Cache question...?

Hello, I'm a newbie, and have few short questions...
Let's say that I have following instructions executed:

1> xor ecx, ecx
2> mov eax, [esi]
3> mov [esi], 33
4> mov ecx, [esi]
5> mov [esi], eax

Is it possible, or even very likely to happen, that line 4 will access
cache, not main memory, and what could happen when value placed in
esi, points to not exesting address (exceeds RAM size, and have no
match in other memories, when paging is disabled) ? Is it possible
then, that ecx = 33 ?

.

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