Page Fault Interrupt Problems

This is a statement referring to problem caused by page fault:(from Silberschatz 7th ed P-310 last para)

'We cant simply restart instructions when instruction modifies several different location Ex:when a instruction moves 256 bytes from source to dest and either src or dest straddles on page boundary , then,after a partial move, if a page fault occurs, 'we can't simply restart the instructions'

My question is:: Why not? Simply restart the instruction again do the same copy after page is in. Is there any problem in it?

[edit]Can anyone explain What exactly happens in case source and destination locations are overlapping? [开发者_JAVA技巧/edit] P.S=> sorry for the late edit.

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Because the counter register (e.g. ECX) has been changed since the start of the operation.