why isnot __m64 working?

why isnot __m64 working?

dear experts,
my environment: pxa250,intel c++ compiler,embedded c++4.0
I want to use __m64 type.
__m64 temp1,temp2;
;;;
temp1 = temp2;
why canot the value of temp2 give temp1 by above method?
Thanks in advance.

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