the interface" (memInt) represent? And what are the operators Send and Reply actually doing?  At first I thought they were implemented by the memory but they are not. 
• is InternalMemory supposed to a spec of what's labeled MEMORY in the figure?
• It seems to me that WriteThroughCache essentially just extends (in the object oriented sense) MemoryInterface apart from having to re-express Req and Rsp on account of TLA not making the Frame assumption. In what sense is it "implementing" Memory?

thanks!

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